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Application Note: PTT Keying

The firmware of the Pi may configure GPIO pins as inputs with pull-ups.

Once the kernel boots, the driver correctly configures the pins. This is not generally a problem, except possibly for PTT.  This will manifest itself by the PTT keying the transmitter before boot-up of the Raspberry Pi.

GPIO 12 & 23 are used to drive PTT and the pull-up is sufficient to turn the transistor on and key the radio, even though the red PTT LED is not lit.

This only happens during boot-up or shutdown, but may be an issue in some applications. Adding a 2.2k pull-down on the Pi's 40 Pin Header solves the issue.

Mini DIN-6 Connector:

Pin-16 to Pin-14

HD15 Connector:

Pin-30 to Pin-32

Note: An original UDRC only uses GPIO 12, so Pin-30 to Pin-32 is prescribed.

Image of 40 Pin Pinout