Note - Engenuics
Contents
1. notes_mpgl1_chapter10 docx Release 1 0 Page 17 of 20 ENGENUICS www engenuics com MPG LEVEL I 10 4 2 3 PWMAudioOn The easiest way to turn the PWM peripheral on and off is with the match 5 register when it is set to zero the PWM peripheral will keep the line low It is assumed that the frequency of the desired PWM signal is set prior to calling PWMAudioOn and all the function does is reset MRS back to half of the MRO register 10 4 2 4 PWMAudioOff On this particular processor turn the PWM signal off by setting MR5 to 0 The frequency last set in the peripheral will not be disturbed since the period is kept in MRO Most importantly this guarantees that the pin voltage is low so you do not leave a DC bias on the buzzer The only problem with this method is that the PWM peripheral is still on and thus consuming current even though it is not driving the output If you had a low power application then functions to turn the PWM on and off would need to also manage the PWM power control bit and make sure the buzzer line was low which probably means you would need to change the pin mode back to GPIO so you could set it to zero 10 4 3 Audio Bits You now have everything you need for the audio driver and the code should build and be ready for test As a last bit of info on the subject here is a quick music lesson that might come in handy 1 You should fully understand that different mus2. Indexing on the current loop counters lets you generically make changes to each LED with a single line of code Since the loop index is an unsigned integer it must be type casted to the enumerated type LedNumberType or else the compiler will generate a warning Note that type casting enums to ints or visa versa is not an ideal programming practice so you must be careful on how the enums are set up so that it works with what you plan to do especially in cases like this where a loop index is assumed to traverse the elements of an enum The ON check and action is left for you do This exists as an else if because we do not want to run the code if we have already determined that the LED is off Ditto for the subsequent check that manages a blinking or PWMing LED Modulation code requires a bit more work and it is slightly different for the case of just blinking the LED and the case of managing PWM since blinking is an adjustment of frequency and PWM is an adjustment of duty cycle Otherwise LED is being modulated in some way else LED is in normal mode if LGasLeds i eMode NORMAL Decrement counter toggle and reload if counter reaches 0 if LGasLeds i ul6Count 0 LedToggle LedNumberType i GasLeds i ul6Count LGasLeds i eRate notes_mpgl1_chapter10 docx Release 1 0 Page 8 of 20 Ks ENGENUVICS www engenuics com MPG LEVEL I LED i
3. that a square wave is a very powerful tool when it comes to achieving analog capability in a digital system Through frequency and duty cycle adjustments of the digital pulses external systems can be modulated to give the appearance of analog voltages this is what PWM is all about When you blink an LED at 1Hz you are in fact generating a square wave albeit a very slow one The LED is on for 500ms and off for 500ms Since the signal is high for half of the time we call this 50 duty cycle Now would be a good time to remind you that the period of a wave is the time between repetitions of the signal and the inverse of period is frequency Duty cycle is the ratio of the time the pulse is high to the total period Figure 10 1 1 shows some details of a 75 duty cycle PWM 1 Frequency Overall PWM Period 4 q 4 Overall PWM Period High Time 3 LowTime 1 Duty cycle ___High Time ___ Overall PWM Period _ _ o Figure 10 1 1 PWM signal details 75 duty cycle If you sped the signal up to 1 kHz the digital behavior of the LED is exactly the same The LED is on for 500us and off for 500us Since LEDs are very fast at reacting to changes in signal voltage the LED is in fact going from fully on to fully off at the drive rate which you can verify by attaching an oscilloscope and looking at the waveform However if you are looking at the LED while this is happening you will notes_mpgl1_chapter10 docx Re
4. IDE File Edit View Project Tools Window Help Cea amp COLOR CYCLE TIME VV Y y AD DA Workspace deva ehdw O1 c chapter10 c main c cstartup s DIAE 157 MODULE SUMMARY 158 159 interrupts c i Bl P 160 Module ro code ro data rw data E leds2 c ach E main c 162D Personal Engenuics MPG MPG Level 1 Firmware Chapter 10 E utilities c 163 buttons o 1 096 80 104 E Output i 164 chapterl0 o 692 18 29 D Ipc2148 log 165 cstartup o 248 166 deva ehdu 01 0 712 D Ipc2148 0ut 167 interrupts o 836 168 leds2 o0 1 500 L2148 EFW main n 13A Figure 10 5 1 Map file showing chapter10 o0 object size Closing thought You could port this code to a purpose built PCB and create a very neat night light or other great lighting effects in your home or car This is a great example of how the learning you do with a development board can be applied to a real life product notes_mpgl1_chapter10 docx Release 1 0 Page 20 of 20
5. bit bashing PWMs is not that difficult though does require a certain amount of processor resources to monitor timing and adjust duty cycles It is functionality that begs for a peripheral to take care of automatically PWM is such a fundamental capability of an embedded system you can almost guarantee that any microcontroller or microprocessor you choose will have at least one PWM peripheral capable of running at least a few PWM channels Regardless of the processor you are using a PWM peripheral is pretty much just a timer peripheral with a few extra features and in some cases less features than a timer peripheral The most notable difference between the two is that a PWM peripheral can be set up so that different events can occur at notes_mpgl1_chapter10 docx Release 1 0 Page 10 of 20 ENGENUVICS www engenuics com MPG LEVEL I different times in the waveform generation period and thus allows for varying duty cycles whereas timers usually only have one event per period On some processors PWM functionality is built in to all timer peripherals rather than having discrete PWMs and timers Using the PWM peripheral on the LPC214x processor is straight forward Just like you did with the Timer and GPIO peripherals you need to examine the datasheet to determine what registers are required to get the peripheral running the way you want Open up the LPC214x User Manual and to Chapter 16 that covers the PWM peripheral The description of the
