The ESP32 SDIO Card host and slave peripherals share two sets of pins, as shown in the table below. The first set is usually occupied by the SPI0 bus, which is responsible for the SPI flash holding the code to run. This means the SDIO slave driver can only run on the second set of pins while the SDIO host is not using it.
The SDIO slave can run under three modes: SPI, 1-bit SD, and 4-bit SD modes. Based on the signals on the interface, the device can determine the current mode and configure itself to adapt to that mode. Later, the slave driver can communicate with the slave device to properly handle commands and data transfers. According to the SDIO specification, the CMD and DAT0-3 signal lines should be pulled up whether in 1-bit SD, 4-bit SD or SPI mode.
- 1-bit SD mode: Connect CLK, CMD, DAT0, DAT1 pins, and the ground.
- 4-bit SD mode: Connect all pins, and the ground.
- SPI mode: Connect SCLK, MOSI, MISO, Interrupt, #CS pins, and the ground.
..note::
Please check if CMD and DATA lines DAT0-DAT3 of the card are properly pulled up by 10 KOhm - 90 KOhm resistors, which should be ensured even in 1-bit mode or SPI mode. Most official modules do not offer these pullups internally. If you are using official development boards, check :ref:`compatibility_overview_espressif_hw_sdio` to see whether your development boards have such pullups.
Most official modules have conflicts on strapping pins with the SDIO slave function. If you are using an ESP32 module with 3.3 V flash inside, when you are developing on the module for the first time, you will need to perform an eFuse burn-in prior to development. This will adjust the pin configuration of the module to make the module compatible with SDIO functionality. See :ref:`compatibility_overview_espressif_hw_sdio` for details on how to configure this.
- Modules: All modules except ESP32-WROVER、ESP32-WROVER-I、ESP32-S3-WROOM-2, and please check `Modules Overview <https://www.espressif.com/en/products/modules>`__ for module list
- Kits: ESP32-PICO-KIT, ESP32-DevKitC (up to v4), ESP32-WROVER-KIT (v4.1 [also known as ESP32-WROVER-KIT-VB], v2, v1 [also known as DevKitJ v1])
You can tell the version of your ESP23-WROVER-KIT version from the module on it: v4.1 are with ESP32-WROVER-B modules, v3 are with ESP32-WROVER modules, while v2 and v1 are with ESP32-WROOM-32 modules.
The host initializes the slave into SD mode by sending the CMD0 command with the DAT3 pin set to a high level. Alternatively, the host initializes the SPI mode by sending CMD0 with CS pin low, which is the same pin as DAT3.
After the initialization, the host can enable the 4-bit SD mode by writing CCCR register 0x07 by CMD52. All the bus detection processes are handled by the slave peripheral.
- A transfer is initiated by a command token from the host and may consist of a response and multiple data blocks. The core mechanism of the {IDF_TARGET_NAME} SDIO slave driver involves data exchange and communication through transfers.
The register names in **{IDF_TARGET_NAME} Technical Reference Manual** > **SDIO Slave Controller** [`PDF <{IDF_TARGET_TRM_EN_URL}#sdioslave>`__] are organized from the host's perspective. For instance, ``RX`` registers indicate sending, while ``TX`` registers denote receiving. In our driver implementation, we've chosen not to utilize the terms ``TX`` or ``RX`` to prevent any potential ambiguities.
- FIFO: A designated address within Function 1 that can be accessed using CMD53 commands for reading or writing substantial volumes of data. The address corresponds to the length intended for reading from or writing to the slave in a single transfer: **requested length** = 0x1F800 – address.
- Ownership: When the driver assumes ownership of a buffer, it means that the driver has the capability to perform random read/write operations on the buffer (often via DMA). The application should not read/write the buffer until the ownership is returned to the application. If the application reads from a buffer owned by a receiving driver, the data read can be random; similarly, if the application writes to a buffer owned by a sending driver, the data sent may be corrupted.
Requested length is different from the transfer length. In the context of {IDF_TARGET_NAME} SDIO slave DMA, the operation is based on the **requested length** rather than the **transfer length**. This means the DMA controller will process the data transfer according to the **requested length**, ensuring that only data within the **requested length** is transferred. The **transfer length** should be no shorter than the **requested length**, and the rest part is filled with 0 during sending or discard during receiving.
- Receiving buffer size: The buffer size is pre-defined between the host and the slave before communication starts. The slave application has to set the buffer size during initialization by the ``recv_buffer_size`` parameter in the ``sdio_slave_config_t`` structure.
- Interrupts: The {IDF_TARGET_NAME} SDIO slave supports interrupts in two directions: from host to slave (referred to as slave interrupts) and from slave to host (referred to as host interrupts). For more details, refer to :ref:`interrupts`.
- Registers: Specific addresses in Function 1 accessed by CMD52 or CMD53.
The host should initialize the {IDF_TARGET_NAME} SDIO slave according to the standard SDIO initialization process (Sector 3.1.2 of `SDIO Simplified Specification <https://www.sdcard.org/downloads/pls/pdf/?p=PartE1_SDIO_Simplified_Specification_Ver3.00.jpg&f=PartE1_SDIO_Simplified_Specification_Ver3.00.pdf&e=EN_SSE1>`_), which is described briefly in :ref:`esp_slave_init`.
