Add a DS1307 clock to your AVR microcontroller
Contents
1. Boarduino LED on PB5 define ClearBit x y x amp _BV y equivalent to cbi x y define SetBit x y x _BV y equivalent to sbi x y eee INCLUDES include lt avr io h gt deal with port registers include lt util delay h gt used for _delay_ms function include lt string h gt string manipulation routines include lt stdlib h gt M_a TYPEDEFS typedef uint8 _t byte I just like byte amp sbyte better typedef int8_t sbyte M_a MISC ROUTINES void InitAVR DDRB 0x3F 0011 1111 set B B5 as outputs DDRC 0x Q 0000 0000 set PORTC as inputs void msDelay int delay put into a routine to remove code inlining for int i 0 i lt delay i at cost of timing accuracy _delay_ms 1 void FlashLED SetBit PORTB LED msDelay 25 ClearBit PORTB LED msDelay 25 eee HD4478 LCD DRIVER ROUTINES Routines ai LCD_Init initializes the LCD controller J LCD_Cmd sends LCD controller command LCD_Char sends single ascii character to display Td LCD_Clear clears the LCD display amp homes cursor ai LCD_Home homes the LCD cursor LCD_Goto puts cursor at position x y Ly LCD_Line puts cursor at start of line x LCD_Hex displays a hexadecimal value LCD_Integer displays an integer value LCD_String displays a string The LCD module requires 6 I O pins 2 control lines amp 4 data lines PortB is used for data communi2. F_CPU F_SCL 16 2 _ My CPU has a 16 MHz clock and want to run the interface in standard 100 kHz mode So the value of TWBR must be 16 0 1 16 2 160 16 2 72 define F_CPU 16000000L CPU clock speed 16 MHz define F_SCL 10009000L I2C clock speed 100 kHz void I2C_Init at 16 MHz the SCL frequency will be 16 16 2 TWBR assuming prescalar of so for 100KHz SCL TWBR F_CPU F_SCL 16 2 16 0 1 16 2 144 2 72 TWSR set prescalar to zero TWBR F_CPU F_SCL 16 2 set SCL frequency in TWI bit register Here is the protocol for sending data from master to slave MT master transmit mode e Master generates Start Condition status code 0x08 is returned e Master sends slave address Oxd0 DS1307 returns ACK status code 0x18 e Master sends one or more data bytes DS1307 returns ACK status code 0x28 e Master generates Stop Condition no status code returned After each operation the ready bit in TWCR will go to logic 0 and return to logic 1 when the operation is completed Byte sized data is sent received via the special TWDR register The start stop and data transfer conditions are specified by the TWCR control register And the status codes are put in the TWSR register Let s look at the code and compare it to the protocol Here is how to generate a start condition define TW_START xA4 send start condition TWINT TWSTA TWEN define TW_READY TWCR amp 0x80
3. turn cursor off x E to enable LCD_Cmd x 6 cursor direction right LCD_Cmd x 1 start with clear display msDelay 3 wait for LCD to initialize void LCD_Clear clear the LCD display LCD_Cmd CLEARDISPLAY msDelay 3 wait for LCD to process command void LCD_Home home LCD cursor without clearing LCD_Cmd SETCURSOR void LCD_Goto byte x byte y put LCD cursor on specified line byte addr line begins at addr xee switch y case 1 addr 0x40 break line 1 begins at addr 0x40 case 2 addr 0x14 break case 3 addr 0x54 break LCD_Cmd SETCURSOR addr x update cursor with x y position void LCD_Line byte row put cursor on specified line LCD_Goto row void LCD_String const char text display string on LCD while text do until character LCD_Char text send char amp update char pointer void LCD_Hex int data displays the hex value of DATA at current LCD cursor position char st 8 save enough space for result itoa data st 16 convert to ascii hex LCD_Message x add prefix x if desired LCD_String st display it on LCD void LCD_Integer int data displays the integer value of DATA at current LCD cursor position char st 8 save enough space for result itoa data st 10 convert to ascii LCD_String st display in on LCD pe a ae ae gee sp tee os eee eg eee ep
