Xtal question - MC9S08GT16A

[Kerms]

This may seem like a silly question, but I am looking at the example layout of a MC9S08GT16A and I see that the xtal has an RF and an RS

What purpose do those resistors play and are they really necessary? If so what value of RS and RF do you recommend?

Added p/n to subject.

Message Edited by NLFSJ on 2007-12-03 02:48 AM

[Kerms]

Thanks Mac, I had seen the 1M resistor before on an ST chip, but found that it wasn't needed.

Thank you for clarifying.

Added p/n to subject.

Message Edited by NLFSJ on 2007-12-03 02:49 AM

[peg]

Hi,
Interestingly, while Rs (~100k) is normally used with a 32kHz xtal on these devices, it is zero ohms in the oscillator circuit of the DEMOGB60 board.

The details of this can be seen in section A.9 of the datasheet.



Message Edited by peg on 2007-12-03 10:47 AM

[bigmac]

Hello Peg,

With a Vdd of 3.3V, there will be lower drive to the crystal, compared with Vdd of 5V.  This may justify the omission of Rs for your demo board.  It would be interesting to observe how much the drive voltage to the crystal circuit is actuallly reduced with the inclusion of a 100k resistor.

Regards,

Mac

Added p/n to subject.

Message Edited by NLFSJ on 2007-12-03 02:50 AM

[bigmac]

Hello,

The resistor Rf is necessary to bias the oscillator into the linear operating region, required during start-up of the oscillator.  The value is usually non-critical, somewhere between 1M and 10M.

The resistor Rs is sometimes required to limit the drive applied to the crystal, particularly with low frequency (32kHz) crystals, with a value of typically 47k or 100k.  It is not usually required for higher frequency crystals, and should be a short circuit in these cases.

Regards,

Mac