Furthermore, there is an {IDF_TARGET_NAME}-specific upper-level communication protocol built upon the foundation of CMD52/CMD53 to Function 1. Within this particular communication protocol, the master and slave engage in data exchange and communication through the utilization of CMD52/CMD53 commands. For more detailed information, please consult the :ref:`esp_slave_protocol_layer` section.
There is also a component :doc:`/api-reference/protocols/esp_serial_slave_link` designed for {IDF_TARGET_NAME} master to communicate with {IDF_TARGET_NAME} SDIO slave. See example :example:`peripherals/sdio` when programming your host.
The host can trigger an interruption in the slave by writing a single bit to the 0x08D register. As soon as any bit within the register is set, an interrupt is generated, prompting the SDIO slave driver to invoke the callback function specified in the ``slave_intr_cb`` member of the ``sdio_slave_config_t`` structure.
Similar to the previous information, there's an alternative set of functions available. You can call ``sdio_slave_wait_int`` to wait for an interrupt within a certain time, or call ``sdio_slave_clear_int`` to clear interrupts from host. The callback function can work with the wait functions perfectly.
The slave can interrupt the host by an interrupt line at certain time, which is level-sensitive, i.e., the interrupt signal can be triggered by detecting the level change of the interrupt line. When the host see the interrupt line pulled down, it may read the slave interrupt status register, to see the interrupt source. Host can clear interrupt bits, or choose to disable a interrupt source. The interrupt line holds active until all the sources are cleared or disabled.
There are 52 R/W shared registers (8-bit) to share information between host and slave. The slave can write or read the registers at any time by ``sdio_slave_read_reg`` and ``sdio_slave_write_reg``. The host can access (R/W) the register by CMD52 or CMD53.
1. Register the buffer by calling ``sdio_slave_recv_register_buf``, and get the handle of the registered buffer. The driver allocates memory for the linked-list descriptor needed to link the buffer onto the hardware. The size of these buffers should equal to the Receiving buffer size.
3. Get the received data by calling ``sdio_slave_recv`` or ``sdio_slave_recv_packet``. If a non-blocking call is needed, set ``wait=0``.
The difference between two APIs is that, ``sdio_slave_recv_packet`` gives more information about packet, which can consist of several buffers.
When ``ESP_ERR_NOT_FINISHED`` is returned by this API, you should call this API iteratively until the return value is ``ESP_OK``. All the continuous buffers returned with ``ESP_ERR_NOT_FINISHED``, together with the last buffer returned with ``ESP_OK``, belong to one packet from the host.
Call ``sdio_slave_recv_get_buf`` to get the address of the received data, and the actual length received in each buffer. The packet length is the sum of received length of all the buffers in the packet.
If the host never send data longer than the Receiving buffer size, or you do not care about the packet boundary (e.g., the data is only a byte stream), you can call the simpler version ``sdio_slave_recv`` instead.
To minimize data copying overhead, the driver itself does not maintain any internal buffer; it is the responsibility of the application to promptly provide new buffers. The DMA system automatically stores received data into these buffers.
Each time the slave has data to send, it raises an interrupt, and the host requests the packet length. There are two sending modes:
- Stream Mode: When a buffer is loaded into the driver, the buffer length is included into the packet length requested by host in the incoming communications. This is irrespective of whether previous packets have been sent or not. In other words, the length of the newly loaded buffer is included into the length of the packet requested by the host, even if there are previously unsent packets. This enables the host to receive data from several buffers in a single transfer.
- Packet Mode: The packet length is updated packet by packet, and only when previous packet is sent. This means that the host can only get data of one buffer in one transfer.
To avoid overhead from copying data, the driver itself does not have any buffer inside. Namely, the DMA takes data directly from the buffer provided by the application. The application should not touch the buffer until the sending is finished, so as to ensure that the data is transferred correctly.
The sending mode can be set in the ``sending_mode`` member of ``sdio_slave_config_t``, and the buffer numbers can be set in the ``send_queue_size``. All the buffers are restricted to be no larger than 4092 bytes. Though in the stream mode, several buffers can be sent in one transfer, each buffer is still counted as one in the queue.
The application can call ``sdio_slave_transmit`` to send packets. In this case, the function returns when the transfer is successfully done, so the queue is not fully used. When higher efficiency is required, the application can use the following functions instead:
- If the ``wait`` is not ``portMAX_DELAY`` (wait until success), application has to check the result to know whether the data is put in to the queue or discard.
2. Call ``sdio_slave_send_get_finished`` to get and deal with a finished transfer. A buffer should be kept unmodified until returned from ``sdio_slave_send_get_finished``. This means the buffer is actually sent to the host, rather than just staying in the queue.
3. Work with the receiving part of this driver, and point ``arg`` to the receive buffer handle of this buffer, so that we can directly use the buffer to receive data when it is sent::