6. driving signal for an LED must be fast enough to trick your eye into seeing a steady dimmed LED instead of a flickering light The minimum frequency for this is around 30Hz which gives a period of 33 3ms Keep that in mind notes_mpgl1_chapter10 docx Release 1 0 Page 3 of 20 ENGENUICS www engenuics com MPG LEVEL I The second thing to consider is how much PWM resolution we need want in the system For example if you wanted 1 duty cycle control you would need to have a total PWM period of 100 units so that you could adjust the duty cycle in 1 unit increments thereby achieving 1 duty cycle steps Therefore each LED could have 100 levels of brightness and you could have 100 1 million different colors The problem is that we need 100 cycles available to manage that amount of resolution but of course we only have 1ms periods cycles to work with That would mean we could only achieve 10Hz frequency for the LEDs and guaranteed you would see them flicker So we have to compromise Either we move away from our 1ms clock so we can get better resolution or we admit that we really do not need a million backlight colors If higher resolution was really necessary you would have to either adapt the 1ms system tick or use another timer to provide a faster interrupt to dedicate to the PWM However higher resolution is NOT necessary so we proceed with our 1ms system intact To make numbers work out nicely let us choose to
7. generate the PWM signal at 50Hz which equates to 20ms periods for the PWM This gives 5 duty cycle resolution and results in 20 8000 colors That is plenty be assured to create a very nice display as you will see once you have completed the chapter exercise Note that we could probably get away with 25Hz and thus 40 cycles to work with but it is likely that some people would notice flicker in the slower signal With those decisions made writing the driver is almost easy Keep in mind that it still requires a lot of thought and though you may read the following description in a few minutes and agree that it is in fact pretty simple know that designing coding testing and tweaking this driver took about 20 hours to complete when all was said and done We do not have to build a state machine for the LED system because a single function to manage timers and perform updates is all that is needed The function will be called LedUpdate and will address each LED explicitly LedOn LedOff and LedToggle become private functions to the LED driver LedUpdate will use the private functions and manage timers to turn the lights on and off All of the code is captured in specific LED source files 10 2 1 Header file leds_deva7 h A header file is used for the board specific parameters for the LED driver The key information is how many LEDs there are in the system and the names that will be used to identify each LED Using an enumerated type for
8. module indicates that the clock source is PCLK which if you refer back to discussions about clock scaling you know is 1 1 to the main system clock of 12MHz There is mention about single edge and double edge control that is rightfully confusing but also talk about match registers that should make sense even though you do not yet know how everything is implemented If you look at the block diagram that is copied and shown here in Figure 10 3 1 you might feel a bit overwhelmed about the complexity of the PWM peripheral But fear not it is actually quite straightforward to set up and use Part of the complexity comes from the fact that there are 6 PWM generators channels within the peripheral which add a lot to the diagram to make it look so complicated But really there are just six copies of the same PWM architecture so if you figure one out then the system unravels itself You can spend some time tracing through the diagram to make sense of what is going on for one of the channels or you trust that it works and skip that for now Unless you are doing some low level hacking or tweaking it is unlikely you would devote any effort to deciphering the internals of a peripheral If you need PWM outputs in a design make sure you correctly assign the hardware that requires PWM to a pin where PWM output is available Also check that there are no conflicts with the PWM peripheral output for the pin you have chosen notes_mpgl1_chapter10 docx Releas
9. peripheral PWMITC Timer Counter register that holds the current value of the timer for the PWM peripheral You do not typically use this register though you need to be aware of its significance with respect to the behavior of the PWM peripheral PWMPR Prescale register that is optionally used to slow down the clock source to the peripheral Prescale registers are clock dividers and while most use bit wise setting to apply fixed scaling ratios like 1 2 1 8 etc this peripheral uses an entire counter to count up to a value before the main timer increments This register is the value to which the prescale counter counts before the main peripheral clock increments and the prescale counter resets and repeats Set to 0 for no prescale PWMIPC The prescale counter register PWMMCR Match control register for configuring what event occurs for each channel of the PWM when the main counter value reaches the value in a specific match register For each of the six channels you can choose to have an interrupt reset the timer or stop the timer notes_mpgl1_chapter10 docx Release 1 0 Page 13 of 20 ENGENUVICS www engenuics com MPG LEVEL I PWMMRO Match Register 0 is the main match register that is used to set the PWM period All match registers are static values that are typically set once for a given PWM signal The value is compared against the current PWM counter value PWMTC with every clock tick When PWMTC has incremented enough to m
10. process all the LEDs It is written in a way that tries to operate as quickly as possible as it works through all of the LEDs on the board For any given LED the fastest states are ON and OFF so LedUpdate takes care of those checks notes_mpgl1_chapter10 docx Release 1 0 Page 7 of 20 ENGENUVICS www engenuics com MPG LEVEL I first OFF is the first check because that is most likely the state of an LED at any given time in the system The blinking and PWM states are checked last as those state require further tests based on the counters that are tracking a change of state As soon as the current state is found the other states do not have to be checked ON and OFF functionality should be trivial If an LED is off the eRate parameter will be either LED_OFF or LED_PWM_O If so a call to LedOff ensures that it is off Making this call every time LedUpdate is called might not be the best idea because if an LED is already off then why turn it off again However to check the current state and make a decision about whether or not you need to call LedOff would probably take more code and instruction cycles than just calling it every time thus that is how the code is written If LED_OFF make sure it s off if LGasLeds i eRate LED_OFF LGasLeds i eRate LED_PWM_0 LedOff LedNumberType i If LED_ON make sure it s on else if II Turn the LED on
