Unexpected interference on GPIO 26 when E10-1 is attached

When the E10-1 module is attached and GPIO 26 is driven high, there’s an unexpected signal observed that occurs about every 1.3 seconds. This does not occur If the E10-1 is not attached.

I tested this on older reTerminal Raspbian images as well as the latest reTerminal Raspbian image and the same result was observed.

Steps to reproduce:

  1. Attach (stock, unmodified) E10-1 module to (stock, unmodified) reTerminal.
  2. Connect an oscilloscope probe to ground and GPIO 26 on the exposed header pins.

Observations w/ E10-1 attached:

Here’s when GPIO26 is driven from low to high. You can see the activation of the unexpected signal.

Here’s the frequency measurement of the signal:

Zooming into the signal and measuring again:

Observations WITHOUT E10-1:

GPIO 26 driven high. Clean, no unexpected signal.

Documentation research:

I looked at the extension module interface schematics and pinout diagrams and there’s nothing in the schematics that indicate GPIO 26 is used for other functions.

Questions:

@lakshan Is this a known issue on your side? This makes GPIO 26 unusable with the E10-1 module attached.