Ruth Hendrix

Program Running -- PLL

Discussion created by Ruth Hendrix Employee on Jan 26, 2006

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Posted: Dec 7, 2005, 11:48 a.m.

 

The program seems to run reliably only if the pll is in manual mode.

 

If in auto, does not always run or takes a minute or 2 to start.

 

Any ideas?

 

Posted: Dec 7, 2005, 1:53 p.m.

 

Not sure about your uP, but if it is a 32khz xtal with PLL like the 908GP32 then here is my experience :

 

Slow start up (ie. > 0.5 second) is caused by not enought loop gain in the osc circuit, caused by any combitation of:

 

1) too low a feedback resistor (usually 10M or 4M7) any less than 4M7 will cause problems

2) too large a value capacitors on the xtal. I have the odd trouble with 27pF caps (4M7 resistor) on each side on the xtal, and have recently experimented any found a 12pF/ 22pF combination the best.

3) too low a value caps. ( 10pF seems to be the minimum)

4) the series resistor is too small, causing excessive xtal vibrations. (I use 330K as recommended by others )

 

The caps values are small, so will depend on the board layout and type (single/double sided). I am using single sided, uP in a DIP socket.

 

There is a range of values which allow the xtal to oscillate, (looks great on the 'scope) but the level is just not enought to lock the PLL, or worst still it locks to (what appears to be) about 1/2 frequency.

 

Posted: Dec 7, 2005, 2:43 p.m.

 

Thanks for the info. I am using 22M and 22pf caps. Not using series resistor. Is series resistor really needed? uP is ap32.

 

I am seeing osc running, but not program. Right now, layout is breadboard.

 

I had already changed resistor and caps from 1M and 33pf, which did not allow osc to run.

 

Posted: Dec 7, 2005, 3:08 p.m.

 

> Thanks for the info. I am using 22M and 22pf caps.

> Not using series resistor. Is series resistor really needed?

> uP is ap32.

 

With a 32Khz tuning fork type of crystal, the series resistor is definitely needed. Usually without one the crystal is destroyed as soon as oscillation starts. I'm surprised yours shows any oscillations at all.

 

Posted: Dec 7, 2005, 3:33 p.m.

 

There is an errata sheet on this.

 

http://www.freescale.com/files/microcontrollers/doc/errata/DE0001.pdf

 

I would start with the values in this document.

 

I am using 10M, 330K, and 15pf successfully in my product using the AP64.

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