Initial CAN Physical Layer spec
This commit is contained in:
@@ -31,8 +31,9 @@ temperature and pressure sensors, and an external starter switch.
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| Analogue outputs | Reserve 2 MCU/output paths | Future 0-5 V output capability; not a V1 requirement |
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The provisional generic digital I/O counts are planning values, not a frozen
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connector or pin budget. CAN and its physical layer are intentionally outside
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the scope of this document.
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connector or pin budget. CAN is defined separately in
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[CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md); its final connector
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pin allocation remains part of the overall STM32 and harness pin budget.
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## Power-Domain Rules
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@@ -259,4 +260,6 @@ included unless a future peripheral explicitly requires it.
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components for generic digital inputs.
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5. Select the STM32H747 package and complete a pin assignment that preserves
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all timer, ADC, optional DAC, debug, and communications resources.
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6. Define CAN and other communications interfaces separately.
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6. Complete the CAN connector and STM32 pin allocation defined in
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[CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md), alongside any
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other required communications interfaces.
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@@ -0,0 +1,191 @@
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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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@@ -125,7 +125,7 @@ Initial intended I/O categories are:
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| Analogue inputs | Air temperature, coolant/water temperature, pressure sensors, and expansion inputs |
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| Engine outputs | One injector and one ignition-coil primary channel |
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| Auxiliary outputs | Starter-enable logic output to an external high-current MOSFET or IGBT switch |
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| Communications | CAN bus with selectable termination |
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| Communications | One CAN FD vehicle bus with selectable ECU termination; see [CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md) |
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The starter-enable output only commands an external high-current switch. The
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ECU does not carry the starter's approximately 40 A current.
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@@ -141,7 +141,9 @@ selection are finalised:
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3. Required quantity and exact types of spare analogue and digital I/O.
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4. ECU enclosure, board-size, mounting, environmental, and connector
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requirements.
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5. CAN connector and bus topology.
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5. CAN connector family/pin assignment, final cable construction, and
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endpoint locations; the V1 CAN FD physical-layer basis is defined in
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[CAN_PHYSICAL_LAYER.md](IO_MODULES/CAN_PHYSICAL_LAYER.md).
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6. Detailed trigger sensor wiring, connector, and cable-length information.
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## Diagram sources
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@@ -1,6 +1,6 @@
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# Architecture diagram sources
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The architecture Markdown embeds these 22 SVG figures. Matching PNG previews
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The architecture Markdown embeds these 23 SVG figures. Matching PNG previews
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are provided for viewers that do not support SVG. Diagrams describe the working
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architecture; component qualification and schematic-release requirements remain
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in the parent documents.
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@@ -11,6 +11,7 @@ in the parent documents.
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| --- | --- |
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| Power | [Sources](power-sources.svg), [protected branches](power-branches.svg) |
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| Analogue acquisition | [Passive input](analog-input.svg), [thermistor](thermistor.svg), [ratiometric acquisition](ratiometric.svg) |
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| Vehicle CAN FD | [Trunk, termination and service access](can-topology.svg) |
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| Timing inputs | [Crank/cam interface](trigger-input.svg) |
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| Deadman | [Contacts and diagnostics](deadman-inputs.svg), [permit gates](deadman-permit.svg), [output gates](deadman-outputs.svg) |
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| Ignition | [Power](ignition-power.svg), [commands](ignition-command.svg), [current flag](ignition-flag.svg), [grounding](ignition-ground.svg) |
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@@ -20,8 +21,10 @@ in the parent documents.
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## Editing and regeneration
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- The 12 block figures have editable `.drawio` sources. Open them in draw.io.
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`build_blocks.py` recreates their baseline layout and `block-connections.json`.
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- The 13 block figures have editable `.drawio` sources. Open them in draw.io.
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`build_blocks.py` recreates the original 12 block layouts and `block-connections.json`.
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The CAN topology is maintained directly in `can-topology.drawio`, with its
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own `can-topology.connections.json` manifest; the builder does not overwrite it.