4. ready when TWINT returns to logic 1 define TW_STATUS TWSR amp xF8 returns value of status register byte I2C_Start generate a TW start condition TWCR TW_START send start condition while TW_READY wait return TW_STATUS 0x 8 return 1 if found otherwise To m a start load TWCR with 0xA4 and wait That s all there is too it Why 0xA4 If you really must know OxA4 is binary 10100100 The three 1 values correspond to the TWINT TWSTA and TWEN bits of the control register These bits enable the TWI interrupt the start condition and the whole TWI module You will see many people write it like this TWCR 1 lt lt TWINT 1 lt lt TWSTA 1 lt lt TWEN Most think that this self documenting style of coding is preferable so please use it if you like For me start is simply code 0xA4 The next thing to do is send the bus address of the slave we are communicating with For the DS1307 this value will be Oxd0 Here is our code to do that define DS1307 xDO I2C bus address of DS1307 RTC define TW_SEND 0x84 send data TWINT TWEN byte I2C_SendAddr addr send bus address of slave TWDR addr load device s bus address TWCR TW_SEND and send it while TW_READY wait return TW_STATUS 0x18 return 1 if found otherwise Put the DS1307 address into TWDR put the send command in TWCR and wait The next operation sending a data byte looks a
5. returns with address of first device found not found for byte addr start addr lt xFF addr search all 256 addresses if 12C_Detect addr 12C detected return addr leave as soon as one is found return none detected so return void I12C_Start byte slaveAddr I2C_Detect slaveAddr byte I2C_Write byte data sends a data byte to slave TWDR data load data to be sent TWCR TW_SEND and send it while TW_READY wait return TW_STATUS 0x28 byte I2C_ReadACK reads a data byte from slave TWCR TW_ACK ack will read more data while TW_READY wait return TWDR return TW_STATUS 0x28 byte I2C_ReadNACK reads a data byte from slave TWCR TW_NACK nack not reading more data while TW_READY wait return TWDR return TW_STATUS x28 void I2C_WriteByte byte busAddr byte data I2C_Start busAddr send bus address I2C_Write data then send the data byte I2C_Stop void I2C_WriteRegister byte busAddr byte deviceRegister byte data I2C_Start busAddr send bus address I2C_Write deviceRegister first byte device register address I2C_Write data second byte data for device register I2C_Stop byte I2C_ReadRegister byte busAddr byte deviceRegister byte data 0 I2C_Start busAddr send device address I2C_Write deviceRegister set register pointer I2C_Start busAddr RE
6. AVR beginners http www avrbeginners net architecture twi twi html 3 ATMEL AVR315 http www atmel com Images doc2564 pdf 3 C CODING It is possible to bit bang the protocol using any two data lines on your microcontroller However the ATmega328 has a dedicated TWI interface which simplifies the process The first job is to set the frequency of the serial data clock Typically the clock frequency is 10 slow mode 100 standard mode or 400 fast mode kHz The maximum clock rate is determined by the slowest device on the bus as well as bus capacitance As a practical matter most I2C devices run at 100 kHz The DS1307 runs at 100 kHz Before going further keep in mind there are already libraries available for using 12C with your AVR or arduino You do not need to do this yourself A search for 12C master library will turn up a few alternatives Skip this section if you have no interest in learning how to code 12C There are two special registers on the ATmega which control the SCL frequency TWSR and TWBR TWSR is the TWI status register and contains prescalar bits used to divide the CPU clock frequency We do not need a prescalar so we can ignore these bits The TWBR is the bit rate register The SCL frequency is a function of the CPU frequency and this register according to the following formula F_SCL in MHz F_CPU 16 2 TWBR Kinda complicated isn t it To determine the value of TWBR we can rewrite it like this TWBR
7. AD restart as a read operation data I2C_ReadNACK read the register data I2C_Stop stop return data if ganRnereaoseassenGan S ETATS ESOR REAR ase nsasosr RARS Er DS1307 RTC ROUTINES define DS1307 oxDO I2C bus address of DS1307 RTC define SECONDS REGISTER 0xe define MINUTES REGISTER 0x01 define HOURS_REGISTER 0x02 define DAYOFWK_REGISTER 0x03 define DAYS_REGISTER 0x04 define MONTHS_REGISTER 0x05 define YEARS_REGISTER 0x06 define CONTROL_REGISTER 0x07 define RAM_BEGIN 0x08 define RAM_END 0x3F void DS1307_GetTime byte hours byte minutes byte seconds returns hours minutes and seconds in BCD format hours I2C_ReadRegister DS1307 HOURS_ REGISTER minutes I2C_ReadRegister DS1307 MINUTES_REGISTER seconds I2C_ReadRegister DS13 7 SECONDS_REGISTER if hours amp x4 12hr mode hours amp x1F use bottom 5 bits pm bit temp amp 0x20 else hours amp 0x3F 24hr mode use bottom 6 bits void DS1307_GetDate byte months byte days byte years returns months days and years in BCD format months I2C_ReadRegister DS1307 MONTHS_REGISTER days I2C_ReadRegister DS1307 DAYS_ REGISTER years I2C_ReadRegister DS1307 YEARS REGISTER void SetTimeDate simple hard coded way to set the date I2C_WriteRegister DS13 7 MONTHS REGISTER x 8 I2C_WriteRegister DS1307 DAYS_REGISTER x31 I2C_WriteRegister DS1307 YEARS REGIS
