class CAN – controller area network communication bus

CAN implements the standard CAN communications protocol. At the physical level it 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 (works without anything connected):

from pyb import CAN
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


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 busses are:

  • CAN(1) is on YA: (RX, TX) = (Y3, Y4) = (PB8, PB9)
  • CAN(2) is on YB: (RX, TX) = (Y5, Y6) = (PB12, PB13)

Class Methods

classmethod CAN.initfilterbanks(nr)

Reset and disable all filter banks and assign how many banks should be available for CAN(1).

STM32F405 has 28 filter banks that are shared between the two available CAN bus controllers. This function configures how many filter banks should be assigned to each. nr is the number of banks that will be assigned to CAN(1), the rest of the 28 are assigned to CAN(2). At boot, 14 banks are assigned to each controller.


CAN.init(mode, extframe=False, prescaler=100, *, sjw=1, bs1=6, bs2=8, auto_restart=False)

Initialise the CAN bus with the given parameters:

  • if extframe is True then the bus uses extended identifiers in the frames (29 bits); otherwise it uses standard 11 bit identifiers
  • prescaler is used to set the duration of 1 time quanta; the time quanta will be the input clock (PCLK1, see pyb.freq()) divided by the prescaler
  • sjw is the resynchronisation jump width in units of the time quanta; it can be 1, 2, 3, 4
  • bs1 defines the location of the sample point in units of the time quanta; it can be between 1 and 1024 inclusive
  • bs2 defines the location of the transmit point in units of the time quanta; it can be between 1 and 16 inclusive
  • 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

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.


Turn off the CAN bus.


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.


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).[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)

Configure a filter bank:

  • bank is the filter bank that is to be configured.
  • mode is the mode the filter should operate in.
  • 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
CAN.LIST16 Four 16 bit ids that will be accepted
CAN.LIST32 Two 32 bit ids that will be accepted
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.
  • rtr 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.
mode length of rtr array

Clear and disables a filter bank:

  • bank is the filter bank that is to be cleared.

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 four values.

  • The id of the message.
  • 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 fourth element in the tuple).

If list is not None then it should be a list object with a least four elements. The fourth 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 three 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, memoryview(buf)]
# No heap memory is allocated in the following call
can.recv(0, lst)
CAN.send(data, id, *, timeout=0, rtr=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.

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.

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):
  if reason == 0:
  if reason == 1:
  if reason == 2:

can.rxcallback(0, cb0)



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


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


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