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class CAN – controller area network communication bus

CAN implements support for classic CAN (available on F4, F7 MCUs) and CAN FD (H7 series) controllers. At the physical level CAN bus consists of 2 lines: RX and TX. Note that to connect the pyboard to a CAN bus you must use a CAN transceiver to convert the CAN logic signals from the pyboard to the correct voltage levels on the bus.

Example usage for classic CAN controller in Loopback (transceiver-less) mode:

from pyb import CAN
can = CAN(1, CAN.LOOPBACK)
can.setfilter(0, CAN.LIST16, 0, (123, 124, 125, 126))  # set a filter to receive messages with id=123, 124, 125 and 126
can.send('message!', 123)   # send a message with id 123
can.recv(0)                 # receive message on FIFO 0

Example usage for CAN FD controller with all of the possible options enabled:

# FD frame + BRS mode + Extended frame ID. 500 Kbit/s for arbitration phase, 1Mbit/s for data phase.
can = CAN(1, CAN.NORMAL, baudrate=500_000, brs_baudrate=1_000_000, sample_point=80)
can.setfilter(0, CAN.RANGE, 0, (0xFFF0, 0xFFFF))
can.send('a'*64, 0xFFFF, fdf=True, brs=True, extframe=True)
can.recv(0)

The following CAN module functions and their arguments are available for both classic and FD CAN controllers, unless otherwise stated.

Constructors

class pyb.CAN(bus, ...)

Construct a CAN object on the given bus. bus can be 1-2, or 'YA' or 'YB'. With no additional parameters, the CAN object is created but not initialised (it has the settings from the last initialisation of the bus, if any). If extra arguments are given, the bus is initialised. See CAN.init() for parameters of initialisation.

The physical pins of the CAN buses are:

  • CAN(1) is on YA: (RX, TX) = (Y3, Y4) = (PB8, PB9)

  • CAN(2) is on YB: (RX, TX) = (Y5, Y6) = (PB12, PB13)

Methods

CAN.init(mode, prescaler=100, *, sjw=1, bs1=6, bs2=8, auto_restart=False, baudrate=0, sample_point=75,
num_filter_banks=14, brs_sjw=1, brs_bs1=8, brs_bs2=3, brs_baudrate=0, brs_sample_point=75)

Initialise the CAN bus with the given parameters:

  • mode is one of: NORMAL, LOOPBACK, SILENT, SILENT_LOOPBACK

  • prescaler is the value by which the CAN input clock is divided to generate the nominal bit time quanta. The prescaler can be a value between 1 and 1024 inclusive for classic CAN, and between 1 and 512 inclusive for CAN FD.

  • sjw is the resynchronisation jump width in units of time quanta for nominal bits; it can be a value between 1 and 4 inclusive for classic CAN, and between 1 and 128 inclusive for CAN FD.

  • bs1 defines the location of the sample point in units of the time quanta for nominal bits; it can be a value between 1 and 16 inclusive for classic CAN, and between 2 and 256 inclusive for CAN FD.

  • bs2 defines the location of the transmit point in units of the time quanta for nominal bits; it can be a value between 1 and 8 inclusive for classic CAN, and between 2 and 128 inclusive for CAN FD.

  • auto_restart sets whether the controller will automatically try and restart communications after entering the bus-off state; if this is disabled then restart() can be used to leave the bus-off state

  • baudrate if a baudrate other than 0 is provided, this function will try to automatically calculate the CAN nominal bit time (overriding prescaler, bs1 and bs2) that satisfies both the baudrate and the desired sample_point.

  • sample_point given in a percentage of the nominal bit time, the sample_point specifies the position of the bit sample with respect to the whole nominal bit time. The default sample_point is 75%.

  • num_filter_banks for classic CAN, this is the number of banks that will be assigned to CAN(1), the rest of the 28 are assigned to CAN(2).

  • brs_prescaler is the value by which the CAN FD input clock is divided to generate the data bit time quanta. The prescaler can be a value between 1 and 32 inclusive.

  • brs_sjw is the resynchronisation jump width in units of time quanta for data bits; it can be a value between 1 and 16 inclusive

  • brs_bs1 defines the location of the sample point in units of the time quanta for data bits; it can be a value between 1 and 32 inclusive

  • brs_bs2 defines the location of the transmit point in units of the time quanta for data bits; it can be a value between 1 and 16 inclusive

  • brs_baudrate if a baudrate other than 0 is provided, this function will try to automatically calculate the CAN data bit time (overriding brs_prescaler, brs_bs1 and brs_bs2) that satisfies both the baudrate and the desired brs_sample_point.