11. the LED names helps to make the API easier to use LedNumberType takes care of this and hopefully it is obvious what LED each word corresponds to Other typedefs are used to keep track of the LED mode location i e port number and current duty cycle that you will see are necessary for running the driver The code is shown here you need to add typedef for LedPWMDutyType define TOTAL _LEDS u8 11 define LED _PWM_PERIOD u8 20 typedef enum WHITE 0 PURPLE BLUE CYAN GREEN YELLOW ORANGE RED RGB_RED RGB_GREEN RGB_BLUE LedNumberType typedef enum NORMAL 0 PWM LedModeType typedef enum PORTO 0 PORT1 0x08 LedPortType SSSS Add a typedef enum called LedPWMDutyType where LOW 0 and HIGH 1 notes_mpgl1_chapter10 docx Release 1 0 Page 4 of 20 Ts ENGENUVICS www engenuics com MPG LEVEL I Since the behavior of every LED is independent from the rest each LED will need to have a set of data to keep track of its specific settings i e is it blinking or in PWM mode and what rate is it currently set at To manage the PWM function every LED will need a counter variable All of the parameters will be kept in a struct whose typedef is also in the LED header file typedef struct LedModeTyp Mode LedRateTyp Rate ul6 ul Count LedPWMDutyType eCurrentDuty DeviceActiveTyp ActiveState LedPortType ePort LedConfi
12. with the ones that you are already familiar with 10 2 2 1 LedOn LedOff and Led Toggle We wrote functions in Chapter 9 to handle the simple tasks of turning LEDs on or off Since we were only working with the discrete LEDs that are all active high it was not necessary to account for active low LEDs but that is not very generic and one of the LCD backlight LEDs is active low so the function must be updated The old function from Chapter 9 looked like this void LedOn LedNumberTyp Led_ Set pu32SetAddress to the correct FIOxSET register address u32 pu32SetAddress u32 amp FIOOSET LGasLeds eLed_ Set the bit corresponding to the LED of interest by writing to the pointer pu32SetAddress LGau32LedBitPositions u8 eLed_ end LedOn The trick to this function was that pu32SetAddress was loaded with the address of the FIOxSET register for the correct port in which the LED lives The array LGasLeds only contained the offset constant PORTO or PORT1 that would make this adjustment based on the LED of interest Once the address was set the correct bit was indexed to turn the LED on The updated function has two main differences and looks like this void LedOn LedNumberTyp Led_ u32 pu32SetAddress u32 amp FIOOSET LGasLeds eLed_ ePort u32 pu32ClearAddress u32 amp FIOOCLR LGasLeds eLed_ ePort Check for active high or active lo
13. work with the LEDs from this point on The audio capability of the board is accessed by setting a frequency with PWMAudioSetFrequency and then using the PWMAudioOn Off functions to start and stop the buzzer The chapter exercise is in two parts that test the audio and LED skills separately Both of these exercises are entirely C programming tasks like you would receive in a lab though they are way cooler because they run on your own development board You may only write code in the Chapter10 source file You do not need any additional states Both tasks must run at the same time when you are complete Tasks 1 Create a 4 note synthesizer where the notes G4 A4 B4 and D5 will play when BUTTONO thru BUTTONS respectively are pressed Prove that your program works by playing Mary had a little lamb the sheet music is shown above It does not have to be absolutely perfect for example do not worry about what happens when two buttons are pressed assume that only one button will be pressed at the same time Hint every time you call PWMAudioOn the current PWM cycle is reset so if you call it repeatedly you will not get the correct tone 2 Color cycle the RGB backlight Write a tight piece of code to color cycle the three RGB LEDS so they mix together and show off most of the 8000 colors you have available Though you could write this fairly brute force take some time to design a solution that is code efficient The PWM notes_mpgl
14. 1_chapter10 docx Release 1 0 Page 19 of 20 ENGENVICS www engenuics com MPG LEVEL I rates for each LED to cycle through a rainbow of colors in the correct ROYGBIV order should be as follows a Red LED ramps up from 0 to 100 Green LED ramps up from 0 to 100 Red LED ramps down from 100 to 0 Blue LED ramps up from 0 to 100 Green LED ramps down from 100 to 0 Red LED ramps up from 0 to 100 Blue LED ramps down from 100 to 0 Repeat from b sm mo a0 Do not forget to put the call in to initialize and run the Chapter 10 application in main c If you want a hint here are the variables from Chapter10SMldle that the solution uses to accomplish the task static ul6 ul6Counter COLOR_CYCLE_TIME static u8 au8LEDOrder RGB_RED RGB_GREEN RGB_RED RGB_BLUE RGB_GREEN RGB_RED RGB_BLUE static u8 au8LEDDirection 0x01 0x01 0x00 0x01 0x00 0x01 0x00 static u8 u8RGBIndex 0 static u8 u8RGBCurrentLevel 0 static u8 u8RGBCounter 0 Remember that there is probably a ton of different ways to program this solution The code space required for this implementation was 316 bytes determined by building the code with and without the color cycling code and comparing the size of the chapter10 0 object files You can see the size of your code by looking at the map file for the project and finding the MODULE SUMMARY Figure10 5 1 shows a screen shot of the object file size including the cycling code IAR Embedded Workbench
15. ENGENUVICS www engenuics com MPG LEVEL I PWM PWM PWM Chapter 10 PWM PWM This chapter examines yet another very important and fundamental service that an embedded system often provides Pulse Width Modulation PWM We will look at what PWM means some applications and how to implement it on the development board both using brute force techniques and the PWM peripheral The LED application will be updated to make use of your new PWM skills and an audio driver will be written so you can start producing ridiculously irritating noise from your buzzer The chapter exercise creates a very nice color cycling backlight on the LCD 10 1 PWM Concepts Digital systems are great because there are only two voltage states high and low that carry any information The potential difference between the two provides a lot of noise immunity making for very robust systems However since the world is analog digital systems must have a way to step out of the binary realm and into the infinite states of analog There are an infinite number of analog voltage levels that are used to adjust volume or intensity and time varying signals are the essence of analog communication and audio Digital systems can achieve some forms of analog output through digital to analog conversion but only at discrete levels The only wave that a digital processor is capable of producing is a square wave since it can only toggle a digital output high or low However it turns out
