8085 program for hexadecimal counter

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.

Problem Analysis:

1. 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.
2. 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.

Program:

Address	Label	Mnemonics
2000H		MVI B, FFH
2002H	NEXT	DCR B
2003H		MVI C, COUNT
2005H	DELAY	DCR C
2006H		JNZ DELAY
2009H		MOV A, B
200AH		OUTPORT#
200CH		JMP NEXT

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 T_L = 14*Clock period*COUNT
=> 14*(0.5*10^-6)*COUNT
=> (7*10^-6)*COUNT

Delay outside the loop includes-
DCR B : 4T
MVI C, COUNT : 7T
MOV A, B : 4T
OUTPORT : 10T
JMP : 10T
Total : 35T

T_O= 35*Clock period => 17.5 microseconds

So, 1ms= (17.5+ 7*COUNT)microsecond

Therefore, COUNT=(140)₁₀