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.. currentmodule:: machine
.. _machine.SPI:
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
:ref:`machine.SPI <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:: 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.
.. _machine.SoftSPI:
.. class:: 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
-------
.. method:: 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 dependent on the platform hardware. The actual
rate may be determined by printing the SPI object.
.. method:: SPI.deinit()
Turn off the SPI bus.
.. method:: 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.
.. method:: 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.
.. method:: SPI.write(buf)
Write the bytes contained in ``buf``.
Returns ``None``.
Note: on WiPy this function returns the number of bytes written.
.. method:: 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
---------
.. data:: SPI.CONTROLLER
for initialising the SPI bus to controller; this is only used for the WiPy
.. data:: SPI.MSB
SoftSPI.MSB
set the first bit to be the most significant bit
.. data:: SPI.LSB
SoftSPI.LSB
set the first bit to be the least significant bit
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