Texas Instruments MSC1210 manual Value Comparison Cjne, Lcall Debouncekey

Value Comparison (CJNE)

16.14 Value Comparison (CJNE)

CJNE (compare, jump if not equal) is a very important instruction. It is used to compare the value of a register to another value and branch to a label based on whether or not the values are the same. This is a very common way of building a switch…case decision structure or an IF…THEN…ELSE structure in assembly language.

The CJNE instruction compares the values of the first two parameters of the instruction and jumps to the address contained in the third parameter, if the first two parameters are not equal.

CJNE A,#24h,NOT24	;Jumps to the label NOT24 if accumulator isn’t 24h
CJNE A,40h,NOT40	;Jumps to the label NOT40	if accumulator is
	;different than the value contained in Internal
	;RAM address 40h
CJNE R2,#36h,NOT36	;Jumps to the label NOT36	if R2 isn’t 36h
CJNE @R1,#25h,NOT25	;Jumps to the label NOT25	if the Internal RAM
	;address pointed to by R1	does not contain 25h

As shown, the MCU compares the first parameter to the second parameter. If they are different, it jumps to the label provided; if the two values are the same then execution continues with the next instruction. This allows for the programming of extensive condition evaluations.

For example, to call the PROC_A subroutine if the accumulator is equal to 30H, call the CHECK_LCD subroutine if the accumulator equals 42H, and call the DEBOUNCE_KEY subroutine if the accumulator equals 50H. This could be im- plemented using CJNE as follows:

	CJNE A,#30h,CHECK2	;If A is not 30h, jump to CHECK2 label
	LCALL PROC_A	;If A is 30h, call the PROC_A subroutine
	SJMP CONTINUE	;When we get back, we jump to CONTINUE label
CHECK2:	CJNE A,#42h,CHECK3	;If A is not 42h, jump to CHECK3 label
	LCALL CHECK_LCD	;If A is 42h, call the CHECK_LCD subroutine
	SJMP CONTINUE	;When we get back, we jump to CONTINUE label
CHECK3:	CJNE A,#50h,CONTINUE	;If A is not 50h, we jump to CONTINUE label
	LCALL DEBOUNCE_KEY	;If A is 50h, call the DEBOUNCE_KEY subroutine
CONTINUE: {Code continues here}		;The rest of the program continues here

As shown, the first line compares the accumulator with 30H. If the accumulator is not 30H, it jumps to CHECK2, where the next comparison is made. If the ac- cumulator is 30H, however, program execution continues with the next instruc- tion, which calls the PROC_A subroutine. When the subroutine returns, the SJMP instruction causes the program to jump ahead to the CONTINUE label, thus bypassing the rest of the checks.

The code at label CHECK2 is the same as the first check. It first compares the accumulator with 42H and then either branches to CHECK3, or calls the CHECK_LCD subroutine and jumps to CONTINUE. Finally, the code at CHECK3 does a final check for the value of 50H. This time there is no SJMP instruction following the call to DEBOUNCE_KEY because the next instruction is CONTINUE.

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