Initial CAN Physical Layer spec
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# CAN Physical Layer
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## Purpose
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This module defines the NeoECU V1 physical interface to the vehicle CAN FD
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network. It covers the MCU interface, transceiver, harness topology,
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termination, protection, grounding, layout, and required fault behaviour. It
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does not define CAN identifiers, payloads, or firmware scheduling.
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CAN is a telemetry and auxiliary-communications interface. It is not part of
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the hardware engine-permit path: loss, overload, or bus-off of CAN shall never
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remove or assert engine permission.
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This is a schematic-design basis, not a released schematic. Final connector
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part numbers, MCU pins, protection component values, and validated harness
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details remain to be confirmed.
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## V1 network definition
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| Item | V1 decision |
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| --- | --- |
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| Protocol | ISO 11898-2 high-speed CAN FD, with Classical CAN frames also supported |
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| Nominal/arbitration bit rate | 500 kbit/s |
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| CAN FD data-phase bit rate | 2 Mbit/s |
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| External buses implemented | One vehicle CAN bus |
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| Expected nodes | ECU plus one or two vehicle nodes; a debug tool may join through a service connector |
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| Harness extent | Approximately 2--2.5 m end-to-end |
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| ECU position | Mid-bus node, not a physical endpoint |
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| Harness | 120 ohm nominal twisted pair for `CAN_H` and `CAN_L` |
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| Shield | Not required initially; assess only if the final route must run near ignition or other demonstrated high-noise wiring |
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All V1 nodes connected to this bus shall support CAN FD at the configured
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rates. A Classical-CAN-only node cannot join a CAN FD network that transmits
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FD frames.
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## STM32H747 interface and ownership
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The STM32H747 provides two independent CAN FD controllers, `FDCAN1` and
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`FDCAN2`, with shared message RAM. V1 implements only the one external vehicle
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bus on `FDCAN1`. `FDCAN2` remains available for a future separate network; it
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does not imply a second physical bus or a second transceiver in V1.
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The Cortex-M4 is the intended owner of CAN telemetry and auxiliary
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communications. The Cortex-M7 retains deterministic engine control. Firmware
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shall define the inter-core message and fault-reporting boundary before either
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core accesses FDCAN resources; neither core may independently initialise or
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reconfigure a live controller.
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`PA11` and `PA12` are reserved for the USB-C USB-FS D-/D+ path used by the
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STM32 system-memory DFU recovery route. They shall not be assigned to
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`FDCAN1_RX` or `FDCAN1_TX`. The final `FDCAN1` pin pair shall be selected as
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part of the complete STM32 package and pin allocation, preserving timer, ADC,
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debug, and required serial interfaces. Candidate pairs include `PB8/PB9` and
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`PD0/PD1` where available in the selected package.
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The board shall retain accessible `BOOT0`, `NRST`, and SWD/J-Link provisions.
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USB DFU is a recovery path, not a replacement for normal SWD debugging.
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## Transceiver and local interface
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Use the automotive-qualified **TI TCAN1044AV-Q1** CAN FD transceiver family.
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Select the exact package and orderable suffix at PCB-layout and procurement
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time; the electrical requirements in this module apply to the selected member.
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| Transceiver connection | Required implementation |
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| --- | --- |
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| `VCC` | Supply from the protected 5 V logic rail, with the local decoupling specified by the selected-device datasheet. |
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| `VIO` | Supply from `+3V3_MAIN` so `TXD`, `RXD`, and mode logic interface directly to STM32 GPIO. |
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| `TXD` / `RXD` | Connect to the selected `FDCAN1_TX` / `FDCAN1_RX` GPIO alternate functions. The ECU shall drive recessive when it is not intentionally transmitting. |
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| `STB` | Connect to an MCU GPIO and pull externally high to `+3V3_MAIN`. The transceiver therefore defaults to standby while the MCU is reset, booting, or unpowered; firmware drives it low only after it is healthy and has configured FDCAN. |
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| CAN wake capability | Retain the `STB`/`RXD` hardware capability, but V1 has no requirement for an incoming CAN frame to wake a sleeping ECU. |
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| `CANH` / `CANL` | Route only through the defined protection/EMC network to the vehicle-I/O/CAN connector. |
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The TCAN1044AV-Q1 uses the 5 V rail for its bus driver and `VIO` for its 3.3 V
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logic interface. Its high-impedance unpowered behaviour is required so an
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unpowered ECU does not load a live vehicle bus. Galvanic isolation is not part
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of V1: all vehicle nodes require a defined common circuit reference.
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## Topology, termination, and service access
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The vehicle wiring shall be one continuous linear trunk, with a physical bus
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endpoint at the rear and another at the front:
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[Editable diagram](../diagrams/can-topology.drawio) · [PNG preview](../diagrams/can-topology.png)
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The two segments shown above are trunk segments, not long stubs. The ECU
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transceiver joins the trunk locally on the PCB. Do not make the ECU a 0.5--1 m
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branch from a separate harness junction.
