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

SPI is a synchronous serial protocol that is driven by a controller. 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, CS (Chip Select), to select a particular device on a bus with which communication takes place. Management of a CS signal should happen in user code (via machine.Pin class).

Both hardware and software SPI implementations exist via the machine.SPI and machine.SoftSPI classes. Hardware SPI uses underlying hardware support of the system to perform the reads/writes and is usually efficient and fast but may have restrictions on which pins can be used. Software SPI is implemented by bit-banging and can be used on any pin but is not as efficient. These classes have the same methods available and differ primarily in the way they are constructed.

Example usage:

from machine import SPI, Pin

spi = SPI(0, baudrate=400000)           # Create SPI peripheral 0 at frequency of 400kHz.
                                        # Depending on the use case, extra parameters may be required
                                        # to select the bus characteristics and/or pins to use.
cs = Pin(4, mode=Pin.OUT, value=1)      # Create chip-select on pin 4.

try:
    cs(0)                               # Select peripheral.
    spi.write(b"12345678")              # Write 8 bytes, and don't care about received data.
finally:
    cs(1)                               # Deselect peripheral.

try:
    cs(0)                               # Select peripheral.
    rxdata = spi.read(8, 0x42)          # Read 8 bytes while writing 0x42 for each byte.
finally:
    cs(1)                               # Deselect peripheral.

rxdata = bytearray(8)
try:
    cs(0)                               # Select peripheral.
    spi.readinto(rxdata, 0x42)          # Read 8 bytes inplace while writing 0x42 for each byte.
finally:
    cs(1)                               # Deselect peripheral.

txdata = b"12345678"
rxdata = bytearray(len(txdata))
try:
    cs(0)                               # Select peripheral.
    spi.write_readinto(txdata, rxdata)  # Simultaneously write and read bytes.
finally:
    cs(1)                               # Deselect peripheral.

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.

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.

class machine.SoftSPI(baudrate=500000, *, polarity=0, phase=0, bits=8, firstbit=MSB, sck=None, mosi=None, miso=None)

Construct a new software SPI object. Additional parameters must be given, usually at least sck, mosi and miso, and these are used to initialise the bus. See SPI.init for a description of the parameters.

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.

Note: on WiPy this function returns the number of bytes read.

SPI.write(buf)

Write the bytes contained in buf. Returns None.

Note: on WiPy this function returns the number of bytes written.

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.

Note: on WiPy this function returns the number of bytes written.

Constants

SPI.CONTROLLER

for initialising the SPI bus to controller; 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