MPG mode switch not working

[stevempotter]

I have recently updated my Teensy4.1 firmware in order to begin using the USB connection on the Teensy for the computer-IOsender link, rather than the ethernet port, which was becoming unreliable due to physical problems with the jacks.

I would like to continue to use my custom made pendant with MPG mode switching on pin 41.
in my_machine.h
#define MPG_ENABLE 1
and boards/T41U5XBB_map.h line 120 #define AUXINPUT5_PIN (41u)

But every time I flash in a version with the above settings,
the Teensy exhibits "flapping": It can't make a proper USB connection to the computer, and its current draw is only 57mA instead of the nominal 94mA. The PC keeps beeping every few seconds, saying it is connecting to COM5 and disconnecting and reconnecting. Needless to say, I can't even start IOsender without a stable COM port connection. Or talk to the pendant either.

Here is a link to my firmware with the settings I have tried. It was downloaded from github on Jan 14, 2026.

Dropbox .zip file of grblHAL firmware code:
https://www.dropbox.com/scl/fi/zdl5m9gp82udldkpkmqey/POTTER-2026-01-22-grblHAL_Teensy4.zip?rlkey=zqe4w6mafo2x4u6aphco2fghn&st=szd1owci&dl=0

Let me know if there is anything else I need to provide to help debug this.

I am using pins 0,1 for the UART connection to the pendant, and Phil Barrett's original breakout board, T41U5XBB.

[terjeio]

If you comment out the keypad plugin does the issue persist?

The "flapping" is likely due to a hardfault causing the processor to reboot. I have seen this before with bad code, but cannot replicate it now.

[stevempotter]

Commenting out the line #define KEYPAD_ENABLE did not help. Still I saw the USB flapping.
However, setting it to 1 instead of 2 seems to have fixed the problem. YAY!!
I had it set to 2 because I am not using I2C at all and because of the comment "// 2: uses a serial stream for input. If MPG_ENABLE is set > 0 the serial stream is shared with the MPG."
So maybe you could clarify how MPG and keypad interact with each other in these comments. Can one actually use the UART for either keypad or MPG?

[terjeio]

However, setting it to 1 instead of 2 seems to have fixed the problem. YAY!!

That is really odd. I have a large commit nearly ready - and I cannot replicate the "flapping" with that. Best would be to wait for it and debug with the same version I have?

Can one actually use the UART for either keypad or MPG?

Yes. If both are enabled received characters are processed by the keypad when MPG input is disabled and sent for normal processing when enabled.

[stevempotter]

Setting the keypad_enable to 1 fixed the problem, so now the pendant is working.
If you want to try to figure out what went wrong, you can flash the firmware repo I shared above in its entirety, as is. That is the version that had the USB flapping problem.

I look forward to seeing what is new in your "large commit"...