Timer1 not running on PIC16F628

Question

I have this little project that requires exact clock so I'm using 32768 Hz quartz on RB6 & RB7 on my PIC16F628. Both pins are grounded using 33 pF capacitors.

My code that configures the timer1 is as follows (HI-TECH C compiler):

TRISA = 0b11100011; //x3F;      // bit=0 - output; bit=1 - input
TRISB = 0b11110000; //x3F;      // bit=0 - output; bit=1 - input
nRBPU = 1; //affects inputs only
TMR1ON = 0;     // Disable Timer 1
T1CKPS1 = T1CKPS0 = 0;   // 1:1 Prescale value
T1OSCEN = 1; //start tmr1 oscillator
TMR1CS = 1;  //0: internal clock; 1: wait external clock
nT1SYNC = 0; //sync by external clock
TMR1 = 0x8000;
PEIE = 1;
TMR1IE = 1;
ei(); //sets GIE
//T1CON = 0b00001111;
TMR1ON = 1;     // Enable Timer 1

The config bits:

__CONFIG(WDTE_OFF & MCLRE_ON & PWRTE_ON & BOREN_OFF & LVP_ON & FOSC_INTOSCIO & CP_OFF & CPD_OFF);

The problem - it never runs. I have an LED in the ISR that should be lighted up. Also I used an oscilloscope to investigate. Changed the capacitors. Tried another crystal. Tried all possible approaches. Nothing helped.

Answer 1

The directive FOSC_INTOSCIO means that the part is using its internal oscillator, and the lines normally used for the crystal are set as I/O lines. That's why your crystal isn't oscillating.

This device has an internal oscillator frequency of approximately 4MHz, so it's quite possible that your ISR is executing faster than your eye can perceive the LED flashing.

You will want to use something like LP_OSC instead of FOSC_INTOSCIO (I don't have that exact header file, so I cannot tell you which exact one to use).

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Further to the comment below: according to the datasheet, the choices for the timer clock source are: an external clock from RB6 or the internal oscillator / 4. What this means is that you can use the internal 4 MHz oscillator for the main clock, and bring a clock signal to RB6 for the timer. However, since you're not using the built-in oscillator hardware, you'll have to externally generate a clock from the crystal and apply it to RB6.

Luckily Microchip explains how in the datasheet: