SPI Master (Serial Peripheral Interface)
========================================

The RP2040 has two hardware SPI interfaces, ``spi0 (SPI)`` and ``spi1 (SPI1)``.
These interfaces are supported by the ``SPI`` library in master mode.

SPI pinouts can be set **before SPI.begin()** using the following calls:

.. code:: cpp

    bool setRX(pin_size_t pin);
    bool setCS(pin_size_t pin);
    bool setSCK(pin_size_t pin);
    bool setTX(pin_size_t pin);

Note that the ``CS`` pin can be hardware or software controlled by the sketch.
When software controlled, the ``setCS()`` call is ignored.

The Arduino `SPI documentation <https://www.arduino.cc/en/reference/SPI>`_ gives
a detailed overview of the library, except for the following RP2040-specific
changes:

* ``SPI.begin(bool hwCS)`` can take an options ``hwCS`` parameter.
By passing in ``true`` for ``hwCS`` the sketch does not need to worry
about asserting and deasserting the ``CS`` pin between transactions.
The default is ``false`` and requires the sketch to handle the CS
pin itself, as is the standard way in Arduino.

* The interrupt calls (``attachInterrupt``, and ``detachInterrpt``) are not implemented.


SPI Slave (SPISlave)
====================

Slave mode operation is also supported on either SPI interface.  Two callbacks are
needed in your app, set through ``SPISlave.onDataRecv`` and ``SPISlave.onDataSent``,
in order to consunme the received data and provide data to transmit.

* The callbacks operate at IRQ time and may be called very frequently at high SPI frequencies.  So, make then small, fast, and with no memory allocations or locking.


Examples
========

See the SPItoMyself example for a complete Master and Slave application.