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Bit Banged MCP2515 driver

this code is an example how you should not write Linux kernel modules. The CAN interface device driver is tightly interlocked with the GPIO SPI bitbanging code. The interclock avoids task switching which speed up the whole driver. Drawback: driver blocks CPU during reading the MCP2515. On high CAN-Bus load the driver is claiming lots of CPU cycles.


  • speed up bitbanging (tend to get ugly though)


Challenge: get the RX0BUF in 47 us starting at INT. 1/47us is the fastest CAN frame repeat rate possible - 1 MBit with min 47 bits (no bit stuffing)

SPI Performance


  • interrupt
  • reading (0x03) CANINTF (0x2c) and EFLG ( ++ -> 0x2d)
  • response buffer 0 is full ( 0x.. 0x.. 0x01 0x..)
  • reading RXB0 buffer (0x90 ...)


In this sequence the interrupt is cleared fast enough, because no data bytes needs to be read (DLC=0)

SPI Performance

Statistics for a fully saturated CAN-Bus at 250k:

root@ar9331:~# ip -s -d link show can0
4: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc fq_codel state UNKNOWN mode DEFAULT group default qlen 10
    link/can  promiscuity 0
    can state ERROR-ACTIVE restart-ms 0
	  bitrate 250000 sample-point 0.875
	  tq 250 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
	  mcp2515-banged: tseg1 3..16 tseg2 2..8 sjw 1..4 brp 1..64 brp-inc 1
	  clock 8000000
	  re-started bus-errors arbit-lost error-warn error-pass bus-off
	  0          0          0          0          0          0
    RX: bytes  packets  errors  dropped overrun mcast
    6216354    5313978  0       0       0       0
    TX: bytes  packets  errors  dropped carrier collsns
    5          1        0       0       0       0

Errors and overrun: none :-)

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