Hello

I have started programming a 20 MHz 16f877 chip (using Hi-Tech C).  I
wish to have an interrupt formed every 1ms.  So I have Timer 1
generating an interrupt.  However the by preloading the timer I can
get an interrupt happening every 985s, if I the loading value.  The
next step I get is a period of 1037us.  Surely I should be able to get
closer to 1ms than this?

Initially I tried using a sample routine that came with the complier,
and while this worked, I was not entirely happy with it so I have
built my own.  But I can't get accuracy high enough, at the moment I
am still 2% out?

The following is my interrupt routine.

if(TMR1IF){
 Count++;
 Scratch        = TMR1H;                        // Getting timer1 value.
 Scratch<<=8;                           // Shifting to most sig half.
 Scratch        = TMR1L + Scratch;
 Scratch        = Scratch + (-2587);    // Adding timer 1 to the scratch
    // pad and then adding the preload value.
    // Vary this number to change interrupt length
 Scratch2 = Scratch;                    // Take a copy, one copy will be
    // used to generate the new TMR1L and the other TMR1H.
 TMR1H  = Scratch>>8;
 Scratch2<<8;
 TMR1L  = Scratch2>>8;          // Cleared top 8 bits and added
    // the remainder in the TMR1L
 TMR1IF = 0;                            // clear flag and start again
}

and the settings I have used.

 TRISB  = 0;            /* all bits output */
 PORTB  = 0;            // to start with anyway
 T1CKPS1        = 0;            //
 T1CKPS0        = 0;            // prescale by 1
 TMR1CS = 0;            // use internal clk
 T1OSCEN        = 0;            // Need C0 & C1 for other stuff

 PEIE   = 1;            // enable peripheral interrupts

 TMR1IE = 1;            // interrupt on overflow
 TMR1IF = 0;            // clear flag to start with

 TMR1ON = 1;            // Start timer
 GIE    = 1;            // away we go


Any suggestions on where I might be able to improve the code to get a
1ms interrupt happening that I can rely on?  I am relatively new to C
and brand new to PICs so all comments are apprieciated.

Thanks
Bryce.

Bryce Reeve
Electronics Designer
DEC International
558 Te Rapa Rd
PO Box 20055
Hamilton
New Zealand

Phone +64 7 849 8755
Fax   +64 7 849 0164

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu