David Frech

Using external xtal osc for bus clock, but keeping internal ref/FLL for BDM (ICS on S08QG)

Discussion created by David Frech on Sep 27, 2009
Latest reply on Sep 28, 2009 by David Frech

I'm a bit confused about the options available with the ICS on the QG. I want to do something that seems simple and obvious: run the bus clock (ICSOUT) *directly* from the external crystal oscillator, using a crystal > 5MHz (11.0952M to be exact - I want to generate 115,200 baud accurately), but *leave* the internal reference clock running and driving the FLL, so the default (~8M) BDM clock is available.

 

It would *seem* this is possible, since the bits that select ICSOUT (CLKS in ICSC1) and the bit that selects the internal reference source (IREFS in ICSC1) are *separate* ... but when I try to switch CLKS to the external clock, leaving IREFS set to 1, BDM goes haywire. (I can get bytes in and out, but they are garbage.)

 

It worries me a bit that this isn't one of the sanctioned, canonical modes of the ICS. It's *like* FBE, but the descriptions of FBE all seem to say that I have to choose the external reference as the source for the FLL - but I can't do this, since it's >5M.

 

I'm confused. Am I missing something?

 

If I had a <5M xtal on hand that I could use - one that worked nicely with 115200 - I would use it, but the only possibility is the 11.0592. I may also explore trimming the internal clock to 18.432M (bus clock of 9.216M), but I'd like to understand the ICS better.

 

Anyone have any experience to share?

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