AMD Geode/Video 3

Video 3 will be about how I messed up, and fixing VGA brightness.

Outline

Mention failure:

• Soldering failed, but that's not very interesting: My understanding of the circuit was wrong
• I thought the capacitors were inline with the circuit
• Removing them successfully would not have given me signal access
• I would've realized this if I checked the caps were connected to GND
• Replacing the inductors weren't technically necessary either
• Resistance and diode mode would show shorts or pull-downs
• Would it happen on all three lines?

Writing code:

• Entering test mode gives a blank screen
• Datasheet seems wrong: "MBD_MSR_DIAG[18:16] = 101h" should be "MSR_DIAG_VP[18:16] = 0b101"
• DAC status registers always say the DAC is below 0.35V

I did what I should have done in the first place: Measured when the display WORKS!

• The DVREF shunt looks okay and produces the correct voltage
• The DVREF shunt has a 12nF cap in parallel
• The DVREF shunt has 13k to the 3V3 rail
• The DRSET resistor is 1.21K to GND
• The VGA resistors seem fine and correct values
• When external DVREF is used, the shunt drops to 800mV, the rail somehow drops to 960mV

Editing the DVREF required source code hacking? DVREF stuff

Shorting between an existing 3.3V rail nearby caused the screen to suddenly display in full brightness.

I followed the rail and found a blown resistor, but I also wondered if the capacitor was somehow draining. So I removed both. This wasn't a completely correct fix: The bypass capacitor is in parallel to the circuit, it couldn't drop voltage like this normally. Had I measured it I could've confirmed it was 3.3V. The voltage drop here is 2.34V! So there's some in series resistor that must be dropping voltage based on current or use. The burned resistor looks like a good candidate.

At this point I believe the DAC is working, so the problem might be related to clocking.

I added in a fresh clock battery and it had no effect.