I've found a few minor design issues which I don't think will affect us in
the short run, but may not be what we want:
1) I already told you (Bob) about the addresses for the Max temp sensors.
As set, they collide with two of the possible 4 addresses on an ADS7828, a
commonly-used and space-tested A/D converter that some satellite might
use. We talked about this before and it should all be straightened out.
2) I just noticed that PWR_SW_AX5043 and PWR_SW_SSPA are connected to
AD1IN_8 and AD1IN_5, pins 78 and 83. They are used as input to the
MAX4995SAL power switch/limiter. While you could use these AD1IN pins as
an input GPIO (read the voltage and subject it to a threshold for high or
low), I don't think it is possible to use these for output. AD1EVT might
be usable for output but I'm not sure what effect it would have on the
bult-in a/d converter. I suppose you could use the nearby MIBSPI5 pins.
They appear to be usable as GPIO pins and are physically near the ADC pins.
I think the current way it is will work for the moment, since if I read the
datasheet right, the "ON" pin or the MAX4995AL really should be ON_N.
That is, a low value on it turns the power on, and you have a pulldown
there. So I think the 5043 and the SSPA will always be on since the ADC
inputs should be high impedance.
3) This may just be a doc issue. I think that "PWR_FLAG" on the schematic
can't be a circuit, right? OTherwise you'd have lots of weird stuff
connected together. If they are actually test points, I think they should
be labelled as such (TP1, TP2, etc) and marked in silk screen on the board.
4) And finally, from what I have read (and seen on other projects), I don't
think the SPI busses need pullup resistors EXCEPT for MISO, and that one
should be VERY weak. (One of our devices does not use MISO very often but
it appears not to be able to pull much current, thus requiring the really
weak pullup. But as I recall a builtin pullup either was not enough, or it
could not be used when the pin was used as an SPI device). I can put a
scope on the MISO line for the AX5043 to see if it is getting pulled down
enough with the 10K. Likewise, I'll see what the other pins look like when
connected to the push/pull output of the SPI master.
Don't take these critiques as anything other than helpful. I'm no expert
here...I just found these things in the process of trying to debug the
software, and in a couple cases from tracking down some really PITA
problems on Fox and/or Golf!
73,
Burns Fisher, WB1FJ
*AMSAT(R) Engineering -- Flight Software*
