Comma.ai Panda Board
Thought for 5 sec
Use TI TCAN3404-Q1 for each of the three CAN channels.
Why it fits:
- CAN FD: supports data rates up to 8 Mbps
- Automotive qualified: AEC-Q100
- 3.3 V MCU interface: compatible with the STM32H725
- Wide bus fault protection: ±58 V
- Low-power standby mode
- Small VSON package: suitable for the 85 × 50 mm board
Recommended orderable part:
- TCAN3404DRBRQ1
- Package: 8-pin VSON/DRB, 3 × 3 mm
- Quantity: 3
Use one TCAN3404-Q1 per channel, followed by the channel’s common-mode choke and PESD2CAN protection. Verify that the footprint includes the exposed thermal pad and connect it to ground with nearby vias.
Recommand NXP product
Thought for 14 sec
I recommend the NXP TJA1044GT/3 for each CAN channel.
- Automotive AEC-Q100
- CAN FD up to 5 Mbit/s
- Designed for direct use with 3.3 V microcontrollers, including STM32H725
- Standby and wake-up support
- SO8 package, easier to assemble than VSON
- Bus fault protection and excellent automotive EMC performance
Use three TJA1044GT/3 transceivers, one per CAN-FD channel. For improved signal integrity on long or highly branched automotive networks, consider the newer NXP TJA1443 CAN SIC transceiver, but it is larger and requires more support/configuration.
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Project Specification
Project Overview
Current schematic-phase Red Panda-compatible automotive USB/CAN-FD interface. Status: Review. The design targets comma.ai panda firmware direction using STM32H725, three CAN-FD buses, USB-C device connectivity, and a protected vehicle-battery supply.
Intended Use
Prototype/validation hardware for vehicle network interfacing. It is not safety-certified and must not be used as a sole safety mechanism or to control a vehicle without independent engineering and regulatory review.
What the Device Should Do
- Run STM32H725 panda-derived firmware.
- Exchange USB 2.0 device traffic over USB-C.
- Independently transmit/receive three CAN-FD channels.
- Operate from OBD-II vehicle battery power with reverse, overcurrent, surge, and EMI protection.
- Support SWD programming and visible firmware status.
Main Features
- STM32H725ZGT6, LQFP-144.
- Three TCAN3404-Q1 3.3 V, AEC-Q100, 8 Mbps CAN-FD transceivers.
- USB-C USB 2.0 receptacle, dual 5.1 kΩ Rd, USBLC6-2SC6 ESD, divided VBUS sensing.
- LM65645-Q1 65 V-input automotive buck set to fixed 3.3 V.
- OBD-II 16-pin male connector as the closest verified current Panda-family vehicle interface.
- Three firmware GPIO LEDs and 10-pin Cortex SWD.
System Architecture
See the Block Diagram project file.
Hardware Subsystems
Automotive Power
OBD pin 16 → 750 mA/24 V PPTC → 60 V Schottky reverse-polarity diode → 24 V TVS clamp and ceramic filtering → LM65645-Q1 buck → 3V3. The regulator uses the datasheet 5.6 µH / 400 kHz direction, 100 nF BOOT-to-SW, 1 µF VCC bypass, 2×4.7 µF input ceramics, and 4×33 µF nominal output bank.
MCU Core
All VDD/VDDLDO/VDDA/VREF+/VBAT/VDD33USB pins are powered; all VSS/VSSA/VSSSMPS pins grounded; each VCAP has an independent 2.2 µF low-ESR capacitor. BOOT0 is pulled low, NRST is pulled up and filtered, and unused GPIOs are explicitly no-connect. Current public firmware uses internal clock infrastructure; HSE/LSE pins are left no-connect rather than inventing unsupported crystal values.
CAN-FD
Three independent channels use PD0/PD1 (FDCAN1), PB5/PB6 (FDCAN2), and PD12/PD13 (FDCAN3), with active-high standby pins mapped to public Red Panda control GPIO direction PG11, PB3, and PD7. Each channel has supply decoupling, automotive common-mode choke, PESD2CAN TVS, and split 2×60.4 Ω + 4.7 nF termination network. Termination parts are intended DNP/selectable by assembly option; do not populate on an already terminated vehicle bus.
