Communication bus between Raspberry Pi 5 and Raspberry Pi Pico 2¶

This bus has several constraints:

It must use as little of the Pico 2's processing time as possible.
It should keep the system wiring simple.
It must use pins that are available on the Pico 2.

UART is a suitable solution: it is supported by both boards and can operate asynchronously, allowing the Pico 2 to perform other work between received bytes. A full-duplex UART connection requires at least three conductors: TX, RX, and a common ground. Two signal wires are sufficient only when both boards already share a ground through another connection, such as their power supply.

Two types of data are sent from the Pi to the Pico: punctual commands and frames.

Current UART frames¶

Raspberry Pi 5 to Raspberry Pi Pico 2¶

Cadence: 30-50 Hz

Position	Variable	Unit	Range	Description
0	`sequence`			Incrementing frame counter.
1	`head_angle_1`			Head angle 1.
2	`head_angle_2`			Head angle 2.
3	`head_angle_3`			Head angle 3.
4	`v_target`	normalized	`[-1, 1]`	Target speed: `-1` backward, `1` forward.
5	`yaw_rate_target`	normalized	`[-1, 1]`	Rotation around the center of gravity: `-1` left, `1` right.
6	`height_cmd`	normalized	`[0, 1]`	Torso height: `0` low, `1` high.
7	`roll_cmd`	normalized	`[-1, 1]`	Torso roll: `-1` left side down, `1` right side down.
8	`armed`			Authorizes motor actuation.
9	`balance_enabled`			Requests balance control mode.
10	`tilt_setpoint_deg`	degrees		Target balance angle.
11	`emergency_stop`			Immediately requests a safe stop.
12	`choreography_id`			Identifier of a prerecorded movement sequence; `0` means no choreography.

Raspberry Pi Pico 2 to Raspberry Pi 5¶

Cadence: 10 Hz

Position	Variable	Description
0	`sequence`	Incrementing telemetry-frame counter.
1	`torso_inclination_x`	Torso inclination on the X axis.
2	`torso_inclination_y`	Torso inclination on the Y axis.
3	`torso_inclination_z`	Torso inclination on the Z axis.
4	`forward_speed`	Measured forward/backward speed.
5	`lateral_speed`	Measured left/right speed.
6	`height`	Measured height.
7	`robot_state`	Current robot state, such as off, booting, ready, standing up, standing, sitting down, sitting, or fault.
8	`status_flags`	Controller and hardware status flags.
9	`left_wheel_speed`	Measured left wheel speed.
10	`right_wheel_speed`	Measured right wheel speed.

Frame behavior¶

sequence increments for every frame and wraps to zero after reaching its maximum value. The receiver can use it to detect duplicate or missing frames.

emergency_stop is latched: setting it back to 0 does not re-arm the robot. Re-arming requires an explicit safe sequence after the emergency-stop condition has been cleared.

choreography_id starts a choreography only when its value changes from 0 to a non-zero value. Keeping the same value in subsequent frames must not restart the choreography.

Current UART Commands¶

Every frame must end with \n.

USB - `Serial`¶

Via the 'commander'

Command	Target variable / object	Description
`R<FOC_command>`	`motor_right`	Sends a SimpleFOC command to the right drive motor.
`L<FOC_command>`	`motor_left`	Sends a SimpleFOC command to the left drive motor.
`S<angle>`	`tilt_setpoint`	Sets the target balance angle in degrees.
`B<value>`	`motor_balance`	Scales the right motor voltage to compensate motor mismatch.
`Y<value>`	`yaw_trim`	Adds a constant steering correction to both motor commands.
`P<value>`	`Kp_tilt`	Sets the proportional gain of the tilt PID controller.
`I<value>`	`Ki_tilt`	Sets the integral gain of the tilt PID controller.
`D<value>`	`Kd_tilt`	Sets the derivative gain of the tilt PID controller.
`V<value>`	`Kv_outer`	Sets the position-hold gain of the outer velocity loop.
`A<value>`	`Kvel_p`	Sets the velocity damping gain of the outer loop.
`W<value>`	`Kyaw_p`	Sets the proportional gain for unwanted rotation damping.
`Z<value>`	`Kyaw_i`	Sets the integral gain for heading hold.
`X`	`log_buffer`	Prints the captured balance-run log as CSV data.
`?`	Commander command list	Prints all commands registered in this Commander instance.
`@<mode>`	`verbose`	Selects the Commander response verbosity mode.
`#<n>`	`decimal_places`	Selects the number of digits printed after the decimal point.

Raspberry Pi - `Serial1` (`commander_ctrl`)¶

Command	Target variable / object	Description
`F<value>`	`v_target`	Sets normalized forward speed from `-1` to `1`.
`T<value>`	`yaw_rate_target`	Sets normalized turn rate from `-1` to `1`.
`H<value>`	`height_cmd`	Sets the leg height command from `0` to `1`.
`S<angle>`	`tilt_setpoint`	Sets the target balance angle in degrees.
`P<value>`	`Kp_tilt`	Sets the proportional gain of the tilt PID controller.
`I<value>`	`Ki_tilt`	Sets the integral gain of the tilt PID controller.
`D<value>`	`Kd_tilt`	Sets the derivative gain of the tilt PID controller.
`?`	Commander command list	Prints all commands registered in this Commander instance.
`@<mode>`	`verbose`	Selects the Commander response verbosity mode.
`#<n>`	`decimal_places`	Selects the number of digits printed after the decimal point.