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Running it overwrites manual layout edits, so update the generator when those
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edits should be reproducible, or export the edited `.drawio` directly.
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- The 10 circuit/logic figures are generated by `build_circuits.py`, with a
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@@ -0,0 +1,132 @@
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{
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"title": "Vehicle CAN FD topology",
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"source": "IO_MODULES/CAN_PHYSICAL_LAYER.md: Topology, termination, and service access",
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"nodes": {
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"title": {
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"label": "Vehicle CAN FD \u00b7 continuous linear trunk",
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"bounds": [
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30,
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20,
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1100,
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45
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]
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},
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"subtitle": {
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"label": "500 kbit/s arbitration \u00b7 2 Mbit/s data phase \u00b7 approximately 2\u20132.5 m end-to-end",
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"bounds": [
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30,
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75,
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1100,
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40
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]
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},
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"rear": {
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"label": "Rear endpoint\n120 \u03a9 across CAN_H / CAN_L",
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"bounds": [
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30,
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170,
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290,
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105
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]
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},
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"ecu": {
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"label": "ECU \u00b7 local PCB junction\nMid-bus connection",
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"bounds": [
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440,
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170,
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280,
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105
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]
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},
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"front": {
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"label": "Front endpoint\n120 \u03a9 across CAN_H / CAN_L",
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"bounds": [
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840,
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170,
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290,
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105
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]
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},
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"distance1": {
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"label": "0.5\u20131 m",
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"bounds": [
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320,
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130,
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120,
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40
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]
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},
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"distance2": {
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"label": "1\u20131.5 m",
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"bounds": [
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720,
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130,
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120,
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40
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]
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},
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"ecu_note": {
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"label": "ECU transceiver joins locally on PCB.\nOptional ECU 120 \u03a9 termination: OPEN in V1;\nenable only if the ECU becomes an endpoint.",
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"bounds": [
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340,
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305,
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710,
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90
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]
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},
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"service": {
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"label": "Debug / service connector\nCAN_H \u00b7 CAN_L \u00b7 CAN_0V\nVery short PCB branch\nNo termination",
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"bounds": [
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440,
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||||
480,
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350,
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||||
110
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||||
]
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||||
},
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"busport": {
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"label": "Same local ECU bus",
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"bounds": [
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60,
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||||
490,
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||||
270,
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||||
80
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]
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||||
},
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"foot": {
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||||
"label": "Lines above represent the CAN_H / CAN_L twisted pair, not individual conductors.\nAll nodes also share the defined CAN_0V reference. Avoid a long attached service cable.",
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||||
"bounds": [
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||||
30,
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||||
625,
