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Project Specification
Project Overview
  • Name: RallyComm I/O Box Rev A
  • Status: Approved for schematic capture
  • Production-intent vehicle USB dock interface for an aluminum 60 x 42 x 55 mm enclosure.
  • The device is not a phone, radio, audio interface, or wireless product.
Intended Use
  • Connect vehicle 12 V, dual CAN buses, and switched keypad 12 V through a Deutsch pigtail.
  • Present an STM32 USB Full-Speed CDC device to a Sonim XP10 acting as USB host.
  • Independently provide 5 V / 3 A charge-only power to the XP10 rear charging cradle.
What the Device Should Do
  • Survive reverse battery and clamp automotive load-dump transients.
  • Receive vehicle CAN by default, with MCU transmit physically gated off at reset.
  • Communicate bidirectionally on keypad CAN.
  • Switch protected vehicle battery power to the keypad.
  • Operate USB-C strictly as data-device wiring without sourcing VBUS.
  • Source CHG_5V only through the USB-A charge connector.
Main Features
  • STM32G0B1CBT3 MCU using crystal-less HSI48 USB.
  • LM65645-Q1 4.5 A automotive synchronous 12 V to 5 V buck conversion.
  • TLV76733-Q1 3.3 V regulation.
  • TPS2595-family 3 A charge-rail eFuse.
  • TPS1HA08B-Q1 high-side keypad switch.
  • Two TCAN3404-Q1 CAN transceivers.
  • TMP117 temperature sensor, SWD, battery sensing, and two status LEDs.
System Architecture
Diagram



  
    
  
  
    
  






























Vehicle 12 V
Fuse + TVS + reverse block
VBAT_PROT
5 V buck
+5V
3.3 V LDO
+3V3
3 A eFuse
CHG_5V USB-A
Keypad high-side switch
KP_PWR
STM32G0B1
USB-C data only
Vehicle CAN RX-default
Keypad CAN
Hardware Subsystems
  • Input protection: J1 VBAT through 0453005.MR, SMAJ24AHE3 TVS, and SS3P6L-M3 series reverse block to VBAT_PROT.
  • Power conversion: LM65645SRZTRQ1 fixed 5 V, 2.2 MHz buck with Coilcraft XGL4030-222MEC 2.2 uH; corrected TLV76733QWDRBRQ1 to +3V3.
  • Charge output: TPS25954x/TPS2595 configured near 3 A with UVLO near 4.3 V; fault to PA2.
  • Keypad power: TPS1HA08BQPWPRQ1 from VBAT_PROT to KP_PWR, controlled by PA5.
  • Compute: STM32G0B1CBT3, LQFP-48, HSI48 USB, optional DNP 16 MHz crystal only.
  • CAN: Vehicle TCAN3404 with ESD, CMC, standby pull-up, and SN74LVC1G126 TX gate with OE pull-down. Keypad TCAN3404 allows TX.
  • USB: USB-C header carries GND, USB_DP, USB_DM, CC1, and CC2 only. USB-A header carries CHG_5V and GND only.
  • Sensor/debug: TMP117 at 0x48 and four-pin SWD header.
Interfaces and Connections
  • J1: Molex 53261-0871, 8-pin vehicle/keypad pigtail.
  • J2: 1.27 mm 10-pin header to Bulgin USB-C panel cable; no VBUS, VCONN, SBU, or SuperSpeed wiring.
  • J3: 1.27 mm 4-pin SWD (+3V3, SWDIO, SWCLK, GND).
  • J4: Molex 53261-0471 to USB-A charge-only panel cable.
Power and Runtime Expectations
  • Input: 9-16 V nominal automotive supply; 24 V TVS clamp architecture.
  • Charge output target: 5 V / 3 A.
  • Keypad output target: protected 12 V at approximately 1.5-2.5 A.
  • USB-C must never source VBUS and must never connect to CHG_5V.
Power Tree and Power Budget
  • Peak direct keypad load: up to 2.5 A from VBAT_PROT.
  • 5 V expected peak is 3.3 A and the buck design target is 3.5 A. LM65645-Q1 provides a 4.5 A rating (28.6% margin above the design target).
  • The specified 5 A input fuse provides margin for simultaneous keypad and charging loads at low vehicle voltage; final thermal and transient validation remains required.
Manufacturing and Assembly Expectations
  • 4-layer, 48.00 x 32.00 x 1.60 mm PCB.
  • Production SMD assembly; exact specified MPNs and official packages.
  • Maximum component height 3.0 mm except permitted low-profile right-angle/pigtail-zone parts within the defined 12 mm zone.
  • Four plated GND mounting holes: 2.20 mm drill, 4.0 mm pad at (2.50,2.50), (45.50,2.50), (2.50,29.50), (45.50,29.50) mm.
Firmware-Relevant Hardware Requirements
  • USB FS device/CDC using PA11 DM and PA12 DP with HSI48.
  • FDCAN mappings exactly as specified for vehicle and keypad buses.
  • Vehicle TX enable PB3 defaults low through 10 kOhm.
  • CAN standby pins default high through 10 kOhm.
  • ADC sensing on PA0 and PA1; eFuse fault on PA2; keypad control on PA5.
  • SWD on PA13/PA14; BOOT0 pulled down and NRST pulled up with 100 nF filter.
Physical Design Expectations
  • Origin at lower-left; +X toward USB connectors and +Y toward SWD/USB-A pigtail.
  • Placement zones: X=0-12 mm input; X=12-32 mm MCU/CAN/keypad; X=32-48 mm regulators/USB.
  • In1.Cu continuous GND; In2.Cu power islands; USB 90 Ohm differential and CAN 120 Ohm differential routing targets.
Important Design Decisions
  • USB-C is self-powered data-device signaling only: independent 5.11 kOhm Rd on CC1 and CC2, with no board connection to cable VBUS.
  • USB-A is charge-only and does not short D+ to D-.
  • Vehicle CAN has no fitted 120 Ohm termination and cannot transmit until firmware explicitly enables U8.
  • Keypad power is always sourced from VBAT_PROT, never from a 5 V or USB rail.
  • No USB hub, PD controller, audio, wireless, second MCU, coin cell, or external battery-pack circuitry.
Assumptions
  • Where the specification permits a TPS25954x or TPS2595 variant, select the available automotive-suitable library part that supports the required current limit and UVLO implementation, and record it as a substitution/selection.
  • Datasheet-recommended support values take precedence where the supplied text says to follow the manufacturer's typical circuit.
Change Notes
  • Rev A requirements imported from the user-provided specification on 2026-08-18.

  • Project Overview

  • Intended Use

  • What the Device Should Do

  • Main Features

  • System Architecture

  • Hardware Subsystems

  • Interfaces and Connections

  • 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