topic Activating The External Oscillator on QG8 in 8-bit Microcontrollers

Activating The External Oscillator on QG8

Davo — Sun, 29 Apr 2007 18:20:53 GMT

I am trying to get a 32.768kHz watch crystal to oscillate on a QG8. However despite trying various ICS register options I have had no luck - the chip continues to run on the internal oscillator. I know this as I can trim the oscillator and see a frequency change on a port I am using as a freq-out pin.

Here is the code I am using. I have tried toggling HGO on and off as well. I am using FFL Engaged External (FEE) mode.

lda   #%00000110    ;Set EREFS and ERCLKEN
  sta   rICSC2
  lda   #%00000000
  sta   rICSC1

With a 32,768Hz crystal this equals a bus clock of 8,388,608 Hz.

Advice appreciated.

Davo

Re: Activating The External Oscillator on QG8

tonyp — Sun, 29 Apr 2007 20:14:00 GMT

It could be because you're not waiting for the oscillator to start (BRCLR instruction) that the switch isn't made.

I use this code and it works:

mov       #%00000110,ICSC2
brclr     OSCINIT.,ICSSC,*
mov       #%00000000,ICSC1

Re: Activating The External Oscillator on QG8

Davo — Mon, 30 Apr 2007 19:47:44 GMT

Tony, thanks for the advice. This works fine.

Should I be in High Gain mode or Low Gain mode for a 32.768kHz crystal ?

Re: Activating The External Oscillator on QG8

tonyp — Mon, 30 Apr 2007 20:56:46 GMT

That depends on what drive level your crystal requires. In my case, it works as it was given to you (with HGO=0) which consumes less power. I wouldn't set it to high-gain if I get a reliable oscillation otherwise.

Re: Activating The External Oscillator on QG8

Alban — Mon, 30 Apr 2007 22:22:04 GMT

Hello,

32kHz crystals tend to oscillate easily and don't need the high gain.
Above 1MHz the story would differ and I would go for high gain.
Above 4MHz, I would not even wonder and switch the high gain straight away.

I though the datasheet would mention something like this.
Anyway, the user should see the stability of his/her oscillator and act accordingly.
Oscillators are not really part of the microcontroller part, but part of the analog world.

Alban.