changing osc on the fly

[mackrackit]

It is poor engineering practice to operate a device outside of the manufacturer's specs - take a look at the graph above where it shows that the minimum voltage is 4.2V for frequencies >25Mhz.

But I am not an engineer

I look at the graph as showing the primary frequency.
I have not built anything to run on less than 3.3 volts. I do a lot at 3.3 though. Have for some time. Makes working with SD cards and other low power things much easier. One power supply.

[rmteo]

But I am not an engineer

I look at the graph as showing the primary frequency.

Makes sense then.

[Charles Linquis]

If you need to run at 4Mhz on the internal OSC, put this in your code.

ASM
       movlw 0x62                              ; %0110 0010  = 4 Mhz, internal osc block
       movwf OSCCON
       movlw 0x80                              ; %1000 0000  = PLL disabled
       movwf OSCTUNE
ENDASM

I have a development board that runs at 40Mhz on a 10Mhz XTAL + PLL, but sometimes I need to slow things down to 4Mhz and run on the internal oscillator to see how they will run on "little" parts. This code does the trick.

[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.