  • brs_sample_point given in a percentage of the data bit time, the brs_sample_point specifies the position of the bit sample with respect to the whole data bit time. The default brs_sample_point is 75%.

The time quanta tq is the basic unit of time for the CAN bus. tq is the CAN prescaler value divided by PCLK1 (the frequency of internal peripheral bus 1); see pyb.freq() to determine PCLK1.

A single bit is made up of the synchronisation segment, which is always 1 tq. Then follows bit segment 1, then bit segment 2. The sample point is after bit segment 1 finishes. The transmit point is after bit segment 2 finishes. The baud rate will be 1/bittime, where the bittime is 1 + BS1 + BS2 multiplied by the time quanta tq.

For example, with PCLK1=42MHz, prescaler=100, sjw=1, bs1=6, bs2=8, the value of tq is 2.38 microseconds. The bittime is 35.7 microseconds, and the baudrate is 28kHz.

See page 680 of the STM32F405 datasheet for more details.

CAN.deinit()

Turn off the CAN bus.

CAN.restart()

Force a software restart of the CAN controller without resetting its configuration.

If the controller enters the bus-off state then it will no longer participate in bus activity. If the controller is not configured to automatically restart (see init()) then this method can be used to trigger a restart, and the controller will follow the CAN protocol to leave the bus-off state and go into the error active state.

CAN.state()

Return the state of the controller. The return value can be one of:

  • CAN.STOPPED – the controller is completely off and reset;

  • CAN.ERROR_ACTIVE – the controller is on and in the Error Active state (both TEC and REC are less than 96);

  • CAN.ERROR_WARNING – the controller is on and in the Error Warning state (at least one of TEC or REC is 96 or greater);

  • CAN.ERROR_PASSIVE – the controller is on and in the Error Passive state (at least one of TEC or REC is 128 or greater);

  • CAN.BUS_OFF – the controller is on but not participating in bus activity (TEC overflowed beyond 255).

CAN.info([list])

Get information about the controller’s error states and TX and RX buffers. If list is provided then it should be a list object with at least 8 entries, which will be filled in with the information. Otherwise a new list will be created and filled in. In both cases the return value of the method is the populated list.

The values in the list are:

  • TEC value

  • REC value

  • number of times the controller enterted the Error Warning state (wrapped around to 0 after 65535)

  • number of times the controller enterted the Error Passive state (wrapped around to 0 after 65535)

  • number of times the controller enterted the Bus Off state (wrapped around to 0 after 65535)

  • number of pending TX messages

  • number of pending RX messages on fifo 0

  • number of pending RX messages on fifo 1

CAN.setfilter(bank, mode, fifo, params, *, rtr, extframe=False)

Configure a filter bank:

  • bank is the classic CAN controller filter bank, or CAN FD filter index, to configure.

  • mode is the mode the filter should operate in, see the tables below.

  • fifo is which fifo (0 or 1) a message should be stored in, if it is accepted by this filter.

  • params is an array of values the defines the filter. The contents of the array depends on the mode argument.

mode

Contents of params array for classic CAN controller

CAN.LIST16

Four 16 bit ids that will be accepted

CAN.LIST32

Two 32 bit ids that will be accepted

CAN.MASK16
Two 16 bit id/mask pairs. E.g. (1, 3, 4, 4)
The first pair, 1 and 3 will accept all ids
that have bit 0 = 1 and bit 1 = 0.
The second pair, 4 and 4, will accept all ids
that have bit 2 = 1.

CAN.MASK32

As with CAN.MASK16 but with only one 32 bit id/mask pair.

mode

Contents of params array for CAN FD controller

CAN.RANGE

Two ids that represent a range of accepted ids.

CAN.DUAL

Two ids that will be accepted. For example (1, 2)

CAN.MASK

One filter ID and a mask. For example (0x111, 0x7FF)

  • rtr For classic CAN controllers, this is an array of booleans that states if a filter should accept a remote transmission request message. If this argument is not given then it defaults to False for all entries. The length of the array depends on the mode argument. For CAN FD, this argument is ignored.

mode

length of rtr array

CAN.LIST16

4

CAN.LIST32

2

CAN.MASK16

2

CAN.MASK32

1

  • extframe If True the frame will have an extended identifier (29 bits), otherwise a standard identifier (11 bits) is used.

