Hi,
Yesterday I spent some time debugging a Roach1 with
something strange with the PPC temperature sensor.
The PPC temperature reported is about 50 deg higher
than that reported for the Xilinx FPGA or Actel Fusion
device (which actually has the ADC).
As in 80 vs 30 when just the Xport is running.
Could this be caused by the temp sensor having a good
connection to 1 and only 1 of the 2 PPC's PNP transistor leads ?
Is there any easy software workaround ?
Would it be OK to use such a workaround and not monitor PPC's temp ?
Would you accept delivery of a Roach1 that did not monitor
the PPC's temp but was otherwise AOK ?
Or should we remove the Actel Fusion BGA device, inspect&repair
as necessary, install a new Actel and continue debugging ?
Thanks
Matt
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Before I had a chance to look at the board the Actel
Fusion device U60 was replaced. Both the current and
previous devices were programmed with the latest and greatest
design. I don't know for certain but suspect the previous
Actel part behaved identically to the device now
installed.
We made some ohmmeter measurements and for a while we
thought that ATRTN1 (J31-7) was 43 ohms to GND vs 7.x ohms
to ground on a board that reported good temps. Later
we redid the measurements and the strange board also reported
7.x ohms. The reported temps were still bonkers.
The voltages at J31-7 vs J31-8 were about .69 volts and
that matched the voltages for the Xilinx temps on J31-3 vs J31-4.
Those voltages decreased and the reported temperatures increased
as expected after the board was fully powered up. But still
the reported values were too high by 50 for the PPC.
The board would only stay powered up if we disabled the
automatic failure condition shutdown function We checked
the other reported temps, voltages and currents and they
were all fine. The board would also stay up if we temporarily shorted
J31-7 to J31-8 which leads to a reported temp of -271. The -271
is nonsensical but understandable. As it is higher
than the low temp shutoff threshold of -280 all the other
autoshutoff protections can remain enabled.
The Actel Fusion device is a 256 pin BGA so it's virtually impossible
to probe even though the PPC temp sensor input connections
are on or near the edge of the package.
Using an X-Ray inspection device and comparing vs a known
good board we found the very short length of etch that delivers
the ATRTN1 signal from a via (from the PPC IC and to J31-7) to the
PCB bad for the Actel's T6 pin looks to be mainly missing.
So maybe the Actel's ADC is getting a valid version of just one of
the PPC's PNP transistor leads and thus reporting a value with a
strange offset ?
We tried blinding pushing in a fine wire to make a connection from J31-7
to the tiny U60-T6 solder ball but never improved the reported value.
It would have been a big surprise if that actually worked but hey
no guts no glory.
Do you have any ideas before we remove the Actual Fusion part at U60
and inspect&repair the PCB trace from the PCB pad to the breakout
via ?
other ideas:
1) tweak the autoshutdown code running on this 1 Roach1 to deal
with the ~ 50deg offset in reported PPC temps ?
2) jumper the PPC temp signals so Vdiff is 0V (-271 deg) or
perhaps some other voltage like 1.0VCC so that
PPC temperature monitoring is lost but the rest of the
protections are up and running.
3) no need to disable entirely the failure mode shutdown function
so don't want to persue that path.
4) ?
For now, I believe, additional board bringup and debugging will continue
with J31-7&8 shorted together...