USB-C
PA11/PA12 are USB DM/DP. Both connector orientations are joined, protected by USBLC6-2SC6, and supplied with independent 5.1 kΩ CC pull-downs. VBUS is divided 100 kΩ/100 kΩ to PA0 for sensing and powers VDD50USB; VDD33USB is on 3V3.
Interfaces and Connections
OBD-II / Vehicle Connector Mapping
Table
| Pin | Function |
|---|---|
| 16 | Vehicle battery input |
| 4 | Power ground |
| 5 | Signal ground |
| 6 / 14 | CAN1 H / L (SAE J1962 standard CAN pair) |
| 3 / 11 | CAN2 H / L (Panda conventional secondary pair) |
| 1 / 9 | CAN3 H / L (Panda conventional tertiary pair) |
| others | No-connect |
The exact proprietary Red Panda enclosure, connector molding, PCB outline, and USB-C-to-OBD relative geometry were not publicly verified. J2 is a real Comtech C-OBD-II-16M library part and is the closest electrically verified family choice; mechanical fit requires later measurement/drawing validation.
Power and Runtime Expectations
Vehicle powered, nominal 12–14.4 V, with cranking/surge exposure. USB is treated as data/VBUS sense, not the primary board-power source.
Power Tree and Power Budget
3V3 estimated 252 mA typical and 600 mA peak. At 3.3 V/0.6 A and 85% efficiency, 6 V cranking input current is about 0.39 A before margin. The 750 mA PPTC is provisionally adequate electrically but must be checked for hot-ambient hold-current derating. TCAN3404-Q1 worst normal dominant current is 55–60 mA each; three simultaneous channels are included in the peak allowance.
Manufacturing and Assembly Expectations
Professional SMD assembly. Automotive temperature and qualification are preferred for active power/CAN parts. D5 and D6 library selections do not have verified AEC qualification metadata and must be upgraded or formally qualified before production.
Firmware-Relevant Hardware Requirements
- USB FS: PA11/PA12.
- FDCAN1: PD0 RX / PD1 TX.
- FDCAN2: PB5 RX / PB6 TX.
- FDCAN3: PD12 RX / PD13 TX.
- CAN standby: PG11, PB3, PD7.
- LEDs: PE4 red, PE3 green, PE2 blue.
- SWD: PA13 SWDIO, PA14 SWCLK, NRST.
Physical Design Expectations
PCB layout intentionally not started. USB-C should be placed on the enclosure-accessible edge and opposite/appropriately oriented to the OBD connector only after public photos and measured mechanical data are reconciled. No exact proprietary geometry is claimed.
Important Design Decisions
- Use a true 3.3 V CAN-FD transceiver to avoid a separate 5 V CAN rail.
- Prioritize three independent CAN-FD channels; omit LIN because current public Red Panda board definition did not clearly require a dedicated LIN transceiver.
- Use internal oscillators unless a pinned firmware clock requirement is later identified.
Assumptions
- OBD pin mapping follows publicly common Panda conventions.
- Split termination is assembly-selectable/DNP, not firmware-switched in this revision.
- Automotive compliance (ISO 7637-2/16750, EMC, ESD, load dump) requires lab validation.
Change Notes
- Created complete schematic blocks, pin mapping, power budget, protection, USB, three CAN-FD channels, MCU support, LEDs, and SWD.
Project Overview
Intended Use
What the Device Should Do
Main Features
System Architecture
Hardware Subsystems
Automotive Power
MCU Core
CAN-FD
USB-C
Interfaces and Connections
OBD-II / Vehicle Connector Mapping
Power and Runtime Expectations
Power Tree and Power Budget
Manufacturing and Assembly Expectations
Firmware-Relevant Hardware Requirements
Physical Design Expectations
Important Design Decisions
Assumptions
Change Notes
