We are using the MC9S12DT256. Normally it is has a colpit clock circuit, which is working without any problems at 16MHz.
Now we need to have a large clock signal amplitde to synchronize it with other ICs on the same board. So I want to run it in Pierce-clock mode. So I changed PE7/XCLKS_not to ground and placed two capacitance from XTAL and EXTAL to ground (15pF) and parallel to the Crystal a resistor of 3MOhm. And I removed the series capacitance.
But now the processor is runing on a low clock frequency (internal clock), and I measure no clock signal on EXTAL and XTAL pins. I also see no DC voltage on these pins (VDDPLL is above 2V).
So my question is:
- Is our processor able to run in Pierce mode?
- Do I need to change more than the clock circuit, and pull-up/down resistor connected to PE7? (some code perhaps?)
- Should there be a DC voltage on the EXTAL and XTAL pins?