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SPI/HDLC Adaptor

spi-hdlc-adapter is an adapter tool for using a SPI interface as if it were an HDLC-lite encoded bidirectional asynchronous serial stream. It uses the SPI protocol outlined in Appendix A.2 of the Spinel protocol document.


spi-hdlc-adapter [options] <spi-device-path>


  • --stdio: Use stdin and stdout for HDLC input/output. Useful when directly started by the program that will be using it.
  • --pty: Create a pseudo terminal for HDLC input/output. The path of the newly-created PTY will be written to stdout, followed by a newline.
  • --raw: Do not encode/decode packets using HDLC. Instead, write whole, raw frames to the specified input and output FDs. This is useful for emulating a serial port, or when datagram-based sockets are supplied for stdin and stdout (when used with --stdio).
  • --mtu=[MTU]: Specify the MTU. Currently only used in raw mode. Default and maximum value is 2043. Must be greater than zero.
  • --gpio-int[=gpio-path]: Specify a path to the Linux sysfs-exported GPIO directory for the I̅N̅T̅ pin. If not specified, spi-hdlc-adapter will fall back to polling, which is inefficient.
  • --gpio-reset[=gpio-path]: Specify a path to the Linux sysfs-exported GPIO directory for the R̅E̅S̅ pin.
  • --spi-mode[=mode]: Specify the SPI mode to use (0-3). Default value is 0.
  • --spi-speed[=hertz]: Specify the SPI speed in hertz. Default value is 1000000 (1MHz).
  • --spi-cs-delay[=usec]: Specify the delay after C̅S̅ assertion, in microseconds. Default is 20µs. Note that this may need to be set to zero for spi-hdlc-adapter to work with some SPI drivers.
  • --spi-align-allowance[=n]: Specify the maximum number of 0xFF bytes to clip from start of MISO frame. This makes this tool usable with SPI slaves which have buggy SPI blocks that prepend a variable number of 0xFF bytes to the start of MISO frame. Default value is 0. Maximum value is 16. This should be set to 7 for chips in the SiLabs EM35x family.
  • --spi-small-packet=[n]: Specify the smallest packet we can receive in a single SPI transaction. Packets sent by the slave which are smaller than or equal to this size will require only a single SPI transaction to be successfully transmitted. Increasing this value will (up to a point) decrease latency for smaller packets at the expense of overall bandwidth. Default value is 32. The minimum value is 0. The maximum value is 2043.
  • --verbose: Increase debug verbosity.
  • --help: Print out usage information to stdout and exit.

spi-device-path is a required argument since it indicates which SPI device to use. An example path might be /dev/spidev1.0.

The GPIO paths are to the top-level directory for that GPIO. They must be already be exported before spi-hdlc-adapter can use them.


If an MCU reset is detected by the reset bit being set on a SPI frame, the special vendor-specific HDLC-lite symbol 0xF8 is emitted. If --gpio-reset is specified, the HDLC client can trigger an MCU reset by sending the symbols 0x7E 0x13 0x11 0x7E or by sending SIGUSR1.

When started, spi-hdlc-adapter will configure the following properties on the GPIOs:

  1. Set I̅N̅T̅/direction to in.
  2. Set I̅N̅T̅/edge to falling.
  3. Set R̅E̅S̅/direction to high.

When resetting the slave device, spi-hdlc performs the following procedure:

  1. Set R̅E̅S̅/direction to low.
  2. Sleep for 30ms.
  3. Set R̅E̅S̅/direction to high.


Some simple usage statistics are printed out to syslog at exit and whenever the SIGUSR1 signal is received. The easiest way to send that signal to spi-hdlc-adapter is like this:

# killall -sigusr1 spi-hdlc-adapter

At which point you will see something like this in the syslogs:

spi-hdlc-adapter[18408]: INFO: sSlaveResetCount=16
spi-hdlc-adapter[18408]: INFO: sSpiFrameCount=2673
spi-hdlc-adapter[18408]: INFO: sSpiValidFrameCount=2668
spi-hdlc-adapter[18408]: INFO: sSpiDuplexFrameCount=3
spi-hdlc-adapter[18408]: INFO: sSpiUnresponsiveFrameCount=5
spi-hdlc-adapter[18408]: INFO: sSpiGarbageFrameCount=0
spi-hdlc-adapter[18408]: INFO: sHdlcTxFrameCount=1454
spi-hdlc-adapter[18408]: INFO: sHdlcTxFrameByteCount=2908
spi-hdlc-adapter[18408]: INFO: sHdlcRxFrameCount=884
spi-hdlc-adapter[18408]: INFO: sHdlcRxFrameByteCount=3875
spi-hdlc-adapter[18408]: INFO: sHdlcRxBadCrcCount=0

Sending SIGUSR2 will clear the counters.