CAN.clearfilter(bank, extframe=False)

Clear and disables a filter bank:

  • bank is the classic CAN controller filter bank, or CAN FD filter index, to clear.

  • extframe For CAN FD controllers, if True, clear an extended filter (configured with extframe=True), otherwise the clear a standard identifier (configured with extframe=False).

CAN.any(fifo)

Return True if any message waiting on the FIFO, else False.

CAN.recv(fifo, list=None, *, timeout=5000)

Receive data on the bus:

  • fifo is an integer, which is the FIFO to receive on

  • list is an optional list object to be used as the return value

  • timeout is the timeout in milliseconds to wait for the receive.

Return value: A tuple containing five values.

  • The id of the message.

  • A boolean that indicates if the message ID is standard or extended.

  • A boolean that indicates if the message is an RTR message.

  • The FMI (Filter Match Index) value.

  • An array containing the data.

If list is None then a new tuple will be allocated, as well as a new bytes object to contain the data (as the fifth element in the tuple).

If list is not None then it should be a list object with a least five elements. The fifth element should be a memoryview object which is created from either a bytearray or an array of type ‘B’ or ‘b’, and this array must have enough room for at least 8 bytes. The list object will then be populated with the first four return values above, and the memoryview object will be resized inplace to the size of the data and filled in with that data. The same list and memoryview objects can be reused in subsequent calls to this method, providing a way of receiving data without using the heap. For example:

buf = bytearray(8)
lst = [0, 0, 0, 0, memoryview(buf)]
# No heap memory is allocated in the following call
can.recv(0, lst)
CAN.send(data, id, *, timeout=0, rtr=False, extframe=False, fdf=False, brs=False)

Send a message on the bus:

  • data is the data to send (an integer to send, or a buffer object).

  • id is the id of the message to be sent.

  • timeout is the timeout in milliseconds to wait for the send.

  • rtr is a boolean that specifies if the message shall be sent as a remote transmission request. If rtr is True then only the length of data is used to fill in the DLC slot of the frame; the actual bytes in data are unused.

  • extframe if True the frame will have an extended identifier (29 bits), otherwise a standard identifier (11 bits) is used.

  • fdf for CAN FD controllers, if set to True, the frame will have an FD frame format, which supports data payloads up to 64 bytes.

  • brs for CAN FD controllers, if set to True, the bitrate switching mode is enabled, in which the data phase is transmitted at a differet bitrate. See CAN.init() for the data bit timing configuration parameters.

If timeout is 0 the message is placed in a buffer in one of three hardware buffers and the method returns immediately. If all three buffers are in use an exception is thrown. If timeout is not 0, the method waits until the message is transmitted. If the message can’t be transmitted within the specified time an exception is thrown.

Return value: None.

CAN.rxcallback(fifo, fun)

Register a function to be called when a message is accepted into a empty fifo:

  • fifo is the receiving fifo.

  • fun is the function to be called when the fifo becomes non empty.

The callback function takes two arguments the first is the can object it self the second is a integer that indicates the reason for the callback.

Reason

0

A message has been accepted into a empty FIFO.

1

The FIFO is full

2

A message has been lost due to a full FIFO

Example use of rxcallback:

def cb0(bus, reason):
  print('cb0')
  if reason == 0:
      print('pending')
  if reason == 1:
      print('full')
  if reason == 2:
      print('overflow')

can = CAN(1, CAN.LOOPBACK)
can.rxcallback(0, cb0)

Constants

CAN.NORMAL
CAN.LOOPBACK
CAN.SILENT
CAN.SILENT_LOOPBACK

The mode of the CAN bus used in init().

CAN.STOPPED
CAN.ERROR_ACTIVE
CAN.ERROR_WARNING
CAN.ERROR_PASSIVE
CAN.BUS_OFF

Possible states of the CAN controller returned from state().

CAN.LIST16
CAN.MASK16
CAN.LIST32
CAN.MASK32

The operation mode of a filter used in setfilter() for classic CAN.

CAN.DUAL
CAN.RANGE
CAN.MASK

The operation mode of a filter used in setfilter() for CAN FD.