changing osc on the fly

[Art]

I was thinking of a routine that could detect the clock speed the pic is running at
so that then the pic program could adjust serial speed, etc accordingly.

Make a delay loop in picbasic and count the number of ticks a hardware timer
makes during the delay loop.. and then the hardware timer value will be
different for different clock speeds.

[rmteo]

Wouldn't a delay need to know clock speed to work correctly?

[Dave]

Art, What would you use as a reference in that situation?

Dave Purola,
N8NTA

[Bruce]

If you have a PIC that supports Internal/External Oscillator Switchover then you shouldn't need a firmware solution to determine osc speed. You already know what it should be after you write to OSCCON & flip the System Clock Select bit.

If you have one running on the internal oscillator, and you need to calibrate it, then Microchip has a few app notes for this using TMR1 with an external 32.768kHz watch type crystal. Or you could just output the internal clock on OSC2/CLKO and test it with an O-scope.

[Art]

Wouldn't a delay need to know clock speed to work correctly?

No, because you use a timer based on RC clock, which is always the same,
but the delay is dependent on osc speed.

So, start timer, execute delay, read timer.

The fact that the delay varies is what I'm counting on.

[rmteo]

Lets say you have a 4MHz clock and you set a delay of 1mS (1000us). A timer (without pre/post scaler) will count to 1000 for the duration of the delay.

Now drop the clock the 1MHz. The timer will now count to 250 (1000/4) each mS. However, the delay will increase to 4mS due to the slower clock, so the timer will again count to 1000.

[Art]

On the 16F877/876 for example, Timer0 can increment on the rising or falling edge of pin RA4/TOCKI
That way you can use an RC osc to control TIMER0 so varying the OSC for the microcontroller will
change the speed of the pause delay, but not the speed of TIMER0.
Art.

[rmteo]

On the 16F877/876 for example, Timer0 can increment on the rising or falling edge of pin RA4/TOCKI
That way you can use an RC osc to control TIMER0 so varying the OSC for the microcontroller will
change the speed of the pause delay, but not the speed of TIMER0.
Art.

That is a little different from this.

Make a delay loop in picbasic and count the number of ticks a hardware timer
makes during the delay loop.. and then the hardware timer value will be
different for different clock speeds.

[Charles Linquis]

In some cases, you can read the configuration bits to determine if you are running on the INT OSC, and whether or not any dividers or PLLs are in use. If you have any kind of external oscillator or RS-232 input, you can figure things out pretty easily.

[Art]

That is a little different from this.

and then the hardware timer value will be
different for different clock speeds.

The hardware timer value you read back after the pause routine will be different for different clock speeds (of the microcontroller osc).