class SPI – a master-driven serial protocol

SPI is a serial protocol that is driven by a master. At the physical level there are 3 lines: SCK, MOSI, MISO.

See usage model of I2C; SPI is very similar. Main difference is parameters to init the SPI bus:

from pyb import SPI
spi = SPI(1, SPI.MASTER, baudrate=1000000, polarity=0, phase=0, nss=SPI.ACTIVE_LOW)

Only required parameter is mode, must be SPI.MASTER. Polarity can be 0 or 1, and is the level the idle clock line sits at. Phase can be 0 or 1 to sample data on the first or second clock edge respectively.

Additional methods for SPI:

data = spi.send_recv(b'1234')        # send 4 bytes and receive 4 bytes
buf = bytearray(4)
spi.send_recv(b'1234', buf)          # send 4 bytes and receive 4 into buf
spi.send_recv(buf, buf)              # send/recv 4 bytes from/to buf

Constructors

class pyb.SPI(bus, ...)

Construct an SPI object on the given bus. bus can be only 1. With no additional parameters, the SPI 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.

Methods

spi.deinit()

Turn off the SPI bus.

spi.init(mode, baudrate=1000000, *, polarity=0, phase=0, bits=8, nss=SPI.ACTIVE_LOW)

Initialise the SPI bus with the given parameters:

  • mode must be SPI.MASTER.
  • baudrate is the SCK clock rate.
  • polarity can be 0 or 1, and is the level the idle clock line sits at.
  • phase can be 0 or 1 to sample data on the first or second clock edge respectively.
  • bits is the width of each transfer, accepted values are 8, 16 and 32.
  • nss is the polarity of the slave select line. Can be SPI.ACTIVE_LOW or SPI.ACTIVE_HIGH.

Note that the SPI clock frequency will not always be the requested baudrate. Printing the SPI object will show you the computed baudrate and the chosen prescaler.

spi.recv(recv, *, timeout=5000)

Receive data on the bus:

  • recv can be an integer, which is the number of bytes to receive, or a mutable buffer, which will be filled with received bytes.
  • timeout is the timeout in milliseconds to wait for the receive.

Return value: if recv is an integer then a new buffer of the bytes received, otherwise the same buffer that was passed in to recv.

spi.send(send, *, timeout=5000)

Send data on the bus:

  • send is the data to send (an integer to send, or a buffer object).
  • timeout is the timeout in milliseconds to wait for the send.

Return value: None.

spi.send_recv(send, recv=None, *, timeout=5000)

Send and receive data on the bus at the same time:

  • send is the data to send (an integer to send, or a buffer object).
  • recv is a mutable buffer which will be filled with received bytes. It can be the same as send, or omitted. If omitted, a new buffer will be created.
  • timeout is the timeout in milliseconds to wait for the receive.

Return value: the buffer with the received bytes.

Constants

SPI.MASTER

for initialising the SPI bus to master

SPI.ACTIVE_LOW
SPI.ACTIVE_HIGH

selects the polarity of the NSS pin