Dave, Naga and I worked on the GFE this evening.

On the goodish news side, we found out that the CAN hardware seems to be
working. On the bad-ish side, we couldn't get the bit timing right for
whatever reason. We're not sure if it's the firmware or the oscillator,
although the oscillator seems pretty much right on.

The big disappointing news of the day is that LPC's crystal oscillator
doesn't behave how we thought. What it should do, like every other
oscillator silicon on a microcontroller running at 3.3 V, is bang the
crystal between 0 and 3.3V. What it does instead is drive the 12 MHz
crystal in a beautiful sine wave from 160 mV to 1.6 V.

Of course, after a lot of head scratching, this makes a lot of since
since the LPC23xx series has a 1.8 V internal core.

What this means is that the 74LVC163A binary counter never, ever
triggers. The counter is supposed to take the 12 MHz crystal output and
divide it down to 1.5 MHz in order to synchronize the switching power
supplies... but the Vih (voltage input, high) rating is 2.3 V minimum,
which is well above the 1.6 V crystal waveform.


So we have some options here:

1. Get the LPC to output a 12 MHz clock (Timer? PWM? SPI?) on some GPIO
line and feed that into the 74LVC163A.

2. Get the LPC to output a 1.5 MHz clock (Timer? PWM? SPI?), bypass the
counter chip completely, and feed that into the SPS chips.

3. Hack in some sort of high speed comparator that converts the 1.6 V
sine wave into a 3.3 V square wave output and feed that into the counter.

I'm currently leaning towards (2), since that's a very easy fix compared
to (3)... Dave, can you easily scare up a 1.5 MHz clock? That's Fosc/8,
for the record.

Any other ideas would be appreciated!



Andrew Greenberg

Portland State Aerospace Society (http://psas.pdx.edu/)
and...@psas.pdx.edu  P: 503.788.1343  C: 503.708.7711

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