GPS

The Super IO carries a GNSS module with a ceramic antenna, on a UART behind a load switch. The firmware powers it, hears it, parses what it says on the board, and shows the result on the Position screen. Nothing about the position leaves the board: an RNode is a modem, and Reticulum has no notion of a device's coordinates.

Power is a load switch

P1.01 is not a module enable. It drives an NMOS that gates a high-side PMOS feeding a switched 3V3 rail out to the expansion connector, rated 500 mA, and the module hangs off that rail. Driving it high is what makes anything appear on the UART.

Off is not merely the switch low. The receiver's output floats when its rail is cut, and a UART left listening to a floating line burns current on the input stage and fills the ring buffer with noise. So the UART driver is dropped when the receiver goes off, which disconnects both pins, and built again when it comes on. oxinode::gps::Gps owns the pins and peripherals rather than a driver; the driver is a thing that exists while the module is powered.

The UART

The module transmits on P0.20 (the nRF52840's RXD) and receives on P0.19, at 9600 baud, 8N1. The two documentation sources name the pair from opposite ends (the schematic names the net from the module's side, the Meshtastic variant names the pin from the MCU's side), and the firmware trusts neither: it opens the UART on the expected pins at 9600 baud, then 115200, then 38400, then the same three with the pins swapped, listening three seconds each, and stops at the first attempt on which a sentence with a good checksum arrives.

0.239562 INFO  gps: power on (P1.01 high)
0.891387 INFO  gps: NMEA at 9600 baud, module TX on P0.20

The first attempt is the one, about 650 ms from rail to first sentence. The probe stays in the product image because it costs nothing when the first attempt is right, and a Super IO revision wired the other way would be a log line rather than a silent screen.

The module is taken as it wakes: its default rate and sentence set. The MCU's transmit pin is connected and idle. The buffered UART driver uses TIMER2, PPI_CH10, PPI_CH11 and PPI_GROUP0 to count received bytes so the ring can be read before a DMA transfer ends.

Who decides whether it is on

The GPS is the largest continuous draw on the board, and the mode switch has a position labelled GPS ON, so a physical control whose position can be seen is where that decision lives.

stateDiagram-v2
    [*] --> Boot
    Boot --> On: switch reads GPS ON
    Boot --> Off: switch reads Power ON, or neither
    On --> Off: switch moves to Power ON
    Off --> On: switch moves to GPS ON
    On --> Off: menu "GPS On/Off"
    Off --> On: menu "GPS On/Off"

• At boot, the switch decides. GPS ON powers the receiver; Power ON does not. A board with no Super IO (both lines floating) has no GPS to power and stays off.
• When the switch moves, it decides again. The task reads it every quarter second.
• Between moves, the menu decides. GPS On/Off on the Position screen flips the receiver regardless of where the switch is, and the switch, unmoved, does not flip it back. A switch caught mid-travel says nothing and changes nothing.

The rule is oxinode_core::gps::Control, twenty lines, every branch a test. The receiver's state is not stored across a reboot; the switch is the persistent default.

NMEA, as much as the screen needs

The framing is oxinode_core::gps::Lexer, one byte at a time, and it is unforgiving on purpose. A line whose checksum fails is dropped whole: a corrupted digit in a latitude is a position a few hundred kilometres out, which is worse than no position. A byte that cannot appear in a sentence (which is what the wrong baud rate produces) abandons the line it was in. A $ mid-line starts over. A line past 82 characters is framing lost.

Three sentences are read by oxinode_core::gps::parse:

• GGA is the fix: time, quality, satellites used, latitude, longitude, altitude. Quality zero is no fix whatever the coordinate fields say.
• RMC carries the date, which GGA does not, and a second opinion on validity. Its coordinates are taken too, with GGA's altitude kept, so a receiver that sends one and not the other still yields a fix.
• GSV carries how many satellites one constellation can see, which is the number worth watching before there is a fix. Each talker (GP, GL, GB, GA, GQ) reports its own view and the screen shows the sum.

Everything else (GSA, GLL, VTG, TXT, proprietary lines) is framed, checked, and ignored. Coordinates are micro-degrees in an i32 and altitude is decimetres; nothing is floating point. The parser is tested against captured sentences from a Quectel-class module through its first fix, a u-blox line with four-place minutes, the southern and western hemispheres, a negative altitude, and eighteen malformed bodies that frame correctly and are wrong inside.

What the receiver knows

oxinode_core::gps::Receiver folds sentences into a Position: status, satellites used and in view, the last fix, its age, the time and the date. It is timed from outside (every call carries the board's millisecond clock), so its rules are testable at any speed:

• Powered and nothing readable for five seconds is no data. Talking with no fix is searching. A fix confirmed within the last five seconds is fix.
• A lost fix is kept and aged. A receiver that had a fix and stopped confirming it shows searching, the last coordinates, and the fix's age. A position a minute stale is still a position; the age is the honest part.
• Power off forgets everything. A position shown after the receiver was deliberately turned off is a claim nobody is standing behind. The module's own memory, not the firmware's, is what makes the next fix quick.

The firmware's part, oxinode::gps::serve, is a task beside the Bluetooth one. It owns the load switch and the UART, runs the probe, feeds bytes to the receiver, and publishes the Position into a mutex the modem loop copies out once per redraw. The two meet nowhere else except the menu item, which raises a signal. Nothing on the radio's path waits for the receiver.

The screen

GPS              fix
Sats            7/11
Fix age          3 s
UTC         13:47:09
Date      2026-09-08
Lat        47.376887
Lon         8.541694
Alt          408.0 m

Every value is a dash until it is known. A receiver that is off shows off, seven dashes, and a line saying the switch or the menu turns it on. The widest a coordinate can be, -180.000000, fits its row, and a test says so.

Not measured

The receiver's current draw. It can be turned off from the menu and the switch, and the UART is released when it is, so both halves of the measurement are one reading away from anyone with a meter in the battery lead.