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RP2350 GPIO Pull-down Latching Bug

ralphjy

I just saw this bug while searching the web.  I haven't been able to get a Pico 2 so I bought a Seeed Xiao RP2350 board which I just received yesterday.  Maybe it's a good thing that these boards have not been more available...

Here is the errata excerpt from the rp2350 datasheet

image

At least this bug is documented.  I haven't powered my board yet so I have not had a chance to encounter the problem.

Here is a link to a post on Hackster - Hardware Bug in RP2350 leads to unexpected pull-down behavior.

Parents Reply
  • in reply to scottiebabe

    A simple test, drive an IO pad with a source impedance of 10 kOhms

    image

    With the Input buffer disabled and Pull Up/Down Disabled

    mem32[0x40038000+4]=0x000

    image

    The IO pad doesn't load the 10kOhm source, as it should Ie being high-impedance.

    With the Input buffer enabled and Pull Up/Down Disabled

    mem32[0x40038000+4]=0x040

    image

    The IO pad is sourcing and sinking current and loading the source resistance.

    As a high impedance input it should not source or sink ANY current.

Children
  • in reply to scottiebabe

    Cool work on verifying/clarifying this Bug/Issue!

  • in reply to scottiebabe

    Is that the same scenario as the bug report? I read it as: only impacting with the pull-down enabled.

  • in reply to genebren

    They didn't do a great job explaining the issue. The issue is the input buffer sources current into the IO pin, so the electrical characteristics table is wrong.

    image

    The pin latches in pull-down mode because this input buffer sources more current than the pull-down resistor can sink.

    The leakage of the input buffer looks something like the following.

    image

    The built-in pulldown resistors are strong enough to overdrive this weird input buffer, so one could also just add their own 10K pull-down resistor on board. 

  • in reply to Jan Cumps

    It's the root cause of the latching effect.

    image

    image

    The input buffer leakage is greater than what the pull-down resistor can sink, so the IO stays high until you either

    Yank the pin low with a bigger pulldown resistor

    or disable the input buffer.

  • in reply to scottiebabe

    Nice work verifying the issue. Not that it matters, but out of curiosity - which board(s) do you have?

    It would be interesting if they will share what current path is turned on when the buffer input switches high. Maybe it’s related to the enable circuit.

    I guess it’s not a horrible hardware or software workaround for someone with just a few boards.  Seems like vendors got a lot of board variants out quickly.  Wonder what they will do?

  • in reply to ralphjy

    > Wonder what they will do?

    They announced that they won't fix silicon or adapt the API, but will use documentation to deal with it.

  • in reply to ralphjy

    Thanks!

    I guess I got lucky with timing and managed to picked up a few Pico 2's

    image

    The GPIO pin only has a connection to the RP2350, which should be the same as most RP2350 boards.

    The input buffer has some strange input bias/leakage current

    image

    I too would be interested to know what the actual cause is.

    If whatever is driving the IO pin has a source resistance less than 10kOhms (most digital outputs are), than it isn't really a showstopper.

    Trying to measure the charge time of a capacitor or a push button connected to VDD (with out a pull-down resistor) could be problematic...

  • in reply to Jan Cumps

      This isn't a 'fix' its im blind and anybody that bought the piece of crap lost money time, and investment!  fixing the Docs will not fix the underling issue! the silicon and or their firmware is Bogged Up! 

    What they should do: is to give anybody that has one to replace it. Its called a RECALL!!  ~~ Cris

    or Simpler Stop using Pis!!! 

  • in reply to phoenixcomm

    I bought a few but have not spent any time with them so far. I might one day make use of them, but on the whole I'm going to use the older Pico instead. For sure the new chip will be very useful for those that buy knowing the issue exists and work around it.

    The bug is severe enough that I would hope that any of the reputable sellers would immediately refund anyone who wants a refund, if they purchased unaware of the issue. (And if they don't I hope people report them to trading standards or equivalent in other countries; users deserve to be dealt with fairly). I hope they now it make clear when purchasing, that there is an issue with the chip, so that users are aware that they may wish to buy an older Pico model instead.