Post by thread:[PIC]: delay

Hi everyone, I am trying to generate a delay for the 20 MHz PIC16F877 that I am using to interface it to the LCD. I think I don't understand how to generate the delay because I know that my hardware is working perfectly. Can some one please tell me how to calculate the delay in seconds for the following code. Thank you Sarah LOOPD equ 2Ah . . . delay movlw .90 movwf LOOPD repeat nop decfsz LOOPD goto repeat return _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

20 Mhz is 200nS per instruction cycle. You are making 90 passes through four cycle loop. 200nS*90*4=72uS, 72.8 with overhead. On Sat, 3 Feb 2001, Tweedy Bird wrote: {Quote hidden}> Date: Sat, 3 Feb 2001 17:00:36 -0500 > From: Tweedy Bird <spam_OUTkatkoota5TakeThisOuTHOTMAIL.COM> > Reply-To: pic microcontroller discussion list <.....PICLISTKILLspam@spam@MITVMA.MIT.EDU> > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: [PIC]: delay > > Hi everyone, > > I am trying to generate a delay for the 20 MHz PIC16F877 that I > am using to interface it to the LCD. I think I don't understand > how to generate the delay because I know that my hardware is > working perfectly. Can some one please tell me how to calculate > the delay in seconds for the following code. > Thank you > Sarah > > LOOPD equ 2Ah > . > . > . > > delay movlw .90 > movwf LOOPD > repeat nop > decfsz LOOPD > goto repeat > return > > > > > _________________________________________________________________________ > Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

>20 Mhz is 200nS per instruction cycle. You are making 90 passes through >four cycle loop. > >200nS*90*4=72uS, 72.8 with overhead. Actually, the code below would be 72.6us with overhead (the last time through the loop, the 'goto' is only a one-cycle nop). > > delay movlw .90 > > movwf LOOPD > > repeat nop > > decfsz LOOPD > > goto repeat > > return --Andrew _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

Hi! For me, the easiest way to calculate the delay is to write the code into a project I call TEST.PJT (all processor definitions, include files and the command line are already there ). Then I step with F6 to the beginning of the delay loop, reset the stopwatch and run the code through the loop with RUN TO HERE command. Then I simply change start value of say LOOPD and try again until I'm happy with my delay. Works fine when you don't need to be too exact ( few instruction cycles up or down ). How long must your delay be? Regards, Samo {Original Message removed}

> I am trying to generate a delay for the 20 MHz PIC16F877 that I > am using to interface it to the LCD. I think I don't understand > how to generate the delay because I know that my hardware is > working perfectly. Can some one please tell me how to calculate > the delay in seconds for the following code. > Thank you > Sarah > > LOOPD equ 2Ah > . > . > . > > delay movlw .90 > movwf LOOPD > repeat nop > decfsz LOOPD > goto repeat > return I personally don't like busy-waits much. The timing can't be relied on when there are interrupts, and the processor is completely out to lunch during the wait. Especially when the wait is more than a few milliseconds, I prefer to set up a periodic interrupt. 1mS is often a convenient interrupt period. The interrupt routine decrements a counter if it is not already zero. The foreground code sets the counter then checks for it to become zero. Meanwhile, it can be doing other tasks, as long as it gets back to checking the wait counter fast enough. Note that you can support more than one simultaneous wait counter this way, and they can each be clocked at a separate multiple of the interrupt period. I've had projects that simultaneously used 1mS, 10mS, and 100mS wait counters, all derived from the basic 1mS interrupt. ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, .....olinKILLspam.....embedinc.com, http://www.embedinc.com -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

>delay movlw .90 ; 1T > movwf LOOPD ; 1T >repeat nop ; 1T (1T while looping) > decfsz LOOPD ; 2T (1T while looping) > goto repeat ; 0T (2T while looping) > return ; 2T The timing of your code is: T = 7 + 4 * (90 - 1) = 363 T cycles not counting the 2T cycles required by the call. This is 363 usec with a 4MHz crystal. Peter -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

On Sat, 3 Feb 2001, Drew Vassallo wrote: > Date: Sat, 3 Feb 2001 21:59:23 -0500 > From: Drew Vassallo <@spam@snurpleKILLspamHOTMAIL.COM> > Reply-To: pic microcontroller discussion list <KILLspamPICLISTKILLspamMITVMA.MIT.EDU> > To: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU > Subject: Re: [PIC]: delay > > >20 Mhz is 200nS per instruction cycle. You are making 90 passes through > >four cycle loop. > > > >200nS*90*4=72uS, 72.8 with overhead. > > Actually, the code below would be 72.6us with overhead (the last time > through the loop, the 'goto' is only a one-cycle nop). Thanks, I was including the call but forgetting the skip... but what's a couple hundred light-feet? {Quote hidden}> > > delay movlw .90 > > > movwf LOOPD > > > repeat nop > > > decfsz LOOPD > > > goto repeat > > > return > > --Andrew > > _________________________________________________________________ > Get your FREE download of MSN Explorer at http://explorer.msn.com > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spamBeGonelistservspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body