Question for Mr. Darrel Taylor and Mister-e Timer1 Reload

[richard]

if using the appropriate reload and preload as calculated is not keeping good time then it means you xtal frequency is slightly off (as pedja089 suggested 1 to 5 % error is to be expected) , you might try trimming the capacitors on the crystal (might take a bit of experimenting if you haven't got a frequency counter) , changing the preload + or minus 1 will probably be too coarse an adjustment . you need to keep in mind that xtal ocillators aren't atomic clocks and that the osc frequency will be temperature dependant and will drift over time .
another possibility is adding or subtracting the "error" say once per day, this was used in a slicon chip magazine project (pic toc )many years ago and was suprisingly good at time keeping .

[mikebar]

Ok, thank you Richard.
I'll try to get a freq meter and see what exactly going to the oscillator.

The chip I'm using is PIC 18F4685. What about to connect a 32768Hz watch quartz in order to feed the Timer1?
I've had no experience yet with this approach so I don't know yet what exactly I'm talking about.
All I need is to interrupt each exactly 100ms. It is possible to have that using the Timer1 feeded by an external source? And if so, using what setup?

Thanks in advance.
Regards,
Mike.

[pedja089]

10Hz interrupt is problem to derive from 32768Hz. 16Hz or 32Hz etc isn't problem. To get 1sec just count 16 interrupts, and that is all.
I posted RTC code, with external crystal, 32 interrupts per second, and it consumes less than 10uA if you put pic in sleep mode.

[mikebar]

NICE!
Where I can see that code?

Thank you.

[pedja089]

I can't find that topic...
Here is cut/paste from my code for PIC18LF14K50

        DEFINE  OSC 4         
        Define  INTHAND myint    ' Define interrupt handler
        DEFINE  NO_CLRWDT 1

        OSCCON=%01010110
        OSCCON2.2=0
        OSCTUNE.7=1
        OSCTUNE.6=1
        REFCON0=0
        REFCON1=0
        ADCON1=0
        ADCON2=0
        ANSEL =0           
        ANSELH=0
        CM1CON0=0
        CM2CON0=0
        SRCON0=0
        SRCON1=0
        WPUA=0
        WPUB=0
        WDTCON=0   
        SLRCON=0           
        RCON.7=0
        RCON.6=0   
        UCON=0  
        IOCA=0
        IOCB=0
        T0CON=0
        T1CON=%00001111  
        T2CON=0
        T3CON=%00000110
        INTCON=192  
        INTCON2=0  
        INTCON3=0 
        PIR1=0
        PIR2=0
        PIE1=1
        PIE2=0
        IPR1=0
        IPR2=0
        porta=0
        portb=0
        portc=0
        trisa=255
        trisb=255
        trisc=255




'RTC_________________________
Seconds        VAR BYTE bankA  
Minutes        VAR Byte bankA       
Hours          var byte bankA
   



 
 clear

GoTo OverInt  ' jump over the interrupt handler and sub
ASM                  
myint 
 movlb .0
 ;If PIR1.0 =1 then
 BTFSS PIR1,0 
 GOTO End_RTC 
 MOVLW .252 ; TMR1H=252
 MOVWF TMR1H
 CLRF PIR1 ; Clear TMR1 int flag
 INCF _Ticks,F 
 ;if ticks>31 then
 MOVLW .31
 CPFSGT _Ticks  
 GOTO End_RTC   
    INCF _Seconds,F  
    CLRF _Ticks
    ; if seconds > 59 then
    MOVLW .59         
    CPFSGT _Seconds   
    GOTO End_RTC   
       CLRF _Seconds   
       INCF _Minutes,F
       ;if minutes > 59 then
       MOVLW .59         
       CPFSGT _Minutes   
       GOTO End_RTC      
          CLRF _Minutes
          INCF _Hours,F
          ;if hours > 23 then 
          MOVLW .23       
          CPFSGT _Hours   
          GOTO End_RTC    
              CLRF _Hours
End_RTC                                      
;Do your other stuff here
 retfie  FAST  
ENDASM  
OverInt:

Main:
@ Sleep
'program starts here 32 times in sec
goto Main

This is example for PIC's that have fast return stack, and saves and restores WREG, STATUS and BSR registers...

[mikebar]

Thank you a lot.

[pedja089]

Just one more thing that I had to deal with...
Selection of crystal and capacitors from OSC pins to ground is critical to get accurate clock. I tried one by one from 10 to 47pF, and best results are with 47pF. But that will depend from board layout and used crystal.
My suggestion is that you buy a lot of crystals, and select appropriate capacitor for it. And use that combination from now.