Write a program to count continuously in hexadecimal from FFH to 00H in a system with clock frequency 0.5 microseconds. Use register C to set up a delay of 1ms between each count and display output at one of the output ports.
- The hexadecimal counter is set by loading a register with starting number and decrementing it till zero is reached and then again decrementing it to will produce -1, which is two’s complement of FFH. Hence, the register again reaches FFH.
- The 1ms time delay is set up by the procedure shown in flowchart-
The register is loaded with appropriate number such that the execution of above loop produces a time delay of 1ms.
|2000H||MVI B, FFH|
|2003H||MVI C, COUNT|
|2009H||MOV A, B|
The C register is the time delay register which is loaded by a value COUNT to produce a time delay of 1ms.
To find the value of COUNT we do-
TD = TL + TO where- TD = Time Delay TL = Time delay inside loop TO = Time delay outside loop
The delay loop includes two instructions- DCR C (4 T-states) and JNZ (10 T-states)
So TL = 14*Clock period*COUNT
Delay outside the loop includes-
DCR B : 4T
MVI C, COUNT : 7T
MOV A, B : 4T
OUTPORT : 10T
JMP : 10T
Total : 35T
TO= 35*Clock period => 17.5 microseconds
So, 1ms= (17.5+ 7*COUNT)microsecond