Homebrew PCBs

"awakephd" <a_wake@...> writes:
> DJ, may I ask a couple of questions about your schematic? (May I ask
> them even if I reveal all sorts of ignorance on my part? :)

Sure.

> I'm taking VSX-S3 near the upper right corner as a power supply that
> converts the line voltage to 5V. Is this some sort of pre-packaged
> module?

Yes.  Digikey part number 102-1802-ND.  Ah - typo, it's a VSK, not a
VSX.  Fixed.

> Am I right in thinking that you are detecting the 0 voltage
> crossover in the AC using U4, and this drives an interrupt?

Yes, but not quite.  The capacitor and resistor drop the 120 vac down
to a few milliamps through the opto, but the timing is such that the
interrupt triggers about 45% of the way through the cycle.  I use an
interrupt-driven timer in the R8C to re-interrupt at the end of the
cycle, which is the start of the next one.

> What is the purpose of J1 -- for expansion, reprogramming, or ??

Programming the R8C.  It doubles as a serial console for debugging.
See http://people.redhat.com/~dj/m32c/ for programming info.  That
connector is the "E8a" standard connector for r8c, m16c, and m32c (and
I think r32c) chip families, although each chip uses different signals
off it.  The E8a itself only uses reset and mode, buy my linux-hosted
programmer uses tx and rx as well.

> Finally, I think the MAX6675 is the interface to the
> thermocouple. Does this IC send the temperature of the probe over a
> serial interface?

Yes.  Each 16-bit SPI read returns the actual temperature of the probe
in Celsius (fractions of a degree, that is, plus some status bits).
It's already done the conversion and cold-point compensation.