Message Edited by Alban on 2007-10-16 08:26 AM
Hello again,
I got the software running but I could not get your Processor Expert to run, so I wrote a small C-program.
I then put in an IO-pin, PTB1 to go high at the beginning and go low at the end of the interrupt routine.
A second pulse was negated during each interrupt.
PTB0 was added to generate a pulse that was inverted each time the interrupt was executed.
On the oscilloscope photo you can see the the PTB0 freguency at the top.
It is about 28.25 kHz and PTB1 at the bottom toggling at 52.57 kHz.
Yes you are right, there is a quite large amount of jitter , < 5 uS or 5 uS* 28.25 kHz = 14 %, on the interrupt timing which shows on the top and the bottom traces.
This is jitter only, there is not long time error introduced.
I think that the reason is because the Timebase is built by a ripple-though counter which gives the jitter. See MC68HC908GR8, Figure 21-1.
The counter is not guaranteed to run at > 32 kHz, See section 21.2, which is far below the 2.3 MHz the ICS08GR is running at.
The error would probalby be reduced to 32 kHz/2.3 MHz * 14% = 0.2 %.
But it is not possible to get a 32 kHz clock that runs so 28 kHz can be measured. The fastest TBM clock is 4.096 kHz.
Regards,
Ake