8. Add a DS1307 RTC clock to your AVR microcontroller Bruce E Hall W8BH Having a real time clock RTC on your microcontroller can be very handy especially for data logging operations The Maxim DS1307 is a common and inexpensive real time clock It requires only two I O lines for data communication If you want to add a clock to your AVR microcontroller or if you want to learn more about two wire I C interfaces please read on 2 THE I C INTERFACE Atmel calls their version of 12C the two wire interface or TWI It is a serial data protocol which uses two data lines for communication a data line SDA and a clock SCL Devices on the I2C bus can either be masters or slaves Masters initiate data transfers and slaves react only to master requests In this article the AVRmega328 is the master and the RTC is always the slave Slaves are specified by a 7 bit address plus a read write bit The device address for the DS1307 is fixed at Oxd0 The interface circuit is open collector which means that the data lines are passively kept high by resistors em m 4 to Vec Any device on the bus can actively pull a data ae I line low Up to 128 devices can be put on the same data bus oe a ee There are plenty of good articles on TWI I2C programming for AVR microcontrollers Check out the following for a good start 1 Non GNU org http www nongnu org avr libc user manual group__twi__demo html 2
9. GISTER days I2C_ReadRegister DS1307 DAYS_ REGISTER years I2C_ReadRegister DS1307 YEARS REGISTER void SetTimeDate simple hard coded way to set the date 8 13 21013 at 8 51 PM I2C_WriteRegister DS1307 MONTHS_REGISTER x 8 I2C_WriteRegister DS1307 DAYS_ REGISTER 0x31 I2C_WriteRegister DS1307 YEARS_REGISTER 0x13 I2C_WriteRegister DS1307 HOURS_REGISTER x08 0x40 add 0x40 for PM I2C_WriteRegister DS1307 MINUTES_REGISTER 0x51 I2C_WriteRegister DS1307 SECONDS_REGISTER 0x00 There are more efficient ways of reading and writing the time For example using sequential mode access we can begin an 12C read operation with the seconds register at 0x00 The address pointer on the DS1307 auto increments after each read operation We can read in all seven time registers before stopping saving time and code space chose to read each register individually to be a little more readable and generic Data stored in each register is in Binary Coded Decimal BCD format Generally this means that each byte contains two digits The most significant digit is stored in the upper four bits and the least significant digit is stored in the lower four bits For example consider the decimal number 36 Ordinarily we would code this as 0x24 hexadecimal or 0010 0100 binary But in BCD it is stored as 0011 0100 Notice that the upper four bits are 0011 decimal 3 and the lower four bits are 0100 decimal 6 Di
10. TER x13 I2C_WriteRegister DS1307 HOURS_ REGISTER x 8 0x40 add 0x40 for PM I2C_WriteRegister DS1307 MINUTES REGISTER x51 I2C_WriteRegister DS1307 SECONDS REGISTER x Q pe ee gee oe pe ee oe ea ee a ae eae ee df APPLICATION ROUTINES void ShowDevices Scan I2C addresses and display addresses of all devices found LCD_Line 1 LCD_String Found byte addr 1 while addr gt LCD_Char addr I2C_FindDevice addr if addr gt LCD_Hex addr void LCD_TwoDigits byte data helper function for WriteDate input is two digits in BCD format output is to LCD display at current cursor position byte temp data gt gt 4 LCD_Char temp data amp Ox F LCD_Char data void WriteDate byte months days years DS1307_GetDate amp months amp days amp years LCD_TwoDigits months LCD_Char LCD_TwoDigits days LCD_Char LCD_TwoDigits years void WriteTime byte hours minutes seconds DS1307_GetTime amp hours amp minutes amp seconds LCD_TwoDigits hours LCD_Char LCD_TwoDigits minutes LCD_Char LCD_TwoDigits seconds void LCD_TimeDate LCD_Line WriteTime LCD_Line 1 WriteDate ee A J PROGRAM LOOP void MainLoop while 1 LCD_TimeDate put time amp date on LCD msDelay 10 one second between updates MAIN PROGRAM int main void InitAVR LCD_I
