Unexpected interference on GPIO 26 when E10-1 is attached

Products & Technology Single Board Computers
#1

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.

with-e10-gpio-26-low-to-high
with-e10-gpio-26-low-to-high800×480 11.3 KB

Here’s the frequency measurement of the signal:

with-e10-gpio-26-high-freq
with-e10-gpio-26-high-freq800×480 23 KB

Zooming into the signal and measuring again:

with-e10-gpio-26-high-zoomed
with-e10-gpio-26-high-zoomed800×480 14.7 KB

Observations WITHOUT E10-1:

GPIO 26 driven high. Clean, no unexpected signal.

without-e10-gpio-26-high
without-e10-gpio-26-high800×480 10.1 KB

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.

extension-module-interface
extension-module-interface1116×823 235 KB

pinout-diagram
pinout-diagram1000×774 302 KB

Questions:

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

#2

I don’t think it’s an issue. I was watching a video that identified GPIO 26 is used for PWM of the fan on the expansion. I would think this is an expected behavior.

Thank you

#3

my bad - the Wiki shows GPIO 23

Thank you