16. LEDs all of the parameter structs for each LED are kept in an array The size of this array depends on the constant TOTAL_LEDS defined in the header file The code is then written to loop through the whole array addressing each LED one at atime The last four definitions are left for you do to Control array for all LEDs in system initialized for LedInit LedConfigType LGasLeds TOTAL_LEDS PWM LED _PWM_100 0 HIGH ACTIVE _HIGH PORTO White 0 PWM LED_PWM_100 0 HIGH ACTIVE_HIGH PORTO Purple PWM LED_PWM_100 0 HIGH ACTIVE_HIGH PORT1 Blue 0 0 0 PWM LED_PWM_100 0 HIGH ACTIVE_HIGH PORT1 Cyan PWM LED _PWM_100 0 HIGH ACTIVE _HIGH PORT1 Green PWM LED_PWM_100 0 HIGH ACTIVE_HIGH PORT1 Yellow notes_mpgl1_chapter10 docx Release 1 0 Page 5 of 20 ENGENUICS www engenuics com MPG LEVEL I PWM LED_PWM_100 0 HIGH ACTIVE_HIGH PORT1 Orange Add the remaining 4 definitions You can see that every LED struct is initialized on startup By default the LEDs are all on as part of the LED application startup routine Only the RGB red LED is active low and the port number for every LED is captured as well So everything we need to access and manage every LED is contained in this array We will look through each of the functions in the source file now to go over how they came to be starting
17. There are a lot more details beyond that but that is enough for now An example is the sheet music for Mary had a little lamb shown in Figure 10 4 3 1 notes_mpgl1_chapter10 docx Release 1 0 Page 18 of 20 ENGENUICS www engenuics com MPG LEVEL I Ma ry had a lit tle lamb Its fleece was white as snow Figure 10 4 3 1 Music for Mary and a little lamb Source http www enchantedlearning com music sheetmusic maryhadalittlelamb shtml There are some pretty easy ways to program algorithms that will play simple single note songs over a piezo buzzer Essentially you write two arrays one with the sequence of notes and one with a corresponding duration to play each of those notes There are even some standard algorithms for computer generated music that you could program and then download source files with various songs We will not explore that in this course but if you are interested it would be an easy programming exercise to explore Note that there are three octaves of note constants defined in deva7 ehdw 01 h for you to use 10 5 Chapter Exercise This chapter has improved the core firmware that is running on the development board by adding a better LED driver and providing basic audio functionality all with the help of PWM You should be comfortable with the use of the LED API function LedSet where LedNumber LedMode and LedRate are specified to manage each LED You do not need any other functions to
18. alue of the PWM period the signal transitions back to high Single edge PWMs can be kept high 100 duty cycle by setting the match register larger than the PWM period The signal can be kept low by setting the match register to 0 notes_mpgl1_chapter10 docx Release 1 0 Page 12 of 20 ENGENUVICS www engenuics com MPG LEVEL I With double edge signals it is expected that two transitions will be specified in firmware thus there will be two transitions inside of the total period defined by the user This allows several PWMs within the peripheral to use the same period but select a time within the total period where the signal is high or low This is useful in certain motor control applications or any application in general where the PWMs must be carefully synchronized For example three phases in a motor may require a 33 duty cycle for each phase but never at the same time Therefore you could define the PWM period as 99 with three double edge PWM channels set to be high for periods 1 33 34 66 and 67 99 Since they run from the same PWM peripheral exact synchronization is ensure which will be appreciated by your customer when their motor does not blow apart The rules for single edge and double edge PWM outputs are clearly listed in the processor user guide If you were designing a system that relied on correct operation of the PWM outputs setting the processor up and watching all the channels on an oscilloscope would be a good step to pe
19. atch the value of PWMMRO then the corresponding event configured in PWMMCR 2 0 takes place If this register is set to 0 then the PWM signal is never asserted high 0 duty cycle PWMMRx The remaining Match Registers specify actions of the six PWM channels For single edge PWM signals the value in the corresponding match register is the count at which the PWM output will go from high to low For double edge PWMs two match registers are required to define the signal behavior Generally speaking the value of PWMMRx 1 lt x lt 6 register is always less than the value in PWMMRO If it is greater then that particular channel will never transition low when in single edge mode 100 duty cycle PWMPCR PWM Control Register to configure and activate each of the PWM channels This is where single or double edge mode is selected 1 bit per channel and individual channels can be enabled or disabled with another bit per channel PWMLER Latch Enable Register is a control mechanism to help synchronize updates to the PWM peripheral If you change the duty cycle of one or more PWM channels by changing the respective PWMMRx register the change does not actually take effect until a bit in PWMPCR is set corresponding to the channel that is updated This allows you to update several match registers without impacting the current duty cycles and then simultaneously applying the changes so that all channels adjust at the same time This would be extremely impo
20. ching power supplies are much more efficient than linear regulators power is only sampled as it is needed from the input If there was no capacitance on the 5V rail than the switching frequency of the buck converter which is a few hundred kHz would be visible and you would not have a very good power supply Switching power supplies require careful calculation of their output capacitors so that an acceptable amount of ripple can be attained you will always see some evidence of the switching signal Generally speaking the larger the capacitor the less ripple will be in the system because the cap stores more energy PWM is also used in motor controllers and essentially works the same way to apply an effective voltage to the motor thereby giving you control over the speed at which the motor is running This applies in both AC and DC systems and many microcontrollers have built in PWM peripherals to target applications like this In this way the PWM signal is essentially pulsing a fly wheel to keep it spinning The momentum of the fly wheel is analogous to capacitance in a purely electrical system If you have a dimmer switch on some incandescent light bulbs in your home the same principle applies but this time it is the slow reaction and heat build up of the light bulb that is acting like capacitance in the system to smooth out the PWM action and make it appear as a steady state lower voltage As it turns out the concept of PWM i
