152 lines
6.0 KiB
ReStructuredText
152 lines
6.0 KiB
ReStructuredText
.. currentmodule:: machine
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.. _machine.SPI:
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class SPI -- a Serial Peripheral Interface bus protocol (controller side)
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=========================================================================
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SPI is a synchronous serial protocol that is driven by a controller. At the
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physical level, a bus consists of 3 lines: SCK, MOSI, MISO. Multiple devices
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can share the same bus. Each device should have a separate, 4th signal,
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CS (Chip Select), to select a particular device on a bus with which
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communication takes place. Management of a CS signal should happen in
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user code (via machine.Pin class).
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Both hardware and software SPI implementations exist via the
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:ref:`machine.SPI <machine.SPI>` and `machine.SoftSPI` classes. Hardware SPI uses underlying
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hardware support of the system to perform the reads/writes and is usually
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efficient and fast but may have restrictions on which pins can be used.
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Software SPI is implemented by bit-banging and can be used on any pin but
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is not as efficient. These classes have the same methods available and
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differ primarily in the way they are constructed.
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Example usage::
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from machine import SPI, Pin
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spi = SPI(0, baudrate=400000) # Create SPI peripheral 0 at frequency of 400kHz.
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# Depending on the use case, extra parameters may be required
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# to select the bus characteristics and/or pins to use.
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cs = Pin(4, mode=Pin.OUT, value=1) # Create chip-select on pin 4.
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try:
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cs(0) # Select peripheral.
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spi.write(b"12345678") # Write 8 bytes, and don't care about received data.
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finally:
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cs(1) # Deselect peripheral.
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try:
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cs(0) # Select peripheral.
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rxdata = spi.read(8, 0x42) # Read 8 bytes while writing 0x42 for each byte.
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finally:
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cs(1) # Deselect peripheral.
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rxdata = bytearray(8)
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try:
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cs(0) # Select peripheral.
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spi.readinto(rxdata, 0x42) # Read 8 bytes inplace while writing 0x42 for each byte.
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finally:
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cs(1) # Deselect peripheral.
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txdata = b"12345678"
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rxdata = bytearray(len(txdata))
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try:
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cs(0) # Select peripheral.
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spi.write_readinto(txdata, rxdata) # Simultaneously write and read bytes.
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finally:
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cs(1) # Deselect peripheral.
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Constructors
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------------
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.. class:: SPI(id, ...)
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Construct an SPI object on the given bus, *id*. Values of *id* depend
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on a particular port and its hardware. Values 0, 1, etc. are commonly used
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to select hardware SPI block #0, #1, etc.
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With no additional parameters, the SPI object is created but not
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initialised (it has the settings from the last initialisation of
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the bus, if any). If extra arguments are given, the bus is initialised.
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See ``init`` for parameters of initialisation.
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.. _machine.SoftSPI:
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.. class:: SoftSPI(baudrate=500000, *, polarity=0, phase=0, bits=8, firstbit=MSB, sck=None, mosi=None, miso=None)
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Construct a new software SPI object. Additional parameters must be
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given, usually at least *sck*, *mosi* and *miso*, and these are used
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to initialise the bus. See `SPI.init` for a description of the parameters.
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Methods
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-------
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.. method:: SPI.init(baudrate=1000000, *, polarity=0, phase=0, bits=8, firstbit=SPI.MSB, sck=None, mosi=None, miso=None, pins=(SCK, MOSI, MISO))
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Initialise the SPI bus with the given parameters:
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- ``baudrate`` is the SCK clock rate.
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- ``polarity`` can be 0 or 1, and is the level the idle clock line sits at.
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- ``phase`` can be 0 or 1 to sample data on the first or second clock edge
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respectively.
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- ``bits`` is the width in bits of each transfer. Only 8 is guaranteed to be supported by all hardware.
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- ``firstbit`` can be ``SPI.MSB`` or ``SPI.LSB``.
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- ``sck``, ``mosi``, ``miso`` are pins (machine.Pin) objects to use for bus signals. For most
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hardware SPI blocks (as selected by ``id`` parameter to the constructor), pins are fixed
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and cannot be changed. In some cases, hardware blocks allow 2-3 alternative pin sets for
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a hardware SPI block. Arbitrary pin assignments are possible only for a bitbanging SPI driver
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(``id`` = -1).
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- ``pins`` - WiPy port doesn't ``sck``, ``mosi``, ``miso`` arguments, and instead allows to
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specify them as a tuple of ``pins`` parameter.
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In the case of hardware SPI the actual clock frequency may be lower than the
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requested baudrate. This is dependant on the platform hardware. The actual
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rate may be determined by printing the SPI object.
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.. method:: SPI.deinit()
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Turn off the SPI bus.
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.. method:: SPI.read(nbytes, write=0x00)
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Read a number of bytes specified by ``nbytes`` while continuously writing
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the single byte given by ``write``.
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Returns a ``bytes`` object with the data that was read.
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.. method:: SPI.readinto(buf, write=0x00)
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Read into the buffer specified by ``buf`` while continuously writing the
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single byte given by ``write``.
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Returns ``None``.
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Note: on WiPy this function returns the number of bytes read.
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.. method:: SPI.write(buf)
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Write the bytes contained in ``buf``.
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Returns ``None``.
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Note: on WiPy this function returns the number of bytes written.
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.. method:: SPI.write_readinto(write_buf, read_buf)
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Write the bytes from ``write_buf`` while reading into ``read_buf``. The
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buffers can be the same or different, but both buffers must have the
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same length.
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Returns ``None``.
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Note: on WiPy this function returns the number of bytes written.
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Constants
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---------
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.. data:: SPI.CONTROLLER
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for initialising the SPI bus to controller; this is only used for the WiPy
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.. data:: SPI.MSB
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set the first bit to be the most significant bit
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.. data:: SPI.LSB
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set the first bit to be the least significant bit
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