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CAN module

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Jaume Olivé Petrus edited this page Feb 15, 2020 · 23 revisions

About this

This module contains functions for accessing the Controller Area Network (CAN) module.

CAN modules are encoded into a byte and are platform-dependent. For this reason the CAN module defines a numeric constant for each available CAN module.

In ESP32 only one CAN module is available, and it's defined by the constant can.CAN0.

The functions of this module are organized in the following categories:

Key concepts

A Controller Area Network (CAN bus) is a vehicle bus standard designed to allow MCUs and devices to communicate with each other in applications without a host computer. It is a message-based protocol, designed originally for multiplex electrical wiring within automobiles, but is also used in many other contexts.

ESP32 Notes

The ESP32 CAN module is based on the SJA100 CAN controller, and needs an external CAN transceiver, such as the VP231.

Proposed connection:

ESP32 VP231 Description
GPIO5 / GPIO12 / GPIO25 D CAN TX
GPIO4 / GPIO14 / GPIO35 R CAN RX

CAN TX / CAN RX pin assignment is done in build-time through Kconfig under Lua RTOS -> Hardware -> CAN pin map.

Configuration functions

can.attach(id, speed, [rx queue] )

Attach a CAN device to a CAN module. When the device is attached no filters are defined, and all CAN packets are accepted.

Arguments:

  • id: CAN module identifier, for example can.CAN0.
  • speed: speed for the CAN bus expressed in Kbps. Can be either 1000, 800, 500, 250, 125, 100, 50 or 25.
  • rx queue (optional): the size of the rx queue expressed in number of frames. If not provided rx queue is set to 200 frames.

Returns: noting, or an exception.

-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

can.addfilter(id, from, to)

Add a filter. Only CAN packets that match the filter will be accepted.

Arguments:

  • id: CAN module identifier, for example can.CAN0.
  • from: first id.
  • to: last id.

Returns: noting, or an exception.

-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

-- Accept only packets from 200 to 250
can.addfilter(can.CAN0, 200, 250)

can.removefilter(id, from, to)

Remove a filter. If all filters are removed, all CAN packets will be accepted.

Arguments:

  • id: CAN module identifier, for example can.CAN0.
  • from: first id.
  • to: last id.

Returns: noting, or an exception.

-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

-- Accept only packets from 200 to 250
can.addfilter(can.CAN0, 200, 250)

....

-- Remove filter from 200 to 250
can.removefilter(can.CAN0, 200, 250)

Operation functions

can.send(id, frame id, frame type, len, data)

Send a CAN data frame over the CAN bus.

Arguments:

  • id: CAN module identifier.
  • frame id: frame identifier.
  • frame type: frame type, can be either can.STD for a 11-bit identifier, or can.EXT for a 29-bit identifier.
  • len: length of the frame (maximum of 8 byte according to CAN specs).
  • data: a string with the data to send.

Returns: nothing, or an exception.

-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

-- Send a data frame with standard identifier 100 and contents 1234
can.send(can.CAN0, 100, can.STD, 4, "1234")

frame id, frame type, len, data = can.receive(id, [timeout)

Receives a CAN data frame from the CAN bus. It's the programmer's responsibility to filter the received frames, because filter capabilities are not currently implemented.

Arguments:

  • id: CAN module identifier.
  • timeout (optional): a timeout expressed in msecs. If no CAN message is received within the timeout a can.error.TransmitTimeout exception is throwed. If timeout is nil, or not provided, the functions blocks the current thread until a CAN message is received.

Returns:

  • frame id: frame identifier.
  • frame type: frame type, can be can.STD, or can.EXT.
  • len: length of the received frame.
  • data: a string with the received frame data.
-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

-- Receive data frame
id, type, len, data = can.receive(can.CAN0)

Information functions

can.stats([table])

Get the CAN bus statistics.

Arguments:

  • table: if true, the statistics are placed into a Lua table, if false the statistics are printed on the console.

Returns:

  • if table is false: nothing or an exception.

  • if table is true: a Lua table with the CAN bus statistics, or an exception. The table has the following fields:

    • rx_packets: number of received packets
    • rx_bytes: number of bytes received
    • tx_packets: number of transmitted packets
    • tx_bytes: number of bytes transmitted
    • hw_overruns: number of hardware overruns errors
    • sw_overruns: number of software overruns errors
    • bus_error: number of bus errors
    • arbitration_lost_errors: number of arbitration lost errors
    • passive_errors: number of passive errors
    • irq_errors: number of irq errors
/ > can.stats()
RX packets:147 bytes:0
TX packets:3 bytes:24
hw overruns:0
sw overruns:0
errors:
   hw overruns:0
   sw overruns:0
   bus:0 arbitration lost:0 passive:0 irq:0

Utility functions

can.dump(id)

This function let you sniff CAN packets from the CAN interface. For exit, press Ctrl-c.

Arguments:

  • id: CAN module identifier.

Returns: nothing, or an exception.

-- Attach the CAN device to CAN0 at 500 Kbps
can.attach(can.CAN0, 500)

can.dump(can.CAN0)
can0  00000064  [8] 10 bf ba 52 4d 3f dd 80
can0  00000064  [8] 9e cf e2 09 aa 2f 29 1e
can0  00000064  [8] fa 38 e3 9b 2f 31 0d 17
can0  00000064  [8] 4e af a4 dd b6 94 fb d0
can0  00000064  [8] df e7 61 e2 9b 62 4c c3
can0  00000064  [8] ba 4e ed 46 20 5e 45 9c
can0  00000064  [8] f9 55 e7 60 c3 b0 3f a7
can0  00000064  [8] 9d 5c 17 c3 53 dd ec 15
can0  00000064  [8] d5 c6 39 3c 36 c1 dd b9
can0  00000064  [8] 1c 72 95 d4 01 d3 b3 e6
can0  00000064  [8] 34 5d 14 a7 f9 92 7a 1e
can0  00000064  [8] 56 f8 01 2f 3e a6 3b 1b
can0  00000064  [8] 16 64 a5 85 5e 03 10 43
can0  00000064  [8] 58 87 ec 33 b7 0e a1 7e
can0  00000064  [8] bc 85 ed 8a ec fc ae 61
can0  00000064  [8] 5b da b9 63 19 a7 62 07
can0  00000064  [8] 9a 8c 9d 69 3a 9f b6 6e
can0  00000064  [8] a5 ea 9b 72 a9 d5 cc 73
can0  00000064  [8] 6e 40 73 23 a0 cf 7e 61
can0  00000064  [8] 08 91 93 d4 56 21 f9 57
can0  00000064  [8] 5a b1 2f 5d 3c b0 3a 56
can0  00000064  [8] a2 89 b5 98 cf 18 08 a8
can0  00000064  [8] 2e e4 b8 d8 b6 15 66 72
can0  00000064  [8] 49 3f 2b e7 a6 7a 8e b2
can0  00000064  [8] c3 99 9f fd 0f 63 08 47
can0  00000064  [8] 71 e8 0a 42 74 21 37 81
can0  00000064  [8] 05 f1 e7 41 e1 52 42 40
can0  00000064  [8] 67 c8 2f 5d ea 86 53 55

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