21. e 1 0 Page 11 of 20 ENGENUICS GROW www engenuics com MPG LEVEL I gt LOAD ENABLE Soa STEN f a SOONEST ae LOAD Siia ee on ENABLE IMATCH REGISTER 5 SHADOW REGISTER 5 MMT aTe REGISTER G REGISTER 6 ae REGISTER 6 ae ENABLE ale a sis Lif Haas LATCHENABLE REGISTER CLEAR ENABLE REGISTER CLEAR INTERRUPT REGISTER CONTROL M G 0 INTERRUPT STOP ON MATCH RESET ON MATCH z PWMENA1 6 PWMSEL2 6 pe ENAR MAXVAL TIMER CONTROL REGISTER PRESCALE REGISTER Figure 10 3 1 PWM peripheral controller hardware Source LPC214x User manual UM10139 Rev 3 4 October 2010 p 259 NXP Semiconductors 10 3 1 Single Edge Double Edge Single and double edge PWM signals determine how many transitions you will actively manage with match registers when defining the signal All PWM signals in this peripheral share a common end point at the end of one cycle period where a transition may or may not occur depending on the configuration Even if nothing explicitly happens it is important to understand that this endpoint is the end of a particular PWM period Single edge behavior allows firmware to specify a single high to low transition and leaves the low to high transition to automatically occur at the end of the PWM period The assumption is that the signal is high at time 0 and will go low when the PWM s counter equals the match register value Once the timer reaches the maximum v
22. ely you will see groups of RGB LEDs that are simply dimmed to a level to create the desired color just like mixing paint in art class If each color has 256 levels of intensity in other words 8bits to control the level then you can achieve 256 x 256 x 256 16 7 million colors Not that your eye can distinguish that many different colors but as you have or will no doubt experience when shopping for monitors or displays 24bit or higher color is a major selling feature that you will pay a bunch of money for note that no one really knows for sure how many colors you can resolve and the arguments tend to be more philosophical than logical opinions have it ranging anywhere from a few thousand to several million By the way if you have equal parts red green and blue the result is white 10 2 1 Application design As of Chapter 9 we have a simple set of function calls living in the board specific source file deva7 ehdw 01 We want to write a reasonably generic LED driver that could be ported to other systems LEDs tend to be very board specific as far as their individual pin connections to the processor so the driver we develop will need to account for that and be flexible Once the definitions are in place the code required to turn the LEDs on and off is quite generic The complexity of what we want to achieve provides reasonable justification to pull that code out of the generic file and into its own application files The names of the
23. files will be leds2 c and leds_deva7 h If you have not downloaded the start code for this chapter yet do that now and load up the project in IAR The code will not build yet because there are several places where you will have to finish the code snippets For now spend a few minutes getting familiar with the code namely the new files specific to the LED application and the Chapter 10 exercise The biggest problem of bit bashing is the overhead required to program each part of the function This becomes increasingly difficult as the speed of whatever signal you are trying to create increases or as the complexity of the waveform that you are trying to create goes up For example bit bashing a communication protocol that transmits and receives at MHz speeds is very challenging if the processor is running at only 12MHz In this case you might be stuck writing assembler routines to get a very efficient algorithm and even then you might not be able to do it In designing the LED PWM driver we have to consider the resources that are available and the system rules we can work with Ideally we should take advantage of the known 1ms period of our application calls It so happens that this will work quite nicely though it is not perfect because we will come very close to the limit of what we can do given the speed of the core system The defining factor is the PWM capability of the LEDs As discussed earlier in the chapter the frequency of the PWM
24. gType Since the board specific values are kept in the header file the source file can be almost entirely generic and just work as long as the correct header file is used That is true at least within the LPC214x family of processors on any development board If ported to a different processor then some updates would likely be required 10 2 2 Source file leds2 c The LED driver source file has some board specific Local Globals though they are easily adjustable and will have the same format regardless of where the code is ported too The first is an array of values that define the bit numbers for each LED i e their GPIO bit positions There is a note in the code to remind the programmer that the order of the bit positions in the array must correspond to the order of the LEDs defined in LedNumberType in the header file It is up to you to add the 11 pin definitions to correctly define the array The exact names of the pins from the schematics should be used and these symbols have already been defined for this specific development board in deva7 ehdw 01 h LED locations order must correspond to the order set in LedNumberType in th header file u32 LGau32LedBitPositions Add the pin location constants to define this array The second local global variable needed keeps track of the parameters for each LED To be able to write the code to easily accommodate different development boards with different numbers of
25. ical notes are a function of frequency The lower the frequency the lower the sound Deep bass is sub 100Hz the highest sound a human can hear is around 16kHz depending on how loud your music was when you were a teenager and thus how sensitive your ears are still Dogs can hear much higher frequencies 2 Different speakers are better at reproducing different frequencies i e subwoofers for bass tweeters for very high sound 3 Music is written with notes from A to G which span one octave There are 8 octaves spanning note frequencies start at CO 16Hz all the way to C8 4186Hz In musical terms it does not make much sense to talk about sounds above that frequency though there is theoretically no upper limit 4 In every octave you have the notes A A sharp B flat B C C sharp D flat D D sharp E flat E F F sharp G flat G G sharp A flat of the next octave The symbol for sharp is like C not to be confused with the programming language but no doubt someone thought they were being clever when they named it and the symbol for flat is b like A 5 The exact frequencies of notes or someone relative but are mathematically defined based on a reference value Check out this link if you want to learn more http www phy mtu edu suits NoteFreqCalcs html 6 Music is written by defining single notes or combinations of notes that are played for a certain duration These are shown on a five line staff