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Fit one 120 ohm, 1 %, normally-open termination option across the ECU `CAN_H`
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and `CAN_L` pins, controlled by a clearly labelled solder jumper. It is
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populated only when the ECU is a physical endpoint. The two actual endpoints
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of the installed bus each require one 120 ohm terminator; the intended
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effective resistance measured across a de-energised, correctly terminated bus
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is approximately 60 ohm.
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The debug/service connector joins the same bus and shall expose `CAN_H`,
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`CAN_L`, and `CAN_0V`. It shall not fit termination. Keep the on-board service
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branch very short and avoid a permanently attached, long unterminated service
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cable. A connected debug tool counts as a CAN node and must support the V1
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CAN FD rates.
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## Harness reference and shielding
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`CAN_H` and `CAN_L` shall be one dedicated twisted pair. A shared vehicle
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circuit return must provide a controlled common-mode reference between every
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node. Expose this return as `CAN_0V` at the ECU and service connectors. It may
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be the defined low-current vehicle power-return conductor where that conductor
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is continuous and sized for the connected nodes; it shall not be an
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ignition/injector/starter current-return path.
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`CAN_0V` is an electrical reference, not a termination conductor and not a
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cable screen. No shield is required by the V1 2--2.5 m harness definition.
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Route the twisted pair away from ignition-primary/high-voltage wiring,
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injector switching loops, and high-current power pairs. If a long parallel
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route next to one of those sources becomes unavoidable, reassess the shield
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and common-mode filtering from measured EMC results rather than treating a
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screen as a substitute for a twisted pair or a reference conductor.
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## Connector protection and EMC provisions
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Place a dual-channel, automotive-qualified, low-capacitance CAN/CAN-FD TVS
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device directly behind the vehicle-I/O/CAN connector. It protects `CAN_H` and
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`CAN_L` against connector ESD and assigned cable transients before they reach
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the transceiver. The final part shall be selected after the connector,
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enclosure, and transient environment are known. A suitable class of candidate
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is Nexperia `PESD2CANFD24L-U`; its final standoff, dynamic-clamp, capacitance,
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temperature, and qualification figures shall be checked against the completed
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system rather than inferred from the part label.
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The primary TVS return shall have a short, low-inductance path to the defined
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connector-side `CAN_0V`/vehicle-reference return. It shall not dump cable
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transient current through the `+3V3_MAIN` or analogue-ground paths. The final
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PCB stack-up, connector shell, enclosure, and any frame bond must establish
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the actual high-frequency return path before this routing is frozen.
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Provide an assembly-selectable common-mode-choke option between the
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connector-side protection network and the transceiver. The default V1
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population is a direct/bypassed path, because the selected transceiver has
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strong EMC performance and an unnecessary choke can reduce signal margin.
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The PCB shall support a choke or its defined bypass arrangement without
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changing the differential-pair geometry. Fit and select a choke only if
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hardware EMC testing demonstrates a need.
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Do not add arbitrary series resistance, split termination, or bus capacitance
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without a timing and EMC reason. Any such option must be evaluated against the
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500 kbit/s / 2 Mbit/s bit timing and the final cable capacitance.
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## Layout requirements
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1. Place the connector-side TVS at the connector entry and keep its return
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loop short and wide.
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2. Route `CAN_H` and `CAN_L` as a symmetric differential pair through the
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protection/EMC network to the transceiver. Avoid stubs, unmatched component
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placements, and large asymmetry between the two paths.
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3. Place the transceiver's 5 V and 3.3 V decoupling capacitors immediately at
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their respective supply pins, using the datasheet-recommended values and
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return paths.
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4. Keep the transceiver and MCU-side logic traces clear of ignition,
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injector, switching-regulator, and high-current return loops.
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5. Keep the termination jumper and resistor close to the local bus interface;
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label the assembly state and endpoint condition on the schematic and PCB.
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## Fault behaviour and validation
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Firmware shall report CAN controller error state, error-passive state,
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bus-off, and transceiver standby state to diagnostics. Bus-off recovery and
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retry timing shall be deliberate and rate-limited; it must not cause a reset
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storm or repeatedly disturb the other MCU core. A lost CAN network is a
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telemetry/auxiliary-service failure, not an engine-permit event.
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Before schematic freeze, validate the completed interface with the intended
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CAN FD configuration and installed harness:
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1. Confirm `CAN_H`, `CAN_L`, and `CAN_0V` connector pins, endpoint locations,
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cable type, and real end-to-end length.
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2. Confirm the selected STM32H747 package and `FDCAN1` pin pair without
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conflicting with USB FS DFU, timers, ADC, debug, or required serial I/O.
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3. Select and validate the final TVS against the connector ESD and transient
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environment, including its clamp at the transceiver pins and its return
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path.
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4. Verify normal communication, ECU reset/boot standby behaviour, an
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unpowered ECU on a live bus, debug-tool connection, and correct 60 ohm
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termination measurement.
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5. Test error handling and controlled bus-off recovery with a missing
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endpoint, open/shorted bus wires, and a deliberately disturbed node.
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6. Test CAN operation and emissions/immunity with ignition and injector
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switching active. Populate and evaluate the common-mode choke only if the
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baseline routing/protection does not meet the assigned EMC target.
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