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

Advantages:

• This program is simple and efficient, making it easy to implement and understand.
  
• The program allows for the customization of the initial value of the counter by modifying the values in the C and B registers.
  
• The program can be modified to count in different bases, such as binary or decimal.
  
• The program can be easily modified to count up or down by changing the instruction from DCR to INR.
  

Disadvantages:

• The program assumes that the counter will never exceed 255 in decimal, which may not be suitable for all use cases.
  
• The program does not output the counter value to a display or other output device, which may limit its usefulness in some applications.
  
• The program does not handle exceptional cases, such as overflow situations or invalid input.
  
• The program does not allow for dynamic memory allocation, which may limit its usefulness in more complex applications.