I2C lines large voltage drop?

Hi Ian, I reverted back to an older firmware (because the new one still has the infamous periodic glitch on both I2C lines…) and plugged probes from a fresh arrived cheap LA from these guys:
(Using as a Logic Analyzer - Sipeed Wiki)
Switched BP5 to I2C mode, enabled internal source on 3v3 (500mA) and 10k pullups. When I plug the LA probes SDA and SCL lines are dropping to …2.5V as you can see in photo.
My first thought was that LA is defective and has a low impedance …measured …5 megaohms !

Does your logic analyzer have a bus hold feature? It shouldn’t be enough to overcome the pullups, but mine does that at exactly 2.5volts when the pullups are off.

Also - the latest auto compile has the changes we worked on in the glitch thread, is it no longer working? I can tweak it a bit more, and I have a DSO in this office now.

Hi Ian. Just retested I2C on latest main-2e62963 firmware with a different new logic analyzer (Kingst LA1010) and the infamous glitches are still there even in idle state … on both lines … Just to know LEDs are disabled (is there a way to memorize setting ? or disable them permanently :smiley: )
Voltages on lines drop a bit to 3.1V (10K pullups enabled, 100mA@3v3)

I have a different fix for you, but it is really just a cheat. Instead of grounding between pins, it just sweeps straight through. Then only pins adjacent to a grounded pin have the glitch.

In general, I think this is a corner case issue. The analog mux on your board seems to be much “worse” than any I’ve checked. However, the boards I’ve checked range from “not great” to “ugh”. Only one will trigger an logic analyzer, but they all definitely have the issue in varying degrees.

Supposedly, UPS is delivering the updated board with a hardware fix any minute. I’ll update when I have it in hand.

One kind request Ian, For future REV9 can you put a simple dual/tri row pinhead 0.1" instead of existing I/O connectors ? That would allow users to have logic analyzer in parallel as well as working on SPI/I2C/JTAG lines …

In regards to the LEDs: in the configuration menu c you can disable the LEDs. If there is an flash card in the slot it will save the settings and load then on startup.

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