PIC Microcontoller Delay Method

General Delay Methods

James Cameron says:

These PICs are just too fast, we often need a way to have the processor wait around for a while. A series of NOP (no operation) instructions is straightforward, but wasteful of instruction memory. A counted loop is the next common trick, because that lets us tune it.

But what are some of the more exotic ways to delay?

1. ) The Common NOP, a delay for one instruction cycle

       DELAY   NOP                     ; delay one cycle
   ...
       DELAY4 ; delay 4 cycles
           NOP
           NOP
           NOP
           NOP
   
   

   Advantages: extremely simple, obvious, maintainable, scaleable.
   Disadvantages: is it really there for a reason? For long delays, uses a lot of instruction memory.

2. ) The Common Loop, three times the initial value plus three

           DELAY   MOVLW   D'95'           ; 288 cycle delay
                   MOVWF   COUNTER
                   DECFSZ  COUNTER,F
                   GOTO    $-1
   

   Advantages: fairly simple, maintainable, scaleable up to 771 cycles; after that go for a nested loop.
   Disadvantages: costs one file register; though there is a variant using just the W register.
   

3. ) The Novel GOTO, two instruction cycles in one instruction

                   GOTO   $+1              ; two cycle delay
   

   Advantages: half the space of two NOPs.
   Disadvantages: obscure unless commented.
   

4. ) The CALL to Nowhere, four instruction cycles

                   ORG     0
                   GOTO    MAIN
           FOUR    RETURN                  ; four cycle delay function
                   [...]
           DELAY   CALL    FOUR
   

   Advantages: quarter the space of four NOPs, the RETURN can be reused by other code, good use for those three bytes between the reset vector and the interrupt vector on a PIC 16F84.
   Disadvantages: implementation separate from use, can look odd, uses one stack level.

   Scott Dattalo says:

   ...if you want a 4-cycle single-instruction delay: call some_return_in_your_code Of course, you run the risk of stack overflow on [some processors]
   

5. ) The Double Call to Nowhere, eight or four cycles

                   ORG     0
                   GOTO    MAIN
           EIGHT   CALL    FOUR
           FOUR    RETURN
                   [...]
           DELAY   CALL    EIGHT
   

   Advantages: looks simple, allows various size delays to be rapidly constructed during prototyping.
   Disadvantages: uses two stack levels.

   [ed: See the "Stack Recursive Delay" below]
   

6. ) The Do Something Useful Extension to the Common Loop, five times the initial value plus three

           DELAY   MOVLW   D'95'
                   MOVWF   COUNTER
   
                   MOVF    TRISM           ; fetch TRIS mirror
                   TRIS    TRISB           ; reapply it
                   DECFSZ  COUNTER,F
                   GOTO    $-3
   

   Advantages: good for precise delays that are no multiples of three, allows useful functionality to be placed within the delay.
   Disadvantages: increased convolution of code, lower maintainability.
   

7. ) The Long Delay Using Timer, more of a technique than a code fragment, set the timer, wait for it to roll over.

   Advantages: immune to distortion by interrupts, easily scaled using a prescaler, even possible to tune the delay by modifying the preload value.
   Disadvantages: allocates a timer.

   Paul B. Webster says:

   I feel that many applications (real-time control, clocks) resolve to a fundamental "tick" or hierarchy thereof, often around a millisecond, which method {7} provides. Counting a thousand of these gives a second, at which point a train of countdowns (semaphores) can lead to various housekeeping actions i.e., if T1 then { T1--; if T1 == 0 then action1 };

   On the 1 ms "ticks" also, a debounce uses a counter which counts down from 20 (20 ms) to verify a keypress/ release. Other countdowns in ms are used for playing tunes or "tick", "Click" or "blip" noises.

   It is highly undesirable to pre-load or fiddle with the TMR0 when you are using it this way, firstly because it interferes with the prescaler in a very inconvenient fashion, and secondly as the 1 ms countdowns can be used for a 500 Hz tone while the TMR0 MSB can be copied to a port for a 1 kHz tone, bit 6 for a 2 kHz tone, etc., controlled as above in even numbers of milliseconds.

   You can similarly, wait on those individual bits by polling, for sub- delays. A sub-delay on bit 3 toggling could be used to time a phase accumulator (32 kHz clock) to generate quite a complete range of square wave tones.

   So, you may say that method "uses" a timer-counter, but I submit that in a well-designed application, you get an awful lot of "use" out of it!
   

8. ) The Watchdog Delay, go to sleep and get woken by the watchdog.

   Advantages: Uses the least amount of energy (best when running off batteries). Extremely simple technique. Can be varied by changing the WDT prescaler ratio.
   Disadvantages: difficult to calibrate.
   
   [ed: Set up a type of state machine that records where the processor should contine executing code after the reset. Make sure you clear it in your non-timed routines. Also see Using the watchdog timer to sense temperature]
   

9. ) The Data EEPROM Delay, a typically 10ms delay that can be triggered by writing to data EEPROM and waiting for the interrupt.

   Advantages: tests the endurance of the EEPROM.
   Disadvantages: tests the endurance of the EEPROM.

See also:

http://www.piclist.com/techref/microchip/seepicsrc/psbpix/pause.htm ?

Code:

• delay type (2) mentions a ``nested loop''. Here's some example code for that:

  ;================================================================
  ;= waitXX100msec -> long time delay routine (with clrwdt!)      =
  ; Usage:
  ;    movlw 5 ; bigger values (1..255) delay longer, except 0 is
  max delay (256).
  ;    call waitXX100msec ; delay 4 + w*773 instruction cycles more
  than NOP.
  ; Change log:
  ; 2001-06-05:DAV: David Cary: minor modifications.
  ; 2001-04-04:RB: posted by Roman Black on 2001-04-04 04:06:30 AM
  ;================================================================
  waitXX100msec   movwf   count_outer       ; store the waiting
  time! (msb)
  
  loop_outer:
                  clrf    count_inner		; prepare lsb
  
  loop_inner:
                  decfsz  count_inner       ; dec and loop 256 times
                  goto    loop_inner        ; 1+2 inst x 256 (delay
  period)
  
                  clrwdt                    ; give the dog a bone
                  decfsz  count_outer       ; dec outer loop counter
                  goto    loop_outer        ; 1+1+2 inst x value of
  count_outer
                                            ; (delay period)
  
                  retlw   0                 ; return
  ;================================================================
  ; You can add a third (or fourth) loop with no effort, and
  ; for varying timings you can always load the inner loop
  ; vars with a value instead of clearing them (which=256).
  ;
  ; My personal pref with stepper motors is to set the inner
  ; loop value so that when initial w==256 I get the slowest
  ; speed I need. You then get the best speed range possible
  ; just by varying the initial w byte.
  ; -- Roman Black
  

Questions:

• spamnatalia_coria13 at ~NOSPAM~yahoo.com.mx asks:

  if i need to use 3 or 4 nested loops for a 5 secs delay (for example) how would I know which value (0-255) use for each of the decfsz?? Is there a formula or something ??
  thanks

  +

Comments:

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