11. atus code 0x08 is returned e Master sends slave bus address 0xd0 DS1307 returns ACK status code 0x18 e Master sends address pointer DS1307 returns ACK status code 0x28 e Master generates another Start Condition restart status code 0x10 returned e Master sends slave bus address read bit 0Oxd1 DS1307 returns ACK status code 0x40 e Master requests data byte with NACK DS1307 returns byte status code 0x58 e Master sends Stop condition no status code returned The only new code required for reading is the read operation in the next to last step It looks very similar to the write operation NACK is used to a request of a single or last byte of data define TW_NACK 0x84 read data with NACK last byte define READ 1 byte I2C_ReadNACK reads a data byte from slave TWCR TW_NACK nack not reading more data while TW_READY wait return TWDR Putting it all together here are the routines for reading and writing registers on the DS1307 void I2C_WriteRegister byte deviceRegister byte data I2C_Start I2C_SendAddr DS13 7 send bus address I2C_Write deviceRegister first byte device register address I2C_Write data second byte data for device register I2C_Stop byte I2C_ReadRegister byte deviceRegister byte data 0 I2C_Start 1I2C_SendAddr DS13 7 send device bus address I2C_Write deviceRegister set register pointer I2C_Start I2C_SendAdd
12. cations with the HD4478 controlled LCD The following defines specify which port pins connect to the controller define LCD_RS define LCD_E define DAT4 define DATS define DAT6 define DAT7 pin for LCD R S eg PB pin for LCD enable pin for d4 pin for d5 pin for d6 7 pin For d7 UBWNR The following defines are HD4478 controller commands define CLEARDISPLAY 0x01 define SETCURSOR 0x80 void PulseEnableLine SetBit PORTB LCD_E take LCD enable line high _delay_us 4 wait 40 microseconds ClearBit PORTB LCD_E take LCD enable line low void SendNibble byte data PORTB amp xC3 1100 0011 clear 4 data lines if data amp BV 4 SetBit PORTB DATA if data amp _BV 5 SetBit PORTB DATS if data amp _BV 6 SetBit PORTB DAT6 if data amp _BV 7 SetBit PORTB DAT7 PulseEnableLine clock 4 bits into controller void SendByte byte data SendNibble data send upper 4 bits SendNibble data lt lt 4 send lower 4 bits ClearBit PORTB 5 turn off boarduino LED void LCD_Cmd byte cmd ClearBit PORTB LCD_RS R S line command data SendByte cmd send it void LCD_Char byte ch SetBit PORTB LCD_RS R S line 1 character data SendByte ch send it void LCD_Init LCD_Cmd 0x33 initialize controller LCD_Cmd 0x32 set to 4 bit input mode LCD_Cmd 0x28 2 line 5x7 matrix LCD_Cmd x C
13. ee se eae a See poet ee se se aes I2C TWI ROUTINES On the AVRmega series PA4 is the data line SDA and PAS is the clock SCL The standard clock rate is 100 KHz and set by I2C_Init It depends on the AVR osc freq define F_SCL 100000L I2C clock speed 100 KHz define READ 1 define TW_START OxA4 send start condition TWINT TWSTA TWEN define TW_STOP 0x94 send stop condition TWINT TWSTO TWEN define TW_ACK xC4 return ACK to slave define TW_NACK 0x84 don t return ACK to slave define TW_SEND 0x84 send data TWINT TWEN define TW_READY TWCR amp 0x80 ready when TWINT returns to logic 1 define TW_STATUS TWSR amp xF8 returns value of status register define I2C_Stop TWCR TW_STOP inline macro for stop condition void I2C_Init at 16 MHz the SCL frequency will be 16 16 2 TWBR assuming prescalar of so for 1 KHz SCL TWBR F_CPU F_SCL 16 2 16 1 16 2 144 2 72 TWSR TWBR set prescalar to zero F_CPU F_SCL 16 2 set SCL frequency in TWI bit register byte I2C_Detect byte addr look for device at specified address return 1 found 9 not found TWCR TW_START send start condition while TW_READY wait TWDR addr load device s bus address TWCR TW_SEND and send it while TW_READY wait return TW_STATUS x18 return 1 if found otherwise byte I2C_FindDevice byte start
14. lmost exactly the same Notice that the returned status code will be different however byte I2C_Write byte data sends a data byte to slave TWDR data load data to be sent TWCR TW_SEND and send it while TW_READY wait return TW_STATUS 0x28 return 1 if found otherwise For the DS1307 we will do this Write operation twice once to set the address pointer on the RTC and again to supply the data for that address The last step is the send the Stop condition Here we just set the command register to 0x94 the value for TW_STOP Again this value sets the TW enable TW interrupt and TW stop bits Go ahead use 1 lt lt TWINT 1 lt lt TWEN 1 lt lt TWSTO if you prefer We do not have to wait or check for status codes so it is just a one line command Instead of writing a routine made a macro instead define TW_STOP 0x94 send stop condition TWINT TWSTO TWEN define I2C_Stop TWCR TW_STOP inline macro for stop condition Just a quick note on the status codes I ve written my routines to check the status but ignore the results In my simple setup this works OK You may want to check each code and show error messages when appropriate Reading data is little trickier we have to write to the device first to set its internal address pointer then read to get the data at that address Here is the protocol for receiving data from the slave e Master generates Start Condition st
