[altusmetrum] NMEA input to a TeleMini?

Keith Packard keithp at keithp.com
Sun Sep 3 22:44:00 MDT 2017

Bryan Duke <bryan at thedukes.org> writes:

> Hi everybody. I posted on the Facebook group, but someone there suggested I try the mailing list instead…
> I’m putting a TeleMini in a nosecone for a 24mm rocket that I’m hoping
> to break the TRA G-motor record with. There isn’t enough room in the
> nosecone for anything bigger than the TeleMini, but it looks like
> there will be just enough room for a separate little GPS. Is there a
> way to feed the TeleMini a NMEA stream & have it send GPS coordinates
> over its telemetry? It looks like several of the STM32's pins are
> accessible....I'm ok writing some code to make it happen.

This should be reasonably straightforward. TeleMini has a 6-pin
connector. Four of those are our standard debug connector (debug
data/clock, reset and ground). The other two are normally a regular USB
connection. You can hook those to a USB cable. However, those same two
pins can be re-configured to talk to a serial device, such as a GPS

        Pin 1 is TX
        Pin 2 is RX
        Pin 3 is gnd, and the pad around that is square.
        4,5,6 are the debug connectors.

The source code for AltOS has GPS drivers for u-blox, sirf and skytraq
already available, and the radio driver has all of the necessary
bits to send our regular digital telemetry as well as APRS.

So, if you're up for building the firmware, what I'd suggest is:

 1) Leave the USB running for a while when the device boots. This will
    make reflashing a ton easier as the alternative is to reflash over
    the debug link, which is really slow.

 2) After 'a while' (maybe 10-20 seconds?), turn off the USB and then
    reconfigure the pins to talk to the GPS device.

Normally, we log GPS data to the eeprom, so eventually you'll want to
add that, but it's a bit of work. Doing the GPS telemetry is quite easy
though, along with making APRS work.

There are 33ohm resistors on the board for the USB port; I don't think
those will hurt communication with the GPS chip, but you should be aware
that they are present.

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