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||||
1100,
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||||
55
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||||
]
|
||||
}
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||||
},
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||||
"connections": [
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||||
{
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||||
"from": "rear",
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||||
"to": "ecu",
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||||
"label": ""
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||||
},
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||||
{
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||||
"from": "ecu",
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"to": "front",
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"label": ""
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||||
},
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{
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"from": "busport",
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"to": "service",
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"label": ""
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}
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],
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"continuations": {
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"busport": "ecu"
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},
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||||
"bus_model": "Each trunk line represents the differential CAN_H/CAN_L pair. CAN_0V is a separate shared reference.",
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"termination": {
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"rear": "120 ohm across CAN_H/CAN_L",
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"front": "120 ohm across CAN_H/CAN_L",
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||||
"ecu": "120 ohm option, solder jumper normally open; endpoint use only",
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||||
"service": "none"
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||||
}
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}
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@@ -0,0 +1,53 @@
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||||
<?xml version='1.0' encoding='utf-8'?>
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||||
<mxfile host="app.diagrams.net">
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||||
<diagram name="Vehicle CAN FD topology" id="can-topology">
|
||||
<mxGraphModel page="1" pageWidth="1160" pageHeight="680">
|
||||
<root>
|
||||
<mxCell id="0" />
|
||||
<mxCell id="1" parent="0" />
|
||||
<mxCell id="title" value="Vehicle CAN FD · continuous linear trunk" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=26;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="30" y="20" width="1100" height="45" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="subtitle" value="500 kbit/s arbitration · 2 Mbit/s data phase · approximately 2–2.5 m end-to-end" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="30" y="75" width="1100" height="40" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="rear" value="Rear endpoint 120 Ω across CAN_H / CAN_L" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f4ed;" vertex="1" parent="1">
|
||||
<mxGeometry x="30" y="170" width="290" height="105" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="ecu" value="ECU · local PCB junction Mid-bus connection" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;" vertex="1" parent="1">
|
||||
<mxGeometry x="440" y="170" width="280" height="105" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="front" value="Front endpoint 120 Ω across CAN_H / CAN_L" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f4ed;" vertex="1" parent="1">
|
||||
<mxGeometry x="840" y="170" width="290" height="105" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="rear_ecu" source="rear" target="ecu" value="" edge="1" parent="1" style="edgeStyle=orthogonalEdgeStyle;rounded=0;html=0;startArrow=none;endArrow=none;strokeWidth=3;strokeColor=#416680;fontSize=18;fontFamily=DejaVu Sans;labelBackgroundColor=#ffffff;exitX=1;exitY=0.5;entryX=0;entryY=0.5;">
|
||||
<mxGeometry relative="1" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="ecu_front" source="ecu" target="front" value="" edge="1" parent="1" style="edgeStyle=orthogonalEdgeStyle;rounded=0;html=0;startArrow=none;endArrow=none;strokeWidth=3;strokeColor=#416680;fontSize=18;fontFamily=DejaVu Sans;labelBackgroundColor=#ffffff;exitX=1;exitY=0.5;entryX=0;entryY=0.5;">
|
||||
<mxGeometry relative="1" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="distance1" value="0.5–1 m" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=17;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="320" y="130" width="120" height="40" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="distance2" value="1–1.5 m" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=17;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="720" y="130" width="120" height="40" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="ecu_note" value="ECU transceiver joins locally on PCB. Optional ECU 120 Ω termination: OPEN in V1; enable only if the ECU becomes an endpoint." style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="340" y="305" width="710" height="90" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="service" value="Debug / service connector CAN_H · CAN_L · CAN_0V Very short PCB branch No termination" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#eef0f3;" vertex="1" parent="1">
|
||||
<mxGeometry x="440" y="480" width="350" height="110" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="busport" value="Same local ECU bus" style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=18;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;" vertex="1" parent="1">
|
||||
<mxGeometry x="60" y="490" width="270" height="80" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="service_branch" source="busport" target="service" value="" edge="1" parent="1" style="edgeStyle=orthogonalEdgeStyle;rounded=0;html=0;startArrow=none;endArrow=none;strokeWidth=2;strokeColor=#416680;fontSize=18;fontFamily=DejaVu Sans;labelBackgroundColor=#ffffff;exitX=1;exitY=0.5;entryX=0;entryY=0.5;">
|
||||
<mxGeometry relative="1" as="geometry" />
|
||||
</mxCell>
|
||||
<mxCell id="foot" value="Lines above represent the CAN_H / CAN_L twisted pair, not individual conductors. All nodes also share the defined CAN_0V reference. Avoid a long attached service cable." style="rounded=1;html=0;fontFamily=DejaVu Sans;fontSize=17;fontColor=#172b40;strokeColor=#577086;fillColor=#e8f0fc;strokeColor=none;fillColor=none;align=left;" vertex="1" parent="1">
|
||||
<mxGeometry x="30" y="625" width="1100" height="55" as="geometry" />
|
||||
</mxCell>
|
||||
</root>
|
||||
</mxGraphModel>
|
||||
</diagram>
|
||||
</mxfile>
|
||||
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|
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|
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Reference in New Issue
Block a user