ROTEM

ROTEM is a 50 × 50 mm four-layer CAN FD attitude and heading reference built around an ICM-45686 IMU with a fitted MMC5983MA magnetometer and an STM32U575. It reports orientation to robot code, and it reports how much it trusts that orientation.

Specifications

Outline	50 × 50 mm, four layers, four M3 mounting holes
Input	6–28 V, reverse-polarity protected, PTC and TVS on the input
Bus	CAN FD and classic CAN 2.0B at 1 Mbps, split termination on jumpers
IMU	ICM-45686, ±4000 °/s, ±32 g
Magnetometer	MMC5983MA, fitted; fusion off by default
Logging	64 Mbit onboard flash, black-box recorder
Debug	USB-C device port, self-powered
Device type	4 (Gyro Sensor) in the FIRST device-type table

USB-C does not power the board. Main input is required; VBUS is protected sense only.

Wiring, and the rules that constrain it

These are not suggestions. Each one maps to a rule an inspector checks.

Power it from the Power Distribution hub, on a small fuse. R610 requires every circuit to be powered solely by a single protected connector pair of the PD. R622 then sizes wire by the protective device, not by what the board draws — on a 40 A channel it would demand 12 AWG. The exemption is explicit: manufacturer-supplied wires are legal “provided they are powered by the smallest value fuse or breaker which permits proper device operation.”

Required protection	2 A ATM fuse (or the smallest available that holds)
Supplied pigtail	24 AWG power pair, 28 AWG CAN pair
Draw	200 mA at 5 V design ceiling; 180 mA expected

Quote that fuse value to your inspector. Without it, a board on a 40 A channel is technically non-compliant no matter how little current it draws.

Wire colours. R624 requires the positive lead to be red, yellow, white, brown or black-with-stripe, and the negative black or blue, on all non-signal wiring. The supplied pigtail already complies. CAN is signal level and exempt, but the WPILib convention is yellow for CAN-H and green for CAN-L — follow it.

Frame isolation. R611 requires greater than 120 Ω between either battery post and any point on the robot frame, and it names sensors with grounded enclosures as a common failure. All four M3 mounting holes on ROTEM are isolated from copper — no ground plane, no stitching, nothing. You can bolt it to a metal chassis with metal hardware and stay compliant.

Termination. R716 permits a device to sit inline on the bus and forbids anything that interferes with, alters or blocks communication. ROTEM daisy-chains and does neither. Close both termination jumpers only if the module is at a physical end of the bus — in the standard topology the roboRIO and the PD already terminate it, and adding a third terminator will break the bus.

Status lights

Two RGB indicators. SYS reports the device, CAN reports the bus.

SYS	CAN	Meaning
Off	Off	No power, or no firmware
Blue slow blink	Off	Booting and running self-test
Green steady	Alternating green	Healthy, robot enabled
Green steady	Simultaneous amber blink	Healthy, robot disabled
Green steady	Alternating amber	CAN present, no host software
Green or red	Alternating red	No CAN, or bus-off
Blue alternating	Blue alternating	Firmware update or bootloader
Amber	Amber	Recoverable warning — read the health frame
Red	Red/amber alternating	Hardware or self-test fault

There is deliberately no branded light sequence, no fade and no breathing animation. Every state above means something.

Mounting and orientation

Mount pose is configured on the device and persisted, not corrected in robot code. A board mounted sideways then reports robot-frame orientation to everything on the bus — the configuration tool and the black-box log included — rather than only to the one program that remembered to rotate it.

var config = new RotemConfiguration();
config.mountPose.yawDegrees = 90.0;
imu.getConfigurator().apply(config);

The commit is atomic and lands in wear-levelled flash with a CRC, so a brown-out mid-commit leaves the previous configuration intact. Mount calibration that does not survive a power cycle is a defect, not a limitation.

Magnetometer policy

Fusion is off by default, and that is a considered position rather than an oversight. An FRC field is full of motors, steel and current; a heading that silently shifts when a neighbouring robot drives past is worse than no magnetometer at all, because the failure is invisible.

With fusion off the magnetometer still runs as a drift auditor — it reports estimated heading error since zeroing without touching the published heading. Turning fusion on keeps every correction gated by the disturbance detector and applied gradually. Heading never steps.