Custom Bpod Firmware

This guide is only needed if the default Bpod communication latency (see Timing, Clocks & Latencies) is not good enough for your experiment — most setups can safely skip it.

By default, some Bpod firmware versions flush outgoing USB serial data with a 5 ms timeout, introducing up to 5 ms of communication latency. This only applies to Bpod state machines built on the Teensy 3.6 (hardware versions Bpod 2.0 to 2.4).

  • Older models (Bpod 0.5–1.0): Do not use Teensy microcontrollers.

  • Newer models (Bpod 2.5 / 2+): Use the Teensy 4.x platform and are not affected by this issue.

Lowering the timeout to 1 ms noticeably reduces Bpod Softcode communication latency (see the Timing, Clocks & Latencies section). On this Raspberry Pi, the one-line edit the fix requires has already been made to the Teensy core library (see the note at the end of this guide) — you still need to follow the steps below to build and upload the firmware so the fix actually reaches your Bpod.

1. Open the firmware sketch

On the Raspberry Pi, you will find a local copy of the original Bpod 2.0 (v23) firmware (unmodified from Sanworks’ GitHub). Open it using the pre-installed Arduino IDE 1.8:

/home/pi/Bpod_StateMachine_Firmware-23/Preconfigured/v23/StateMachine-Bpod2_0/StateMachine-Bpod2_0.ino

Teensy board support is also already installed on the Pi (https://www.pjrc.com/teensy/td_download.html).

2. Select the board, USB type, and port

From the Tools menu:

  • Board → Teensy 3.6

  • USB Type → Dual Serial

  • Port → the Bpod’s serial port (on Linux this typically appears as /dev/ttyACM0 or /dev/ttyACM1; if you are unsure, unplug the USB cable and check which port disappears).

3. Upload

Click the upload button (the right-pointing arrow). A successful upload prints “Done uploading” with a “Verify successful” message in the output window.

Where the speed fix actually lives

The firmware sketch itself is untouched — the fix is a one-line edit to the Teensy core library, not the sketch:

~/arduino-1.8.19/hardware/teensy/avr/cores/teensy3/usb_serial.c

Default Teensy core installations ship with:

#define TRANSMIT_FLUSH_TIMEOUT  5  /* in milliseconds */

On this Raspberry Pi, that line has already been changed to:

#define TRANSMIT_FLUSH_TIMEOUT  1  /* in milliseconds */