Daisy-chain readout boards

Multiple 30-pin readout boards can share one USB connection through a Teensy 4.0 controller. The boards communicate with the controller over I²C, while the host uses the same sensor-configuration and readout interfaces as for a board connected directly over USB.

Only the Teensy is on USB; the boards share four bus wires (5 V, SDA, SCL, GND). Pin assignments and pull-up requirements are given in Connect the I²C bus.

The controller supports up to 18 nodes on a 1 MHz bus. In a test with three 30-pin boards, 7,213 bus transactions completed without errors. With two boards streaming, the measured rate was approximately 240 frames/s per board.

1. Flash every board for the bus

Plug each board in by its own USB cable, one at a time, and flash the bus version of the firmware:

uv run opentac flash --bin firmware/prebuilt/NODE16_bus.bin

BOOT0 (S4) on the 30-pin top board. Render of its KiCad file.

A new board has no firmware yet: hold its BOOT0 button while plugging it in, then run the same command. The bus firmware still answers B on USB, so the same command reflashes it later.

In the Arduino IDE, add a file named build_opt.h next to opentac_readout.ino containing this one line, then upload as in Flash the firmware:

-DOT_BUS_NODE -DOT_BOARD_NODE16 -DI2C_TXRX_BUFFER_SIZE=160

The flags must reach every file the build compiles, not only the sketch, or the I²C driver keeps a 32-byte buffer and cuts every frame short. build_opt.h does that in the IDE; firmware/build.sh passes them as build.extra_flags. Delete build_opt.h again before building a normal USB firmware.

2. Flash the Teensy

Load firmware/prebuilt/bus_controller.hex with the Teensy Loader. Or, in the Arduino IDE with Teensyduino: copy firmware/libraries/OpenTactileBus into your Arduino libraries folder, open firmware/bus_controller/bus_controller.ino, choose Board Teensy 4.0 and USB Type Serial, and click Upload.

3. Connect the I²C bus

Each board has two 4-pin JST-SH bus connectors, wired the same way, so the cable goes in on one and out on the other:

The two bus connectors (CN1, CN2) on the 30-pin top board. Render of its KiCad file.

Pin	Signal	Teensy 4.0
1	5 V	VIN (USB power)
2	SDA	pin 18
3	SCL	pin 19
4	GND	GND

• Fit one pair of pull-up resistors, SDA and SCL to 3.3 V, at the Teensy end only. The boards have none on the bus.
• The Teensy is 3.3 V and not 5 V tolerant: keep SDA and SCL at 3.3 V.
• A board takes 5 V from either its own USB or pin 1 (each through a diode), so you can unplug the boards' own USB cables once they are flashed.

4. Set up each sensor

uv run opentac info                                  # lists the boards (nodes) on the bus
uv run opentac setup palm.zip --node pick            # choose the board by its LED
uv run opentac setup thumb.zip --node 1              # or by node number

With --node pick, one board's LED lights red. Press Space until the red LED is on the board that sensor is plugged into, then Enter. Each board remembers its sensor; a board that restarts keeps its node number.

5. View and read multiple sensors

uv run opentac view                 # every board at once, one panel each
uv run opentac view --node 1        # one board

Each board's LED takes the colour of its panel's title.

from opentac.device import Bus

with Bus() as bus:
    for s in bus:                         # one Sensor per board
        print(s.node, s.manifest.name)
    for node, frame in bus.frames():      # every board's frames, as they arrive
        print(node, frame.t_us, frame.taxels.max())

frame.t_us uses the Teensy's clock in microseconds, placing frames from all nodes on a common time base. This supports temporal alignment across sensors; it does not imply that their taxels are sampled simultaneously. Use Sensor(node=2) to access one board on the bus.

Bus limitations

Only run the OpenTac bus controller firmware against these boards. A controller that reads more bytes than a board sends makes the board hold SCL low, and the bus remains blocked until the boards are power-cycled.

The bus firmware is for the 30-pin board only. A 60-pin board needs more memory for its frame buffers than its firmware has left.