8085 program for pulse waveform

Problem – Write a program to generate continuous square wave. Use D₀ bit to output the square wave. The required waveform is:

Explanation –
The alternate pattern of 0/1 bits can be provided by loading the accumulator with AAH(10101010) and rotating the pattern once through each loop. Bit D₀ of the output port is used to proved logic 0 and 1. Therefore, all other bits can be masked by ANDing the accumulator with 01H.

Example –

Accumulator   : 1 0 1 0 1 0 1 0
And with 01H  : 0 0 0 0 0 0 0 1
Output        : 0 0 0 0 0 0 0 0
So output => 0

After RLC :

Accumulator   : 0 1 0 1 0 1 0 1 
And with 01H  : 0 0 0 0 0 0 0 1
Output        : 0 0 0 0 0 0 0 1
So output => 1

Program –

Address	Label	Mnemonics	Comments
2000H		MVI D, AAH	Load bit pattern AAH
2002H	ROTATE	MOV A, D	Load bit pattern in A
2003H		RLC	Change data to AAH to 55H and vice versa
2004H		MOV D, A	Save A
2005H		ANI 01H	Mask bits D7 to D1
2007H		OUTPORT 1	Output the D0 bit
2009H		JMP ROTATE	Jump to ROTATE to change logic level

Program Description –

1. Register D is loaded with AAH(10101010).
2. Bit pattern is moved to accumulator.
3. Bit pattern is rotated left and saved again in register D. This save is necessary as accumulator is used again in the program.
4. Mask all bits but 0th bit.
5. Output A at port 1.

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