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97 lines
4.5 KiB
ReStructuredText
97 lines
4.5 KiB
ReStructuredText
SPI Slave driver
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=================
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Overview
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--------
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The ESP32 has four SPI peripheral devices, called SPI0, SPI1, HSPI and VSPI. SPI0 is entirely dedicated to
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the flash cache the ESP32 uses to map the SPI flash device it is connected to into memory. SPI1 is
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connected to the same hardware lines as SPI0 and is used to write to the flash chip. HSPI and VSPI
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are free to use, and with the spi_slave driver, these can be used as a SPI slave, driven from a
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connected SPI master.
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The spi_slave driver
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^^^^^^^^^^^^^^^^^^^^^
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The spi_slave driver allows using the HSPI and/or VSPI peripheral as a full-duplex SPI slave. It can make
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use of DMA to send/receive transactions of arbitrary length.
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Terminology
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^^^^^^^^^^^
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The spi_slave driver uses the following terms:
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* Host: The SPI peripheral inside the ESP32 initiating the SPI transmissions. One of HSPI or VSPI.
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* Bus: The SPI bus, common to all SPI devices connected to a master. In general the bus consists of the
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miso, mosi, sclk and optionally quadwp and quadhd signals. The SPI slaves are connected to these
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signals in parallel. Each SPI slave is also connected to one CS signal.
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- miso - Also known as q, this is the output of the serial stream from the ESP32 to the SPI master
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- mosi - Also known as d, this is the output of the serial stream from the SPI master to the ESP32
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- sclk - Clock signal. Each data bit is clocked out or in on the positive or negative edge of this signal
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- cs - Chip Select. An active Chip Select delineates a single transaction to/from a slave.
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* Transaction: One instance of CS going active, data transfer from and to a master happening, and
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CS going inactive again. Transactions are atomic, as in they will never be interrupted by another
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transaction.
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SPI transactions
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^^^^^^^^^^^^^^^^
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A full-duplex SPI transaction starts with the master pulling CS low. After this happens, the master
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starts sending out clock pulses on the CLK line: every clock pulse causes a data bit to be shifted from
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the master to the slave on the MOSI line and vice versa on the MISO line. At the end of the transaction,
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the master makes CS high again.
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Using the spi_slave driver
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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- Initialize a SPI peripheral as a slave by calling ``spi_slave_initialize``. Make sure to set the
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correct IO pins in the ``bus_config`` struct. Take care to set signals that are not needed to -1.
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A DMA channel (either 1 or 2) must be given if transactions will be larger than 32 bytes, if not
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the dma_chan parameter may be 0.
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- To set up a transaction, fill one or more spi_transaction_t structure with any transaction
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parameters you need. Either queue all transactions by calling ``spi_slave_queue_trans``, later
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quering the result using ``spi_slave_get_trans_result``, or handle all requests synchroneously
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by feeding them into ``spi_slave_transmit``. The latter two functions will block until the
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master has initiated and finished a transaction, causing the queued data to be sent and received.
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- Optional: to unload the SPI slave driver, call ``spi_slave_free``.
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Transaction data and master/slave length mismatches
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Normally, data to be transferred to or from a device will be read from or written to a chunk of memory
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indicated by the ``rx_buffer`` and ``tx_buffer`` members of the transaction structure. The SPI driver
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may decide to use DMA for transfers, so these buffers should be allocated in DMA-capable memory using
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``pvPortMallocCaps(size, MALLOC_CAP_DMA)``.
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The amount of data written to the buffers is limited by the ``length`` member of the transaction structure:
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the driver will never read/write more data than indicated there. The ``length`` cannot define the actual
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length of the SPI transaction; this is determined by the master as it drives the clock and CS lines. In
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case the length of the transmission is larger than the buffer length, only the start of the transmission
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will be sent and received. In case the transmission length is shorter than the buffer length, only data up
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to the length of the buffer will be exchanged.
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Warning: Due to a design peculiarity in the ESP32, if the amount of bytes sent by the master or the length
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of the transmission queues in the slave driver, in bytes, is not both larger than eight and dividable by
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four, the SPI hardware can fail to write the last one to seven bytes to the receive buffer.
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Application Example
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-------------------
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Slave/master communication: :example:`peripherals/spi_slave`.
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API Reference
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-------------
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.. include:: /_build/inc/spi_slave.inc
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