Lab 6: Generating a Square Wave of Desired Frequency
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1. B The address for Port B is 1004 This location does not belong to the user programmable space in the RAM you can see this on the EVB memory map Any data that is stored at this location will appear at the eight pins of Port B The instruction STAA 1004 will store the value in Accumulator A into 1004 and thus the eight bits in Accumulator A appear at Port B The frequency of the square wave can be measured by connecting the appropriate pins to the oscilloscope The pin assignment of the 60 pin connector P1 is shown in Table 6 1 of the EVB User s Manual We start by writing a program that can do these manipulations We will use Accumulator A to increment the value we send to output Port B We first have to clear Accumulator A then continuously increment it and write the result to address 1004 Program 1 below performs this task Note that the BRA instruction always branches back so this program has an infinite never ending loop We can connect an oscilloscope to the appropriate pin on connector P1 to see the generated square wave and can calculate the frequency of this signal All of the square waves will be observed on bit 0 of Port B abbreviated PB0 You may want to look at the signals generated on the other bits of Port B too ELE 205 Lab 6 Spring 2005 amplitude anny One Pesiod One Period 0 091 sees 0 001 secs ov Figure 1 Square wave of kHz frequency Port C General purpose I O lines 20 252. Port D General purpose I O lines Used with the Serial Communication Interface SCI and Serial Peripheral Interface SPI 27 34 Port A General purpose I O lines 35 42 Port B General purpose output lines 43 50 Port E General purpose input or A D channel input lines Table 1 Pin numbers of MCU ports available on connector P1 Program 1 Address Label Mnemonic Operand Comment C010 CLRA Clear Accumulator A C011 LOOP INCA Increment Accumulator A C012 STAA 1004 Send ACCA to Port B C015 BRA LOOP Branch back always C017 FINISH SWI We never get here How can we calculate the frequency of the square wave The M68HC11 operates with a clock of frequency 2 MHz megahertz i e 2 million cycles per second Hence each cycle has a period of 1 second 2 x 10 cycles 0 5 usec cycle Therefore if an instruction takes 4 cycles it takes 4 x 0 5 2 usec to execute The number of cycles each instruction takes can be found in the Instruction Set Summary Appendix A in the textbook From these tables we see that CLRA takes 2 cycles INCA takes 2 cycles STAA with extended addressing takes 4 cycles and BRA takes 3 cycles Hence the total number of cycles involved in each loop are 2 4 3 9 cycles Note that the loop consists of only INCA STAA and BRA instructions CLRA and SWI are outside the loop The total time involved in executing 9 cycles is 9 x 0 5 usec cycle 4 5 usec Our output square wave goes from up to down
3. or from down to up every 4 5 usec This makes the period T 2 x 4 5 usec 9 0 usec and the corresponding frequency f 1 9 0 usec 0 1111 MHz 3 Generating a square wave of desired frequency In the previous section we generated a square wave This section deals with generating a square 2 ELE 205 Lab 6 Spring 2005 wave of a desired frequency We showed the calculations required for computing the total number of clock cycles in a given loop To generate a square wave of lower frequency lt 0 1111 MHz we need to introduce a wait loop i e a sequence of instructions that takes some time to execute but does nothing The following assembly program shows one such wait loop WAIT LOOP Address Label Mnemonic Operand Comment C010 LDX 50010 load loop counter C011 LOOP DEX Decrement Index Register C012 BNE LOOP Branch till the end of loop C014 FINISH SWI Stop This wait loop does nothing but it spends some time executing Inserting such a loop in Program 1 we can increase the execution time and hence can increase the period of the generated square wave Program 2 shows Program modified with a wait loop Program 2 Address Label Mnemonic Operand Comment C010 CLRA Clear Accumulator A C011 LOOP INCA Increment Accumulator A C012 LDX SN Load Index Register X with a number N COTS WAIT DEX Decrement Index Register C016 BNE WAIT Branch back to wait loop C018 STAA 10
