class SPI – a controller-driven serial protocol

SPI is a serial protocol that is driven by a controller. 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(2, SPI.MASTER, baudrate=600000, polarity=1, phase=0, crc=0x7)

Only required parameter is mode, SPI.CONTROLLER or SPI.PERIPHERAL. 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. Crc can be None for no CRC, or a polynomial specifier.

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 2. 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.

The physical pins of the SPI buses are:

  • SPI(2): (NSS, SCK, MISO, MOSI) = (P3, P2, P1, P0) = (PB12, PB13, PB14, PB15)

At the moment, the NSS pin is not used by the SPI driver and is free for other use.

Methods

SPI.deinit()

Turn off the SPI bus.

SPI.init(mode, baudrate=328125, *, prescaler=-1, polarity=1, phase=0, bits=8, firstbit=SPI.MSB, ti=False, crc=None)

Initialise the SPI bus with the given parameters:

  • mode must be either SPI.CONTROLLER or SPI.PERIPHERAL.

  • baudrate is the SCK clock rate (only sensible for a controller).

  • prescaler is the prescaler to use to derive SCK from the APB bus frequency; use of prescaler overrides baudrate.

  • 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 can be 8 or 16, and is the number of bits in each transferred word.

  • firstbit can be SPI.MSB or SPI.LSB.

  • ti True indicates Texas Instruments, as opposed to Motorola, signal conventions.

  • crc can be None for no CRC, or a polynomial specifier.

Note that the SPI clock frequency will not always be the requested baudrate. The hardware only supports baudrates that are the APB bus frequency divided by a prescaler, which can be 2, 4, 8, 16, 32, 64, 128 or 256. SPI(2) is on AHB1. For precise control over the SPI clock frequency, specify prescaler instead of 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.CONTROLLER
SPI.PERIPHERAL

for initialising the SPI bus to controller or peripheral mode

SPI.LSB
SPI.MSB

set the first bit to be the least or most significant bit