Elapsed Timer findings

[Demon]

...For the 20MHz example where the constant is 03B7h, it's the reverse of this:

FFFFh - 03B7h = C348h (49992 decimal),
49992 + 8 = 50000 (it must take eight instruction cycles to reload the timer)
50000 / 10000 = 5
5 x 4 = 20MHz

... Art.

For the next guy that needs to incorporate the Prescaler into Art's formula (like when you want to calculate Preload for faster oscillators):

65535 - (((( MHz / Prescaler ) / 4 ) x 10000 ) - 8 )

Then set T1CON prescaler accordingly.

[richard]

did you see this

'***************************************************************************
'*  Name    : Elapsed_INT-18.bas                                           *
'*  Author  : Darrel Taylor                                                *
'*  Date    : JUL 11, 2006 : 7/11/2010                                     *
'*  Version : 1.2                                                          *
'*  Notes   : Must have DT_INTS-18.bas loaded first                        *
'*   ver 1.2: Now works at any OSC frequency without using the prescaler   *
'***************************************************************************

the pic16 ver can work up to and including 32MHz without the need of a prescaler if necessary

'****************************************************************
'*  Name    : Elapsed_INT.bas                                   *
'*  Author  : Darrel Taylor                                     *
'*  Date    : Jan 10, 2006                                      *
'*  Notes   : Must have DT_INTS-??.bas loaded first             *
'****************************************************************
DISABLE DEBUG
; syntax =     Handler  IntSource,        Label, Type, ResetFlag?
DEFINE  Elapsed_Handler  TMR1_INT,  _ClockCount,  PBP,  yes
; the above define can be used in the INT_LIST macro, if desired (optional)


Ticks    var byte   ' 1/100th of a second
Seconds  var byte
Minutes  var byte
Hours    var byte
Days     var word


SecondsChanged   var bit
MinutesChanged   var bit
HoursChanged     var bit
DaysChanged      var bit
SecondsChanged = 1
MinutesChanged = 1


Goto OverElapsed


' ------------------------------------------------------------------------------
' To calculate a constant for a different crystal frequency - see this web page
' http://www.picbasic.co.uk/forum/showthread.php?t=2031
' ------------------------------------------------------------------------------
Asm
  IF OSC == 4                       ; Constants for 100hz interrupt from Timer1
TimerConst = 0D8F7h                 ; Executed at compile time only
  EndIF
  If OSC == 8
TimerConst = 0B1E7h
  EndIF
  If OSC == 10
TimerConst = 09E5Fh
  EndIF
  If OSC == 16
TimerConst = 063C7h
  EndIF
  If OSC == 20
TimerConst = 03CB7h
  EndIF
  If OSC == 32
TimerConst = 25539
  EndIF

[Demon]

Nope. The one I just downloaded today (just to be certain) only went up to 20 MHz.

  IF OSC == 4                       ; Constants for 100hz interrupt from Timer1
TimerConst = 0D8F7h                 ; Executed at compile time only
  EndIF
  If OSC == 8
TimerConst = 0B1E7h
  EndIF
  If OSC == 10
TimerConst = 09E5Fh
  EndIF
  If OSC == 16
TimerConst = 063C7h
  EndIF
  If OSC == 20
TimerConst = 03CB7h
  EndIF

Can you get to that number using Art's formula?

The link in the comments is dead.

[Demon]

Maybe I have obsolete includes. These are the versions I have:

'***************************************************************************
'*  Name    : ReEnterPBP.pbp                                               *
'*  Author  : Darrel Taylor                                                *
'*  Version : 3.4  (8/13/2010)                                             *
'*  Notes   : Allows re-entry to PBP from an ASM interrupt                 *
'*          : Must have DT_INTS-14.bas loaded first                        *
'***************************************************************************
'*  ver 3.4 (8/13/2010)                                                    *
'*           Changed Restore to use 1 less stack level                     * 
'*  ver 3.3 (1/16/2010)                                                    *
'*           compatability with PIC16F1 chips                              *
'*  ver 3.2 
'*           Bug Fix, RS1/RS2 were being restored incorrectly              *
'***************************************************************************

'***************************************************************************
'*  Name    : DT_INTS-14.bas                                               *
'*  Author  : Darrel Taylor                                                *
'*  Version : 1.10 (8/13/2010)                                             *
'*  Date    : OCT 13, 2009                                                 *
'***************************************************************************
'* REV 1.10  Fixes Duplicate label error when Handlers cross page boundary *
'*           Fixes error with 16F1's and MPLAB 8.53 (high)                 *
'* REV 1.00  Completely re-written, with optimization and F1 chips in mind *
'* REV 0.93  Fixed CMIF and EEIF problem with older PIC's                  *
'*           that have the Flags in PIR1 instead of PIR2                   *
'* Rev 0.92  solves a "Missed Interrupt" and                               *
'*           banking switching problem                                     *
'***************************************************************************

I have the same version of Elapsed_INT.bas as you.

[richard]

Can you get to that number using Art's formula?

nope

looks like i snuck in 2:1 prescale with nill comments or anything

picmulticalc says it all




i don't know where the pic18 vers came from , its not something i would ever use these days

[Demon]

I'm happy you shared that image. I have the Interrupt Frequency at 10 usec, not 10 msec. I suppose that could have just been a default value.

I'm using the timer to count Ticks between encoder pulses. I control when the Elapsed Timer starts and stops, so do I really want such a fast interrupt that probably will take resources from other operations?

I'm assuming that the Interrupt Frequency triggers code in the background.