LH2 Decoder on RP2040

Valve's Lighthouse v2 positioning system decoder implemented for the RP2040. 100% CPU implementation.

4 TS4231 are connected to a RP2040. The MCU receives and decodes data from two LH2 basestations.

A logic analyser is connected to the system. It shows the arriving data, and how much time it takes the CPU to decode the data.

The decoded data is printed to a serial port trough the USB.

How does it work

The code uses the PIO system to sample the data from the TS4231. Four PIO state machines samples each TS4231 data line for 16us at a constant rate of 32 MS/s. The capture data is automatically moved to a buffer in RAM using the DMA system. After each successful capture, the PIO state machine triggers an interrupt. This signals the CPU to retrieve the data from the buffers and decode it. Each core of the RP2040 monitors and processes two TS4231 sensors

Results and Benchmark

Below is a capture from two sweeps of a basestation.

• Top 4 lines: Envelope pin of the TS4231. It signals the presence of data
• Bottom 4 lines: Benchmark of each function in the decoding function All 8 pulses from a full sweep of the basestation, can be decoded in less than 3ms. Full bench mark available here.

All the decoded information gets printed to the serial console in the following format.

Getting Started

Circuit Pinout

TODO

Build & Flash the code

1. Clone the repository and initialize the SDK submodule

git clone https://github.com/DotBots/lh2_on_rp2040.git
cd lh2_on_rp2040
git submodule update --init
cd pico-sdk
git submodule update --init
cd ..

2. Set the enviroment variable To let the system know where is the SDK

export PICO_SDK_PATH=$PWD/pico-sdk

3. Create a build directory and run CMAKE

mkdir build && cd build

# For the Pico 2 board
cmake .. -D"PICO_BOARD=pico2"
# For the Pico 2W board
cmake .. -D"PICO_BOARD=pico2_w"
# For the Pico 1 board
cmake .. -D"PICO_BOARD=pico"
# For the Pico 1W board
cmake .. -D"PICO_BOARD=pico_w"

4. Build project

make

5. Flash the lh2_receiver.uf2 file to the microcontroller/

References

• How to change the CPU system clock, with set_sys_clock_khz(128000, true) Link
  • this function is available from stdlib.h
  • The script pico-sdk/src/rp2_common/hardware_clocks/scripts/vcocalc.py confirms that 128MHz is an achievable frequency.