Andrea Pastega

32.789 kHz crystal with HCS08DZ60

Discussion created by Andrea Pastega on Aug 8, 2007
Latest reply on Aug 27, 2007 by Andrea Pastega
I'm working over a project employing a HCS08DZ60. We use a crystal oscillator with 32.789 kHz frequency, and then internally I multiply this frequency by 1024 with FLL. I connected this crystal with two 22p cap over ground, a 10M res in parallel to the crystal, and a 100k from XTAL and one of the crystal pins, with the classic pierce configuration.
In the micro I configured for external crystal oscillator, low power mode, and enabled bus clock output on PTA0 for debugging purposes.
The oscillator starts with no problem, but the uP works bad and continues to reset itself.
What I see is that the frequency on PTA0 is sometime stuttering, and when it happens, the uP restarts.
What is really strange is that it does in this way also if I disabled clock monitor feature.
I tried to change a little the configuration by changing the series resistor to 330k and to 47k, with no change.
I tried also to disconnect the crystal and connect in place an external generator coupled with an 10n capacitor on pin EXTAL (input of the uP), giving 1Vpp and a frequency of 32.789 kHz, or other frequencies.
What happens is the same, but also the stability seems to be better if the frequency is around 34 kHz or so, but it could be just a case.
I tried to increase also the level of the signal, with no difference at all.
I would exclude in this way that it is a matter of noise, unstable oscillator or wrong external components, because I used a very stable R&S generator, and in any case the crystal oscillator alone works good.
I tried to switch to internal oscillator, in this way all works good.
It seems to be a problem related to the FLL, I could also suppose that something happens when you use a frequency that in some way is near the internal generated frequency.
Any idea to solve this problem?