Espressif provides several chips that can work as slaves. These slave devices rely on some common buses, and have their own communication protocols over those buses. The ``esp_serial_slave_link`` component is designed for the master to communicate with ESP slave devices through those protocols over the bus drivers.
- ESSL device: a virtual device on the master associated with an ESP slave device. The device context has the knowledge of the slave protocol above the bus, relying on some bus drivers to communicate with the slave.
- ESSL device handle: a handle to ESSL device context containing the configuration, status and data required by the ESSL component. The context stores the driver configurations, communication state, data shared by master and slave, etc.
- The context should be initialized before it is used, and get deinitialized if not used any more. The master application operates on the ESSL device through this handle.
- TX buffer num: a counter, which is on the slave and can be read by the master, indicates the accumulated buffer numbers that the slave has loaded to the hardware to receive data from the master.
- RX data size: a counter, which is on the slave and can be read by the master, indicates the accumulated data size that the slave has loaded to the hardware to send to the master.
The slave updates the TX buffer num to inform the master how much data it can receive, and the master then read the TX buffer num, and take off the used buffer number to know how many buffers are remaining.
4. RX FIFO (slave to master): The slave can send data in stream to the master. The SDIO slave can also indicate it has new data to send to master by the interrupt line.
The slave updates the RX data size to inform the master how much data it has prepared to send, and then the master read the data size, and take off the data length it has already received to know how many data is remaining.
The ESP SDIO slave link (ESSL SDIO) devices relies on the SDMMC component. It includes the usage of communicating with ESP SDIO Slave device via the SDMMC Host or SDSPI Host feature. The ESSL device should be initialized as below:
3. Initialize the ESSL device with :cpp:type:`essl_sdio_config_t`. The ``card`` member should be the :cpp:type:`sdmmc_card_t` got in step 2, and the ``recv_buffer_size`` member should be filled correctly according to pre-negotiated value.
If you are communicating with the ESP SDIO Slave device through SPI interface, you should use the :ref:`SDIO interface <essl_sdio_slave_init>` instead.
1. Call :cpp:func:`essl_get_tx_buffer_num` to know how many buffers the slave has prepared to receive data from the master. This is optional. The master will poll ``tx_buffer_num`` when it tries to send packets to the slave, until the slave has enough buffer or timeout.
1. Call :cpp:func:`essl_get_rx_data_size` to know how many data the slave has prepared to send to the master. This is optional. When the master tries to receive data from the slave, it updates the ``rx_data_size`` for once, if the current ``rx_data_size`` is shorter than the buffer size the master prepared to receive. And it may poll the ``rx_data_size`` if the ``rx_data_size`` keeps 0, until timeout.
