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missing "CCCC..." on serial port of AM335x EVM on power up, is it dead?

shannon mackey
Prodigy 200 points
Other Parts Discussed in Thread: TPS65910, AM3359

normally, upon power up, the AM335 General Purpose EVM would display "CCCCC..." for a brief period before it moved on to regular uBoot messages etc.  Well, it no longer boots, nor do I get any activity at all over the serial port.  It clearly isn't booting.

So far we get power at many test points around the processor, though current is only drawn by the clock and ddr (as determined by the test pin pairs across the shunts).

We had external connections to level shifters on the spi0 pins when the failure occurred.

Beyond half-stepping our way through the schematic, does anyone have any troubleshooting tips for this situation?  Any suggestion (or, consolation) would be appreciated.

  • 0
    TI__Intellectual 2725 points

    What is the Boot mode selected. On EVM, "CCC" is seen if the Boot-mode has UART as a boot device in the boot sequence. May be there is a change in the Boot mode switch.

  • 0 in reply to Gururaja Hebbar
    Prodigy 200 points

    Thanks, but no luck.   The switches remain in the position as they were from the beginning, matching the positions indicated in the Quick Start guide.

  • 0
    Prodigy 140 points

    I would

    1. Verify the boot switch settings against the Technical Reference Manual of AM335X.

    2. Check all the power supply outputs.

    3. Using an oscilloscope check the the 24MHz crystal oscillation. If power supplies are good, but no oscillation, the MPU is probably damaged. Make sure the oscillator is not disabled in anyway.

    4. Use the oscilloscope to check the pin11 of U18 (the UART0_TXD signal) at power up.

    DL

  • 0 in reply to Coder
    Prodigy 200 points

    Thanks for the thoughtful reply.

    1. switch settings are verified correct

    2. I've been identifying seemingly pertinent outputs of U4 (TPS65910) as they relate to inputs of the U2C(AM3359).  Where I get ~1.8 and ~3.3, I'm assuming they are correct.  But, I'm also getting just 0.9 V (not the expected 1.2 V) on VDD_MPU and no current flow (as tested across R46).  Additionally, VDD_CORE is presenting 0.2 V (not the expected 3.3 V) and no current (as tested across R46).  Which doesn't seem right, as VDAC shows a correct 1.8 V but comes later in the TPS65910 switch-on sequence, if I'm reading the datasheet correctly.  Could it be just something wrong in/around U4?

    3. I'm getting no love from my 24MHz crystal (Y2), but that is no surprise at this point.

    4. obviously, nothing.

    Could it be just something wrong in/around U4?

  • 0 in reply to shannon mackey
    Prodigy 140 points

    Remove R46 and R32. See if the voltages are still 0.9V.

  • 0 in reply to Coder
    Prodigy 200 points

    I'm getting, now, 1.09 V on the supply side of both of those resistors. Close for the MPU 1.2V, but not the CORE 3.3V.

  • 0 in reply to shannon mackey
    Prodigy 140 points

    I think they both should be 1.1V. The voltages are ok. Use a 2W and 2 Ohm resistor. Connect one side to the ground and the other side to the power supply. Be careful they can get very hot. See if the output voltage drops. If it does, replace the U4. Otherwise the MPU is drawing too much current. Probably damaged.

  • 0 in reply to Coder
    Prodigy 140 points

    Also check the decoupling capacitors related to these power supplies, could be some contamination short them out. Try removing everything connected to the board.

  • 0 in reply to Coder
    Prodigy 200 points

    Well, darn!  U4 stands up to the load just fine.  Time to move to Plan-B ("B" as in BeagleBone)

    Thanks so much for your detailed assistance!