class SPI – a Serial Peripheral Interface bus protocol (master side)

SPI is a synchronous serial protocol that is driven by a master. At the physical level, a bus consists of 3 lines: SCK, MOSI, MISO. Multiple devices can share the same bus. Each device should have a separate, 4th signal, SS (Slave Select), to select a particular device on a bus with which communication takes place. Management of an SS signal should happen in user code (via machine.Pin class).

Constructors

class machine.SPI(id, ...)

Construct an SPI object on the given bus, id. Values of id depend on a particular port and its hardware. Values 0, 1, etc. are commonly used to select hardware SPI block #0, #1, etc. Value -1 can be used for bitbanging (software) implementation of SPI (if supported by a port).

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.init(baudrate=1000000, *, polarity=0, phase=0, bits=8, firstbit=SPI.MSB, sck=None, mosi=None, miso=None, pins=(SCK, MOSI, MISO))

Initialise the SPI bus with the given parameters:

  • 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 in bits of each transfer. Only 8 is guaranteed to be supported by all hardware.
  • firstbit can be SPI.MSB or SPI.LSB.
  • sck, mosi, miso are pins (machine.Pin) objects to use for bus signals. For most hardware SPI blocks (as selected by id parameter to the constructor), pins are fixed and cannot be changed. In some cases, hardware blocks allow 2-3 alternative pin sets for a hardware SPI block. Arbitrary pin assignments are possible only for a bitbanging SPI driver (id = -1).
  • pins - WiPy port doesn’t sck, mosi, miso arguments, and instead allows to specify them as a tuple of pins parameter.

In the case of hardware SPI the actual clock frequency may be lower than the requested baudrate. This is dependant on the platform hardware. The actual rate may be determined by printing the SPI object.

SPI.deinit()

Turn off the SPI bus.

SPI.read(nbytes, write=0x00)

Read a number of bytes specified by nbytes while continuously writing the single byte given by write. Returns a bytes object with the data that was read.

SPI.readinto(buf, write=0x00)

Read into the buffer specified by buf while continuously writing the single byte given by write. Returns None.

SPI.write(buf)

Write the bytes contained in buf. Returns None.

SPI.write_readinto(write_buf, read_buf)

Write the bytes from write_buf while reading into read_buf. The buffers can be the same or different, but both buffers must have the same length. Returns None.

Constants

SPI.MASTER

for initialising the SPI bus to master; this is only used for the WiPy

SPI.MSB

set the first bit to be the most significant bit

SPI.LSB

set the first bit to be the least significant bit