15. nit I2C_Init LCD_String Ready ShowDevices msDelay 4000 LCD_Clear MainLoop if EL set port direction initialize HD4478 LCD controller set I2C clock frequency show that I2C is working OK display time
16. r DS1307 READ restart as a read operation data I2C_ReadNACK read the register data I2C_Stop stop return data 4 DS1307 SPECIFIC CODING The RTC is pretty straightforward It contains data registers that specify the seconds minutes hours days months and years You write these registers to set the time and read these registers to get the time Here are the data register addresses define SECONDS REGISTER x 0 define MINUTES REGISTER x 1 define HOURS REGISTER 0x02 define DAYOFWK_REGISTER 0x03 define DAYS_REGISTER 0x04 define MONTHS_REGISTER 0x05 define YEARS_REGISTER 0x06 There are a few special cases The seconds register contains a flag to start stop the clock And the hours register has flags for 12 24 hour format and AM PM Otherwise getting the time is just a matter of reading the appropriate registers void DS1307_GetTime byte hours byte minutes byte seconds returns hours minutes and seconds in BCD format hours I2C_ReadRegister DS1307 HOURS REGISTER minutes I2C_ReadRegister DS1307 MINUTES_REGISTER seconds I2C_ReadRegister DS13 7 SECONDS_REGISTER if hours amp 0x40 12hr mode hours amp x1F use bottom 5 bits pm bit temp amp 0x20 else hours amp 0x3F 24hr mode use bottom 6 bits void DS1307_GetDate byte months byte days byte years returns months days and years in BCD format months I2C_ReadRegister DS1307 MONTHS RE
17. splaying the BCD values is not difficult since each digit is separately coded Here is an example for an LCD display using the LCD_Char routine void TwoDigits byte data helper function for WriteDate and WriteTime input is two digits in BCD format output is to LCD display at current cursor position byte temp data gt gt 4 get upper 4 bits LCD_Char temp display upper digit data amp x 0F get lower 4 bits LCD_Char data display lower digit void WriteDate byte months days years DS1307_GetDate amp months amp days amp years TwoDigits months LCD_Char TwoDigits days LCD_Char TwoDigits years void WriteTime byte hours minutes seconds DS1307_GetTime amp hours amp minutes amp seconds TwoDigits hours LCD_Char TwoDigits minutes LCD_Char TwoDigits seconds 5 CONSTRUCTION Instead of breadboarding an ATmega328 directly use the DC boarduino by Adafruit it is breadboard friendly and puts a DC power supply microprocessor external oscillator ISP programming header status LED and reset switch all on a very small circuit board Rees Flee Clack Next you need a DS1307 Again used a small circuit module rather aT a a than the chip The module used is the 15 RTC kit by Smiley age Micros The module adds the required external oscillator and battery we backup Other good ones are available from SparkF
18. un and Adafruit A Ot a a First connect both devices to 5V and ground Then connect the I C data lines as follows DC Boarduino 328 DS1307 module A4 PC4 SDA A5 PC5 SCL Remember than each data line needs a pull up resistor Check your RTC module for these resistors Mine uses a pair of 2 2K red red red resistors If your module does not include these resistors install them on your breadboard between 5V and SDA SCL also tried a pair of 4 7K resistors and they worked fine Here is the breadboard layout The I C bus is represented by the white SDA and blue SCL wires There are two 4 7K pull up resistors on the bus which are partially hidden by the four red data lines The LCD is a 20x4 character HD44780 compatible display See my LCD article for more information on this interface A 10K potentiometer controls the display contrast That s it In my next article l Il show how to interface a more clock like LED display over IC 6 SOURCE CODE i2c 1 Experiments with interfacing ATmega328 to an DS1307 RTC Author Bruce E Hall lt bhall66 gmail com gt Website w8bh net Version 1 1 Date 7 Sep 2013 Target ATTmega328P microcontroller Language C using AVR studio 6 Size 1386 bytes using 01 optimization df Fuse settings 8 MHz osc with 65 ms Delay SPI enable NO clock 8 eee GLOBAL DEFINES define F_CPU 16000000L run CPU at 16 MHz define LED 5
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