time delay in 16f876

[amr_karimm]

hi ,
i want to make a (asm) code for make 1 sec delay for pic 16f876 using (4mhz ) xt oscillator
any one can help me..

 

[patricktran]

I have the same question. According to what other people adviced me, you use TMR0.
TMR0 with prescale 256. So we have 256*256 = 65536 Ms or 65.5 ms
if you loop it 15 times, so 65.5ms * 15 = 0.9825 sec
correct me If I am wrong.
Thanks

 

[csaba911]

Try this if you not short of memory, use MPLAB stopwatch to test it.

del		  EQU	0X74
delf1		EQU	0x75
delf2		EQU	0x76
main	call	DEL1sec
		 goto	main
DEL1sec	movlw	.6	;Delay start here,  
	movwf	del	;1.000001 sec
	movlw	.19        
	movwf	delf2       
	movlw	.174    ;fine tune with this number    
	movwf	delf1 	;173=999.99800 s
			;174=1.000001 s
			;175=1.000004 s
	decfsz	delf1 	;Start decrementing the file registers. 
	goto 	$-1
	decfsz	delf2 
	goto	$-3	
	decfsz	del
	goto	$-5
	return	;Go back if is done

STEVE

 

[patricktran]

Hi, can you please tell me what else do I need to add for the code to make it work, I mean such things as specifying which PIC, including inc file... I am a nuewbie.
Thanks

 

[Nigel Goodwin]

Hi, can you please tell me what else do I need to add for the code to make it work, I mean such things as specifying which PIC, including inc file... I am a nuewbie.
Thanks

If you check my tutorials you will see everything you need, Tutorial 1 introduces timing delays and the reasons for them - an easy way to generate the code for a specific delay is to use the Delay Code Generator on the PICList at http://www.piclist.com.

 

[patricktran]

Thanks for your advices.
I tried to get a delay of 1 sec by using busy loop. I think as follow:
1 sec = 1,000,000 instructions = 256 * 256 *15 = FF * FF * 0F
And I have the code:

; Delay = 1 seconds
; Clock frequency = 4 MHz

; Actual delay = 1 seconds = 1000000 cycles
; Error = 0 %

	
d1   EQU    20h
d2   EQU    21h
d3   EQU    22h
	

			;999997 cycles
	movlw	0xFF
	movwf	d1
	movlw	0xFF
	movwf	d2
	movlw	0x0F
	movwf	d3
Delay1sec_0
	decfsz	d1, f
	goto	$+2
	decfsz	d2, f
	goto	$+2
	decfsz	d3, f
	goto	Delay1sec_0
	nop
   End

This code runs so long, it does not delay 1 sec. What is wrong with that?
Thanks
Pat

 

[patricktran]

Oh, this code I got from the piclist code generator. It does with d1 = 0x08
d2=0x2F d3=0x03. I modified according to what I think: d1=0xFF, d2=0xFF d3=0x0F

 

[falleafd]

;999997 cycles
movlw 0xFF
movwf d1
movlw 0xFF
movwf d2
movlw 0x0F
movwf d3
Delay1sec_0
decfsz d1, f
goto $+2
decfsz d2, f
goto $+2
decfsz d3, f
goto Delay1sec_0
nop
End

As it reduce d1, it goto ... goto.... goto..., that is, it takes 2 + 2 + 2 cycles to do this loop. And it takes d1* ( 1 + 2 + 2+2) cycles to decrease d1 to zero. It's not only d1 cycles. So to d2. Therefore, you get a so long loop.

I've never used

d1 equ 20h.. so I don't remember, but if it's a const, how can you change it? I usually use variable in form that

d1 res 1
d2 res x
....

And as I need a preset value, I set in on Initialize fuctions. I do this because I made a standard form for all of my programs. As I more register, I add it to that block.

 

[patricktran]

d1 equ 20 ; means d1 has an address of 20, which is a general purpose reg.
I think I can change it, cant I? Because it is just a general purpose reg, and therefore, it is a var, not const.
In a loop. I noticed:
1. goto <-> 2 instructions cycle
2. decfsz <-> 1 cycle if we dont skip, and 2 cycles if we skip....
Wow, this is so complicated. How come we calculate to get 1 sec delay by usign busy loop? Can someone give a sample code with explanation?
Thanks :?

 

[kings3eed]

Hi kemo
I'm also a student in the faculty of engineering. I need your help in microcontroller specially for smart card projects. I'm also from Egypt (Alex). I think we could be parteners in a very good workshops. We can alter experiences. I wish to have contact with you as soon as possible

Said