26. ime LedInit is called so you can still make use of them for timing purposes To use them use the IsTimeUp utility function The init function for the LED system does not really need to do very much beyond putting the LEDs ina state that makes sense for the main program to start running likely this would be all OFF However as a means to self test all of the LEDs the user is responsible for verifying that they all light up make things a bit interesting and demonstrate the operation of the LED driver a little display sequence will be shown The plan is to turn on all of the LEDs at full intensity and then fade them out over a few seconds We want to use our LedUpdate function during this initialization so we do not have to waste time writing different code however we do not yet have the super loop 1ms system running so we have to work around that The structure of LedInit is as follows notes_mpgl1_chapter10 docx Release 1 0 Page 9 of 20 ENGENUVICS www engenuics com MPG LEVEL I Loop to progress through the 20 available duty cycles Nested loop to wait 40ms at the current intensity Delay lms using IsTimeUp Call LedUpdate Wait additional 1 5 seconds at full intensity only Use a loop to update the PWM duty cycle of all of the LEDs to the next lowest level Final call to LedUpdate since LEDs will be off Short delay to complete the display The nested loop that runs 40 times performs the acti
27. ke the pin out of standard GPIO mode and connect it to the peripheral While this has already been in place in the code since Chapter 8 this is the first time we are using peripheral functionality that depends on it so it is a good place to draw some attention to it Look at the PINSEL1_INIT value in deva7 ehdw 01 h for PO 21_ BUZZER you can see that it is not OO like all the GPIO digital pins but rather 01 If you check the Table 38 Pin function Select register 1 PINSEL1 in the user guide you see where this comes from No mysteries here just awareness on how the processor works and how to find and use the relative information Figure 10 4 2 shows relevant snippets of the user guide and the header file Table 38 Pin function Select register 1 PINSEL1 address 0xE002 C004 bit description Bit Symbol Value Function Reset value 11 10 P0 21 00 GPIO Pot 0 21 0 01 PWMS ii 0 PO 21 BUZZER PWM5 10 pi 09 1 PO 20 SSP SSEL og roj define PINSEL1 INIT u32 0x650806A9 f Bit Set Description 10 Reserved or AD1 6E1 11 Capture 1 3 Timer 1 Figure 4 10 2 P0 21 user manual left and firmware right information Table 38 Source LPC214x User manual UM10139 Rev 3 4 October 2010 p 61 NXP Semiconductors You must also make sure that the pin is configured as an output since the peripheral is outputting a signal Configuring a data direction register based on what the peripheral connected to it does is usually a safe bet b
28. lease 1 0 Page 1 of 20 ENGENUVICS www engenuics com MPG LEVEL I not see it blinking because your eye cannot respond that quickly In fact your eye can only distinguish changes at about 10Hz though you might see some flicker as fast as 30Hz Instead you will see the LED as if it is running at about half the voltage as your supply rail so it will appear to be half as bright Soa 50 duty cycle if the frequency is high enough results in an effective 50 supply voltage If you actually measure the effective luminous output it probably is not exactly half but as a general rule of thumb the LED brightness is equal to the duty cycle of the PWM signal driving it If you change your driving signal to be on for 250us but off for 750us the period of the signal remains 1ms but now you are running 25 duty cycle and the LED will appear about one quarter of its full brightness As long as you maintain the frequency and just vary the duty cycle on time vs off time then you can make the LED any brightness that you want Not only does this work for varying LED brightness in a digital system it has many other applications as well You have already witnessed PWM in action with the switching power supply on the course development board The integrated circuit that converts the high input voltage 7 30V down to a regulated 5V uses a fast switch and varying duty cycle to down convert the higher voltage to the lower voltage This is why swit
29. lue that is actually used A good rule of thumb is to delete the hex value for any setup value you are working with and retype it when you are done with the comments If you do this and forget to update the hex value at least the compiler will catch your mistake because the value it tries to use is not complete notes_mpgl1_chapter10 docx Release 1 0 Page 16 of 20 ENGENUVICS www engenuics com MPG LEVEL I 10 4 2 2 PWMAudioSetFrequency The ability to set the tone frequency with a single function is powerful and pretty much a must have for any useful API to an audio driver like this Unfortunately it takes a bit of work and good understanding of the PWM peripheral to make that happen From looking at PWM peripheral registers you should now know that the total period and thus the frequency of the PWM signal is defined by the PWMMRO register We have a 1 1 scaling of the main clock 12MHz to the peripheral clock PCLK If we wanted to set the PWM period to provide a 1kHz signal the period must be 1 1kHz 1ms So the question is how many clock ticks at 12MHz does it take to time out Ims The answer is 1ms 1 12MHz 12 000 Generalizing this we can solve period T for any frequency f given PCLK _ PCLK f The calculation really is not difficult but it is not something you want to forget You can see therefore that the discussion and resulting equation is written directly into the code comment Not only does this give
30. on of the 1ms super loop LedUpdate must be called every 1ms to properly implement the PWM functionality that it provides 40ms x 20 different PWM levels makes the LEDs fade out over about 800ms which by test looks the best If you try to slow it down then the 5 duty cycle steps start to become more obvious To make the fade out look smooth when the duty cycle is changed it is important to make the duty cycle change only at the end of a period In other words the delay loop choices are limited to 20ms 40ms 60ms etc Updating the LEDs for the next cycle uses a loop to run through all the LED arrays Again the loop indexes must be typecast to their respective enum types or else the compiler will send up warnings Set the intensity for the next iteration for u8 j 0 j lt TOTAL_LEDS j LedSet LedNumberType j PWM LedRateType i 1 There are two special cases within LedInit where extra delay is added to stretch out some time periods at the beginning and at the end The choice of these time periods is almost entirely arbitrary they just look right That completes the LED driver The code should now build so you can see the fruits of your labor Even if you did not understand all of the code to implement the LED driver the good news is that all you need to know to use the LEDs from this point on is the single API function LedSet 10 3 PWM the Easy Way LPC214x PWM Peripheral As you just saw
