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SDIO Card Slave Driver

Overview

The ESP32 SDIO Card peripherals (Host, Slave) shares two sets of pins as below table. The first set is usually occupied by SPI0 bus which is responsible for the SPI flash holding the code to run. This means SDIO slave driver can only runs on the second set of pins while SDIO host is not using it.

The SDIO slave can run under 3 modes: SPI, 1-bit SD and 4-bit SD modes, which is detected automatically by the hardware. According to the SDIO specification, CMD and DAT0-3 lines should be pulled up no matter in 1-bit, 4-bit or SPI mode.

Connections

+----------+---------------+-------+-------+ | Pin Name | Corresponding | Slot1 | Slot2 | + + pins in SPI +-------+-------+ | | mode | GPIO Number | +==========+===============+=======+=======+ | CLK | SCLK | 6 | 14 | +----------+---------------+-------+-------+ | CMD | MOSI | 11 | 15 | +----------+---------------+-------+-------+ | DAT0 | MISO | 7 | 2 | +----------+---------------+-------+-------+ | DAT1 | Interrupt | 8 | 4 | +----------+---------------+-------+-------+ | DAT2 | N.C. (pullup) | 9 | 12 | +----------+---------------+-------+-------+ | DAT3 | #CS | 10 | 13 | +----------+---------------+-------+-------+

  • 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 D0-D3 of the card are properly pulled up by 10 KOhm resistors. This should be ensured even in 1-bit mode or SPI mode. Most official modules don't offer these pullups internally. If you are using official development boards, check compatibility_overview_espressif_hw_sdio to see whether your development boards have such pullups.

Note

Most official modules have conflicts on strapping pins with the SDIO slave function. If you are using a ESP32 module with 3.3 V flash inside, you have to burn the EFUSE when you are developing on the module for the first time. See compatibility_overview_espressif_hw_sdio to see how to make your modules compatible with the SDIO.

Here is a list for modules/kits with 3.3 V flash:

  • Modules: ESP32-PICO-D4, ESP32-WROOM-32 series (including ESP32-SOLO-1), ESP32-WROVER-B and ESP32-WROVER-IB
  • Kits: ESP32-PICO-KIT, ESP32-DevKitC (till 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.

Refer to sd_pullup_requirements for more technical details of the pullups.

sd_pullup_requirements

The host initialize the slave into SD mode by first sending CMD0 with DAT3 pin high, or in SPI mode by sending CMD0 with CS pin (the same pin as DAT3) low.

After the initialization, the host can enable the 4-bit SD mode by writing CCCR register 0x07 by CMD52. All the bus detection process are handled by the slave peripheral.

The host has to communicate with the slave by an ESP-slave-specific protocol. The slave driver offers 3 services over Function 1 access by CMD52 and CMD53: (1) a sending FIFO and a receiving FIFO, (2) 52 8-bit R/W registers shared by host and slave, (3) 16 interrupt sources (8 from host to slave, and 8 from slave to host).

Terminology

The SDIO slave driver uses the following terms:

  • Transfer: a transfer is always started by a command token from the host, and may contain a reply and several data blocks. ESP32 slave software is based on transfers.
  • Sending: slave to host transfers.
  • Receiving: host to slave transfers.

Note

Register names in {IDF_TARGET_NAME} Technical Reference Manual > SDIO Slave Controller [PDF] are oriented from the point of view of the host, i.e. 'rx' registers refer to sending, while 'tx' registers refer to receiving. We're not using tx or rx in the driver to avoid ambiguities.

  • FIFO: specific address in Function 1 that can be access by CMD53 to read/write large amount of data. The address is related to the length requested to read from/write to the slave in a single transfer: requested length = 0x1F800-address.
  • Ownership: When the driver takes ownership of a buffer, it means the driver can randomly read/write the buffer (usually 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; if the application writes to a buffer owned by a sending driver, the data sent may be corrupted.
  • Requested length: The length requested in one transfer determined by the FIFO address.
  • Transfer length: The length requested in one transfer determined by the CMD53 byte/block count field.

Note

Requested length is different from the transfer length. ESP32 slave DMA base on the requested length rather than the transfer length. The transfer length should be no shorter than the requested length, and the rest part will be filled with 0 (sending) or discard (receiving).

  • Receiving buffer size: The buffer size is pre-defined between the host and the slave before communication starts. Slave application has to set the buffer size during initialization by the recv_buffer_size member of sdio_slave_config_t.
  • Interrupts: the esp32 slave support interrupts in two directions: from host to slave (called slave interrupts below) and from slave to host (called host interrupts below). See more in interrupts.
  • Registers: specific address in Function 1 access by CMD52 or CMD53.

