External oscillator on a PIC16F628A MCU question…

[mbu]

I’m a little lost; maybe some would take the time to help me.

I have a circuit that uses an external 10Mhz crystal oscillator on a PIC16F628 MCU. So if I want 1 second of real clock time do I count 10,000,000 oscillations or is it some fraction of this?

 

[Russlk]

Obviously, at 10 million oscillations per second, you count 10 million of them. If I were doing it, I would use PICBASIC and would have to calculate the difference between 4 mHz oscillations and 10 mHz.

 

[mbu]

Oops! guess I should have worded it like this:

On a PIC timer0 how many ticks/second will a 10Mhz oscillator have? Since the MCU takes 4 cycles an instruction, I was wondering if I need to divide the 10Mhz oscillator by 4 to obtain the timer0 ticks per second.

 

[SPDCHK]

Some helpful tools to help you generate PIC Delays...



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[gramo]

The speed in which a PIC will operate is a division of 4 of the crystal, and TMR0 will increment each instruction cycle depending on the pre-scaler, and a TMR0 overflow interrupt will occur every time TMR0 overflows from 255 to 0.

So a 10Mhz crystal will have the PIC operating at 2.5Mhz. This means that every second, 2.5 million instructions are executed.

The TMR0 interrupt (with a prescaler of 1:2) will occur every 512 cycles, so 2.5Mhz / 512 = 4882.8125 interrupts a second (or 0.0002048 seconds = 204.8uS)

If you change the Pre-Scaler too 1:4 then an interrupt will occur once every 1024 Cycles, so 2.5Mhz / 1024 = 2441.40625 interrupts a second (or 0.0004096 seconds = 409.6uS)

 

[mbu]

Thanks gramo!