31. re two lines of code for you to add void LedSet LedNumberTyp Led_ LedModeTyp Mode_ LedRateTyp Rate_ LGasLeds u8 eLed_ eMod eMode_ SSSS Add two lines of code to update eRate and ul6Count both set to eRate_ if eMode_ PWM LGasLeds u8 eLed_ eCurrentDuty HIGH end LedSet We use a function like this to abstract the details of how the LED data is stored and provide a single entry point into the LED driver for a user to use LedSet becomes the one and only public API function for the system and should be the only function you ever use to access the LEDs in other applications that you write Yes there is added complexity for the user to use LedSet in lieu of LedOn and LedOff if they only want to toggle an LED now and then However the simplicity of blinking or PWMing an LED thanks to LedSet far outweighs the minor overhead of using the same function to simply toggle it now and then 10 2 2 3 LedUpdate The function that puts everything together and actually updates the LEDs is called LedUpdate A call to this function should appear in the main super loop and we consider it protected since the usage is very specific The function expects to run every 1ms and is thus coded based on that assumption If it does not run every 1ms then the PWM bit bashing it is doing will not work properly LedUpdate is a fairly long function and takes a few hundred instruction cycles to
32. rform during the design phase 10 3 2 PWM Registers The registers needed to configure the PWM module are shown in Table 248 of the user manual Though it looks like there are quite a few six of them are just match registers that do the same thing for different channels and the others are setup registers some of which we will use and some we can ignore Do not forget to turn on the power and clock source to the peripheral in the PCONP register Each of the PWM registers is detailed here The information about the purpose of each register is almost verbatim from the user manual though a few clarifications are made As an engineer you will be responsible for using the data sheet to fully understand the peripheral you are working with You have to determine how the peripheral works in general and then what registers are relevant to your application In most cases choosing the register setup values is straightforward though occasionally you will come across some options that do not make immediate sense in which case you will have to try them out and see if the behavior is what you want Some processor documentation does an excellent job and explaining everything others seem to leave out important information that you have to find elsewhere or figure out through trial and error PWMIR Interrupt flag register that will be ignored as PWM interrupts are not required PWMITCR Timer Control Register providing the core on off and reset control of the
33. rtant in applications where duty cycle changes are frequent and to cite the motor control example again are critical in timing and synchronization so you do not blow things apart Configuration of these registers specific to the course development board is described in the next section since all settings are highly application specific 10 4 Development Board Audio Driver Now that you know how to setup the PWM peripheral we can introduce the application to which it will be applied audio Generating a square wave to drive a beeper buzzer is a great way to take advantage of a microcontroller s PWM peripheral even though use of the peripheral in this way is not the way it really is intended to be used PWM is supposed to adjust duty cycle to achieve different results but using the PWM for audio will keep a 50 duty cycle and just change the frequency As a side note piezoelectric pronounced pee EH zoh electric buzzers like the one that is on the course development board respond very nicely to a square wave drive and they can usually be driven directly from the processor pin as they are voltage actuated devices and barely draw any current You cannot drive a magnetic coil speaker directly from a processor because they rely principally on current and have very little DC resistance A speaker coil requires tens if not hundreds of milliamps of drive current which a processor cannot source directly at the extreme case a big sub woofer could p
34. s everywhere 10 2 PWM the Hard Way There is no better way to really understand a concept than to work through an exercise that forces you to explicitly implement every bit of functionality related to it So the first PWM exercise we will do is an upgrade to the LED driver that we have already built for the development board to allow dimming of the lights via PWM This will be a great demonstration of what it takes to bit bash some desired functionality and will give you a bit of an appreciation for the various peripherals that automatically implement common functions We have to do this the hard way because the LPC214x processor does not have enough PWM outputs to drive each LED it would be very unlikely that you would find a processor that would have eleven PWMs notes_mpgl1_chapter10 docx Release 1 0 Page 2 of 20 ENGENUVICS www engenuics com MPG LEVEL I The goal is to create an application that will automatically take care of blinking the LEDs but also provide a facility to dim the lights by using PWM Though dimming of the discrete LEDs will allow for some nice effects the main purpose is to have dimming control for the LCD backlight LEDs Since the backlight has red green and blue lights denoted RGB and are the primary colors of light that light up the screen adding them together with different levels of intensity will allow you to form any color you want This is exactly how LED displays work if you look clos
35. s in PWM mode else Decrement counter toggle and reload if counter reaches 0 if LGasLeds i ul6Count 0 if LGasLeds i eCurrentDuty HIGH iGasLeds i ul6Count LED _PWM_ PERIOD LGasLeds i eRate iGasLeds i eCurrentDuty LOW LedOff LedNumberType i else GasLeds i ul6Count LGasLeds i eRate GasLeds i eCurrentDuty HIGH LedOn LedNumberType i end LED PWM mode end LED modulated The source code in the leds2 c file is missing the normal mode code that is shown above For a bit of a challenge you can try to figure it out yourself or if you want to move on then simply copy the code above 10 2 2 4 LedInit LedInit is discussed last here as it turns out to be the most complicated and relies on LedUpdate As you should know by now every application that is added to the system will need an initialization function to properly set it up before the main program super loop starts to run There are no timing rules during initialization but also no 1ms calls to the function that you can make use of Initialization functions run once and are permitted to take as long as necessary though they should never fail to exit if something does not work and should do everything possible to not crash the system Even though the automatic 1ms timing is not available the system tick timers are in fact running by the t
