I need help to understand this delay code !

[Ghalebi]

Salam everyone,
I am using a PIC16F84A with a 4 MHz oscillator for some purpose .. and someone fed me up with the following code in order to develop a 5 seconds delay ..

;5 second delay.
DELAY5 
       CLRF TMR0                       ;START TMR0.
LOOPB  
       MOVF TMR0,W                  ;READ TMR0 INTO W.
       SUBLW .160                     ;TIME - 160
       BTFSS STATUS,ZEROBIT  ; Check the diffrence 
       GOTO LOOPB                   ; Difference aint ZERO
       RETLW 0                          ;Time is 160, return.

He supposed that the frequency of the oscillator is 32 KHz .. I need to understand how did he make a relation between his assumed frequency and the number 160 to get the 5 seconds delay ..

The TMR0 is set to 1:256 ..

Any kind of help is appriciated .. thanks in advance

 

[Ghalebi]

Please help ..

 

[jankop]

Internal Fosc clock is 32 768Hz, prescaler 1:256

MCU clock Fosc/4 = 8192Hz

Output from prescaler is 8192Hz / 256 = 32Hz

Period is T = 1 / f = 1 / 32Hz = 0,03125s

Delay is T x 160 = 0,03125s x 160 = 5s

This code fragment is bad, test for Zero is'nt advisable.

 

[Ghalebi]

Thanx a lot man, so clear ..

but u didnt like the fragment, could u give ur suggestion to improve it ..

Many Thanks

 

[jankop]

Here is code:

;5 second delay, Fosc 32 768Hz 
DELAY5 
       CLRF TMR0                      ; Clear TMR0. 
LOOPB  
       MOVF TMR0,W                  ; READ TMR0 INTO W. 
       SUBLW .160                     ; TIME - 160 
       BTFSS STATUS,C             ; Check not borrow
       GOTO LOOPB                   ; 
       RETLW 0                         ; Time is 160, return.

It is workable with low prescaler factor too, without infinite loop.

 

[Ghalebi]

thanks again sir,
but whats the difference with checking the Carry bit instead of Zero bit !!

and what about if i am working on Fosc = 4 MHz ..

Regards

 

[jankop]

;--------- VARIABLE DEFINITIONS
#define RAM_Start 0x20
    CBLOCK RAM_Start
counterLo
counterMi
counterHi
    ENDC
;----------------------------------------------
; Fosc = 4MHz, delay 4,999994s
;----------------------------------------------
 delay5                 ;2  (from call)
    movlw 0x70          ;3
    movwf counterLo     ;4
    movlw 0x10          ;5
    movwf counterMi     ;6
    movlw 0xF9          ;7
    movwf counterHi     ;8
loop
    incf counterLo,F    ;   1        
    btfsc STATUS,Z      ;   3     
    incf counterMi,F    ;   4     
    btfsc STATUS,Z      ;   6
    incf counterHi      ;   7
    btfss STATUS,Z      ;   9
    goto loop           ;   11
    retlw 0x00          ;10