TK2633: MICROPROCESSOR & INTERFACING Programming Techniques

DELAY Procedure to use delay is almost similar to counter. A register is loaded with a number; depending on the delay required; the register is subtracted until zero is achieved by putting the conditional jump instruction. December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a single register December 9, 2018 Dr. Masri Ayob: TK2633 figure 2

Time delay using a single register From the example, noticed that the MVI instruction uses 7 Time states for it to execute. Suppose the 8085 µp has a clock frequency of 2MHz, the MVI instruction will execute within 3.5µs. Below is the example to calculate the time of execution. December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a single register If the clock frequency given is 1MHz, then the MVI instruction will take 7µs to execute. To calculate the time delay in a loop, the total T-states and total instructions must be predetermined. In figure 2, register C is loaded with FFH (25510). DCR and JNZ in the loop use 14 T-states. And the loop will repeat 255 times until C gets zero. Then the delay calculation is determined as follows: Where TL = Time delay in the loop. T = time states of the system (1/f) N10 = decimal number December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a single register Most of the time, the delay calculation is almost accurate, however to get better accuracy the calculation for JNZ has to be modified. The T-states for JNZ shows 10/7. It means the 8085 µp requires 10 T-states for JNZ to jump within the loop process, and 7 T-states when JNZ jump out of the loop. Therefore in the final loop (255th loop) the JNZ instruction uses 7 T-states and the following calculation shows how to modified this accuracy. December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a single register Therefore the overall delay including the delay outside the loop is as follows: December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using register pair Longer delay can be achieved by using register pair and 16-bits number. The 16-bit numbers require DCX instruction to decrement. However the DCX instruction will not affect Z flag. Therefore special technique has to be added. Below is the example of using BC register pair as delay register. December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using register pair In the previous example, the instruction LXI B, 2384H, causes B=23H and C = 84H. DCX instruction will decrement the BC by one. MOV A,C and ORA B, are to determine the content of Z flag. The OR instruction set the Z register whenever B and C have the same value that is zero. Therefore this loop will repeat 2384H times. The total T-states in the loop is 24 (for instructions DCX, MOV, ORA and JNZ). The conversion of 2384H times equivalent to its decimal: December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using register pair Suppose the time interval in the system is 0.5µs, the delay in the loop is: The same time delay technique can be used to calculate the delay of nested loop delay technique (loop within a loop). December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a loop within a loop A longer time delay can be achieved using two loops; one loop is inside the other loop. As shown in the instructions below, the C register is used in the inner loop (LOOP1) and the B register is used in the outer loop (LOOP2). Example: December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a loop within a loop Figure 3 December 9, 2018 Dr. Masri Ayob: TK2633

Time delay using a loop within a loop Delay in the LOOP1 is equal to the previous example, TL1 = 1783.5µs. The second loop, LOOP2 is generated 56 times equivalent to 38H in B register. Delay calculation for LOOP2 is as follows: December 9, 2018 Dr. Masri Ayob: TK2633

Counter design with time delay Figure 4 December 9, 2018 Dr. Masri Ayob: TK2633

Counter design with time delay Rajah 5 December 9, 2018 Dr. Masri Ayob: TK2633

Counter design with time delay Example : Write a programme to count number from FFH to 00H in a system with time interval of 0.5 µs. Use C register as delay register with the delay of 1ms for every count. Display the count at the output port. Analysis: The hexadecimal counter is loaded in a register and decremented until zero is reached. After reaches zero the counter will start again at FFH. To get 1ms delay, one value will be loaded into register C, and the delay calculation is as shown in previous example. December 9, 2018 Dr. Masri Ayob: TK2633

Counter design with time delay MVI B, 00H NEXT DCR B MVI C, COUNT DELAY DCR C JNZ DELAY MOV A, B OUT PORT JMP NEXT December 9, 2018 Dr. Masri Ayob: TK2633

Counter design with time delay Delay calculation (TL delay in the loop) : December 9, 2018 Dr. Masri Ayob: TK2633

Exercise Write a program to turn a light ON and OFF every 2 seconds. Use data bit D7 (at OUTPORT) to operate the light. Logic ‘0’ will ON the light, whilst logic ‘1’ OFF the light. The clock frequency of the MPU is 2 MHz. December 9, 2018 Dr. Masri Ayob: TK2633

Thank you Q&A December 9, 2018 Dr. Masri Ayob: TK2633