36. ull tens of amps of current Regardless of the type of speaker buzzer you have never leave a DC voltage applied to it If you are not driving the speaker the output pin to the speaker should be at 0 volts notes_mpgl1_chapter10 docx Release 1 0 Page 14 of 20 ENGENUICS www engenuics com MPG LEVEL I What we would like to have out of all of this is an easy way to use the PWM peripheral to send a tone signal to the piezoelectric buzzer on the development board To do this we need to setup the PWM peripheral and write a few API functions to allow any application in the system to use audio easily The goal is to write a simple interface where the tone frequency can be specified and turned on and off We do not need to write this as an application since the PWM peripheral pretty much takes care of all the work An application might be necessary if you wanted to expand the audio capability to play music or allow priority access to the buzzer 10 4 1 PWM Hardware The PWM channel used for the buzzer is PWMS5 Do you know how to determine this Figure 10 4 1 1 shows the section of the schematic where the buzzer signal connects to the processor This pin was selected carefully in the design phase of the PCB to make sure the PWM resource was available Figure 10 4 1 1 Schematic showing buzzer signal connection Since the function of this pin is assigned to a peripheral you have to make sure that the relevant GPIO registers are set to ta
37. upAudio The PWM peripheral registers that are required for the audio application are all set up in this function which is called during board initialization as part of the existing DEVA7RazorSetup function Open deva7 ehdw 01 c and find the function to see what registers require configuration Note that after this code executes the peripheral is ready to be used but the audio output is not yet enabled so the beeper is quiet The course development board has a dip switch to physically disconnect the beeper Make sure the switch is on to hear the sound If you are testing audio functions to see if the waveform you are sending is correct but you do not want to listen to the beeper you can turn it off but still measure your audio signal at the BUZZ test point Open the deva7 ehdw 01 header file and find the PWM setup values you can search for a few 444 characters that bookmark the code Here you will find the INIT values used in the setup function as well as some definitions for some update mnemonics As usual all bits are explained which though it requires extra effort lets you immediately see the purpose and value of each bit The PWMPCR_INIT value is left for you to complete Make sure you fully document the bits make the correct binary assignments in the comments and then update the hex value that is actually loaded A common mistake to make with header files like this is to update the values of comments without updating the hex va
38. ut you should always read the datasheet to be sure Some peripherals will have their own notes_mpgl1_chapter10 docx Release 1 0 Page 15 of 20 ENGENUVICS www engenuics com MPG LEVEL I output drivers separate from the GPIO driver and require the GPIO data direction register to be set a certain way for the peripheral output to work You should never have to guess at this the information should be somewhere in the user manual although occasionally it is hidden quite well NEVER quess at choices you make as an engineer Read documentation to find the answer you need If you cannot find the answer find someone who knows If you have to figure something out because it is a brand new problem use a systematic process ina controlled environment and try to use as much reliable information as possible to limit the amount of guesswork experimentation you do 10 4 2 PWM Firmware The firmware functionality we need to make all of this work is captured in 4 functions that will be written in the deva7 ehdw 01 source files since they are quite board specific e void PWMSetupAudio void initialize the PWM peripheral for the specific use of buzzer audio e void PWMAudioSetFrequency u16 u16Frequency_ configure the PWM peripheral for the desired audio frequency e void PWMAudioOn void activate the PWM peripheral e void PWMAudioOff void deactivate the PWM peripheral i e turn buzzer off and leave the line low 10 4 2 1 PWMSet
39. w if LGasLeds eLed_ eActiveState Set the bit corresponding to eLed_ if active high pu32SetAddress LGau32LedBitPositions u8 eLed_ else Clear the bit corresponding to eLed_ if active low notes_mpgl1_chapter10 docx Release 1 0 Page 6 of 20 ENGENUVICS www engenuics com MPG LEVEL I pu32ClearAddress LGau32LedBitPositions u8 eLed_ end LedOn Can you spot the changes Better yet are the changes that you see immediately obvious as to why they are necessary The most significant change is that the function now handles active low and active high LEDs The other difference is that the specific LED information like what port it is on and if it is active high or active low is now in an array of structs that was introduced already So the eLed_ function parameter now indexes the LED data array and we have to select the field in the indexed struct to get the data we want namely LGasLeds eLed_ ePort and LGasLeds eLed_ eActiveState The exact same updates are necessary for LedOff You might notice that LedToggle is almost identical to the Chapter 9 code since you do not have to care about the active state of an LED when you are toggling it So the only change is to properly index the LED parameter array 10 2 2 2 LedSet The first new function of the LED driver is called LedSet and is responsible for updating certain parameters that define an LED s current behavior There a
40. you a reminder of what you did to justify the code implemented it documents what was done so if there is an error later on you can see where that error was made The importance and significance of finding root cause to a problem cannot be stressed enough Not that setting buzzer frequency would ever be a significant problem but if you did have an issue and could trace it back to the calculation you derived because you wrote it down you know for sure that you have solved the problem PWMAudioSetFrequency determines the new PWM period based on the frequency argument and adjusts the value for the MR5 match register to maintain 50 duty cycle It then updates the two match registers and flags PWMLER to load the new values void PWMAudioSetFrequency ul6 ul6 Frequency_ u32 u32MROPeriod u32MR5Period The PWM peripheral clock period is 1 PCLK_VALUE and the period of the desired audio frequency is 1 ul6Frequency_ Therefore the MRO match register should be set at 1 ul6Frequency_ 1 PCLK_VALUE PCLK_VALUE ul6Frequency_ u32MROPeriod PCLK_VALUE ul6Frequency_j Load the value for MR5 remember we want 50 duty cycle Setup the new values in the match register kept in shadow registers PWMMRO u32MROPeriod PWMMR5 u32MR5Period Flag that the MRO and MR5 match registers need updating PWMLER _PWM_ENABLE_MRO_UPDATE _PWM_ENABLE_MR5_UPDATE end PWMAudioSetFrequency
Download Pdf Manuals
Related Search