Communication with ESP SDIO Slave

The host should initialize the ESP32 SDIO slave according to the standard SDIO initialization process (Sector 3.1.2 of SDIO Simplified Specification), which is described briefly in esp_slave_init.

Furthermore, there's an ESP32-specific upper-level communication protocol upon the CMD52/CMD53 to Func 1. Please refer to esp_slave_protocol_layer. There is also a component ESP Serial Slave Link </api-reference/protocols/esp_serial_slave_link> for ESP32 master to communicate with ESP32 SDIO slave, see example peripherals/sdio when programming your host.

Interrupts

There are interrupts from host to slave, and from slave to host to help communicating conveniently.

Slave Interrupts

The host can interrupt the slave by writing any one bit in the register 0x08D. Once any bit of the register is set, an interrupt is raised and the SDIO slave driver calls the callback function defined in the slave_intr_cb member in the sdio_slave_config_t structure.

Note

The callback function is called in the ISR, do not use any delay, loop or spinlock in the callback.

There's another set of functions can be used. 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.

Host Interrupts

The slave can interrupt the host by an interrupt line (at certain time) which is level sensitive. 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 will hold active until all the sources are cleared or disabled.

There are several dedicated interrupt sources as well as general purpose sources. see sdio_slave_hostint_t for more information.

Shared Registers

There are 52 8-bit R/W shared registers 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.

Receiving FIFO

When the host is going to send the slave some packets, it has to check whether the slave is ready to receive by reading the buffer number of slave.

To allow the host sending data to the slave, the application has to load buffers to the slave driver by the following steps:

  1. Register the buffer by calling sdio_slave_recv_register_buf, and get the handle of the registered buffer. The driver will allocate 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.
  2. Load buffers onto the driver by passing the buffer handle to sdio_slave_recv_load_buf.

  3. Get the received data by calling sdio_slave_recv or sdio_slave_recv_packet. If 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 don't care about the packet boundary (e.g. the data is only a byte stream), you can call the simpler version sdio_slave_recv instead.

  4. Pass the handle of processed buffer back to the driver by sdio_recv_load_buf again.

Note

To avoid overhead from copying data, the driver itself doesn't have any buffer inside, the application is responsible to offer new buffers in time. The DMA will automatically store received data to the buffer.

Sending FIFO

Each time the slave has data to send, it raises an interrupt and the host will request for the packet length. There are two sending modes:

  • Stream Mode: when a buffer is loaded to the driver, the buffer length will be counted into the packet length requested by host in the incoming communications. Regardless previous packets are sent or not. This means the host can get data of several buffers in one 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.

Note

To avoid overhead from copying data, the driver itself doesn't 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.

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 effeciency is required, the application can use the following functions instead:

  1. Pass buffer information (address, length, as well as an arg indicating the buffer) to sdio_slave_send_queue. If non-blocking call is needed, set wait=0. 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 keep 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.

There are several ways to use the arg in the queue parameter:

  1. Directly point arg to a dynamic-allocated buffer, and use the arg to free it when transfer finished.

  2. Wrap transfer informations in a transfer structure, and point arg to the structure. You can use the structure to do more things like:

    typedef struct {
    uint8_t* buffer;
    size_t   size;
    int      id;
}sdio_transfer_t;

//and send as:
sdio_transfer_t trans = {
    .buffer = ADDRESS_TO_SEND,
    .size = 8,
    .id = 3,  //the 3rd transfer so far
};
sdio_slave_send_queue(trans.buffer, trans.size, &trans, portMAX_DELAY);

//... maybe more transfers are sent here

//and deal with finished transfer as:
sdio_transfer_t* arg = NULL;
sdio_slave_send_get_finished((void**)&arg, portMAX_DELAY);
ESP_LOGI("tag", "(%d) successfully send %d bytes of %p", arg->id, arg->size, arg->buffer);
some_post_callback(arg); //do more things

  3. Working with the receiving part of this driver, point arg to the receive buffer handle of this buffer. So that we can directly use the buffer to receive data when it's sent:

    uint8_t buffer[256]={1,2,3,4,5,6,7,8};
sdio_slave_buf_handle_t handle = sdio_slave_recv_register_buf(buffer);
sdio_slave_send_queue(buffer, 8, handle, portMAX_DELAY);

//... maybe more transfers are sent here

//and load finished buffer to receive as
sdio_slave_buf_handle_t handle = NULL;
sdio_slave_send_get_finished((void**)&handle, portMAX_DELAY);
sdio_slave_recv_load_buf(handle);

    More about this, see peripherals/sdio.

Application Example

Slave/master communication: peripherals/sdio.

API Reference

inc/sdio_slave_types.inc

inc/sdio_slave.inc