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.
Note that this driver does not yet support filter configuration (it defaults to a single filter that lets through all messages), or bus timing configuration (except for setting the prescaler).
Example usage (works without anything connected):
from pyb import CAN
can = pyb.CAN(1, pyb.CAN.LOOPBACK)
can.send('message!', 123) # send message to id 123
can.recv(0) # receive message on FIFO 0
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 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)
Methods¶
- can.init(mode, extframe=False, prescaler=100, *, sjw=1, bs1=6, bs2=8)¶
Initialise the CAN bus with the given parameters:
- mode is one of: NORMAL, LOOPBACK, SILENT, SILENT_LOOPBACK
- 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
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.any(fifo)¶
Return True if any message waiting on the FIFO, else False.
- can.recv(fifo, *, timeout=5000)¶
Receive data on the bus:
- fifo is an integer, which is the FIFO to receive on
- timeout is the timeout in milliseconds to wait for the receive.
Return value: buffer of data bytes.
- can.send(send, addr, *, timeout=5000)¶
Send a message on the bus:
- send is the data to send (an integer to send, or a buffer object).
- addr is the address to send to
- timeout is the timeout in milliseconds to wait for the send.
Return value: None.