4. 04 Send ACCA to Port B CO1B BRA LOOP Branch back always CO1C FINISH SWI We never get here Note that an unknown number N is used in Program 2 For a given frequency we wish to generate we must compute the value of N We compute the execution time involved in executing LOOP by analyzing the loop WAIT and the rest of the instructions in loop LOOP with cycle by cycle calculations WAIT loop timing N Total number of times the loop WAIT is executed DEX takes 3 cycles BNE takes 3 cycles Total cycles in WAIT loop 6N LOOP loop timing INCA takes 2 cycles LDX takes 3 cycles in immediate addressing mode STAA takes 4 cycles in extended addressing mode BRA takes 3 cycles Total cycles in LOOP loop 2 3 6N 4 3 6N 12 cycles Square wave characteristics Period of square wave T 2 x 6N 12 cycles x 0 5 usec cycle 6N 12 usec A desired frequency of f MHz implies N 1 f 12 6 vas ELE 205 Lab 6 Spring 2005 The number N calculated from this formula for a desired frequency fdesirea Should be substituted for N in Program 2 Care should be taken since the resulting N is a real number not an integer The integer value closest to the calculated real number should be chosen The resulting frequency will not always equal the desired frequency faesirea For example fdesirea 25 kHz results in N 4 67 we choose the closest integer N 5 The calculated frequency is faic 23 8 kHz A s
5. ELE 205 Lab 6 Spring 2005 Lab 6 Generating a Square Wave of Desired Frequency 1 Objectives Learn how to send data to the output ports on 68HC11 EVB Generate a square wave of desired frequency 2 Generating a square wave Reading Sections 2 4 4 6 1 and 6 2 of M68HC11EVB Evaluation Board User s Manual A square wave with frequency f 1 kHz kilohertz is shown in the Figure below This is a periodic signal with period T 1 f 1 1000 1 millisecond The amplitude of the signal is 5 V volts This signal can be generated by making a bit 1 when high voltage is required and 0 when low voltage is required If one is added repeatedly to a binary number the least significant bit LSB of the result will alternate between 0 and 1 We use this technique of continually incrementing a number to generate a square wave The problem then becomes write a program that will change the LSB of an accumulator and send the result to one of the output lines of the EVB As can be seen in the Table the EVB has output ports A E They are accessed through connector P1 Connector P1 includes the lines of five ports namely Port A B C D and E and other signals like E clock interrupt request IRQ Vpp ete Except for Port D each port has eight pins Note that Port B is the only port that generates only output lines Hence we will use Port B in this lab We will generate a square wave with a program and send the signal to the Port
6. olution to this problem is to introduce more instructions to change the loop timing A good choice is the NOP instruction no operation This instruction does nothing but use two CPU cycles it doesn t change any registers affect the CCR etc Since it doesn t affect the CPU it is a good choice to use in wait loops If additional instructions are introduced all the cycle by cycle calculations must be redone since the above formula for N is no longer valid Prelab 1 Create Program 3 from Program 2 by introducing two NOP instructions the first before the LDX instruction and the second before the BNE instruction Redo the cycle by cycle calculations and derive the new expression for N required for a given f 2 Let faesirea be 16 kHz Find fcac for both Program 2 and Program 3 the one with the additional NOP instructions Calculate the percentage difference between the desired frequencies and calculated frequencies 3 Code Program 1 Program 2 and Program 3 Make sure AS11 can assemble them without generating any warnings or error messages Bring these source files to lab Lab Work Assemble and run Program 1 Program 2 and Program 3 For Program 1 measure the frequency of the square wave on PBO For Programs 2 and 3 let fdesirea 16 0 kHz Measure the frequency of the square wave on PBO Also record the wave frequencies on PB1 and PB2 Lab Report Show your calculations from prelab and values measured in lab Make a table for the re
7. sults of Program 2 and Program 3 Show the desired frequencies the calculated frequencies and the measured frequencies Show the percentage difference between the desired and measured frequencies and the percentage difference between the calculated and measured frequencies Answer the following 1 Explain the purpose of connector P1 in our lab 2 List the number of bytes and number of cycles required for executing the instructions MUL FDIV IDIV and NOP Note that the number of bytes of an instruction is different from the number of cycles required to execute the instruction 3 Compute the time required for executing loop LOOP once in Program 1 if the clock frequency of the M68HC11 is 5 MHz instead of 2 MHz 4 Which program gave you the frequency closest to the desired frequency of 16 0 kHz Why 5 Derive an expression mathematically and or graphically relating the frequency of the square wave on bit 1 of Port B PB1 to the square wave frequency you observed on bit 0 PBO
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