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esp_spi_slave_protocol.rst

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ESP SPI Slave HD (Half Duplex) Mode Protocol

esp32

Warning

The driver for ESP32 hasn't been developed yet.

SPI Slave Capabilities of Espressif chips

ESP32 ESP32-S2 ESP32-C3
SPI Slave HD N Y (v2) Y (v2)
Tohost intr N N
Frhost intr 2 * 2 *
TX DMA Y Y
RX DMA Y Y
Shared registers 72 64

Introduction

In the half duplex mode, the master has to use the protocol defined by the slave to communicate with the slave. Each transaction may consist of the following phases (list by the order they should exist):

  • Command: 8-bit, master to slave

    This phase determines the rest phases of the transactions. See spi_slave_hd_supported_cmds.

  • Address: 8-bit, master to slave, optional

    For some commands (WRBUF, RDBUF), this phase specifies the address of the shared buffer to write to/read from. For other commands with this phase, they are meaningless but still have to exist in the transaction.

  • Dummy: 8-bit, floating, optional

    This phase is the turnaround time between the master and the slave on the bus, and also provides enough time for the slave to prepare the data to send to the master.

  • Data: variable length, the direction is also determined by the command.

    This may be a data OUT phase, in which the direction is slave to master, or a data IN phase, in which the direction is master to slave.

The direction means which side (master or slave) controls the MOSI, MISO, WP, and HD pins.

Data IO Modes

In some IO modes, more data wires can be used to send the data. As a result, the SPI clock cycles required for the same amount of data will be less than in the 1-bit mode. For example, in QIO mode, address and data (IN and OUT) should be sent on all 4 data wires (MOSI, MISO, WP, and HD). Here are the modes supported by the ESP32-S2 SPI slave and the wire number used in corresponding modes.

Mode command WN address WN dummy cycles data WN
1-bit 1 1 1 1
DOUT 1 1 4 2
DIO 1 2 4 2
QOUT 1 1 4 4
QIO 1 4 4 4
QPI 4 4 4 4

Normally, which mode is used is determined by the command sent by the master (See spi_slave_hd_supported_cmds), except the QPI mode.

QPI Mode

The QPI mode is a special state of the SPI Slave. The master can send the ENQPI command to put the slave into the QPI mode state. In the QPI mode, the command is also sent in 4-bit, thus it's not compatible with the normal modes. The master should only send QPI commands when the slave is in QPI mode. To exit from the QPI mode, master can send the EXQPI command.

Supported Commands

Note

The command name is in a master-oriented direction. For example, WRBUF means master writes the buffer of slave.

Name Description Command Address Data
WRBUF Write buffer 0x01 Buf addr master to slave, no longer than buffer size
RDBUF Read buffer 0x02 Buf addr slave to master, no longer than buffer size
WRDMA Write DMA 0x03 8 bits master to slave, no longer than length provided by slave
RDDMA Read DMA 0x04 8 bits slave to master, no longer than length provided by slave
SEG_DONE Segments done 0x05
ENQPI Enter QPI mode 0x06
WR_DONE Write segments done 0x07
CMD8 Interrupt 0x08
CMD9 Interrupt 0x09
CMDA Interrupt 0x0A
EXQPI Exit QPI mode 0xDD

Moreover, WRBUF, RDBUF, WRDMA, RDDMA commands have their 2-bit and 4-bit version. To do transactions in 2-bit or 4-bit mode, send the original command ORed by the corresponding command mask below. For example, command 0xA1 means WRBUF in QIO mode.

Mode Mask
1-bit 0x00
DOUT 0x10
DIO 0x50
QOUT 0x20
QIO 0xA0
QPI 0xA0

Segment Transaction Mode

Segment transaction mode is the only mode supported by the SPI Slave HD driver for now. In this mode, for a transaction the slave load onto the DMA, the master is allowed to read or write in segments. This way the master doesn't have to prepare a large buffer as the size of data provided by the slave. After the master finishes reading/writing a buffer, it has to send the corresponding termination command to the slave as a synchronization signal. The slave driver will update new data (if exist) onto the DMA upon seeing the termination command.

The termination command is WR_DONE (0x07) for the WRDMA and CMD8 (0x08) for the RDDMA.

Here's an example for the flow the master read data from the slave DMA:

  1. The slave loads 4092 bytes of data onto the RDDMA
  2. The master do seven RDDMA transactions, each of them is 512 bytes long, and reads the first 3584 bytes from the slave
  3. The master do the last RDDMA transaction of 512 bytes (equal, longer, or shorter than the total length loaded by the slave are all allowed). The first 508 bytes are valid data from the slave, while the last 4 bytes are meaningless bytes.
  4. The master sends CMD8 to the slave
  5. The slave loads another 4092 bytes of data onto the RDDMA
  6. The master can start new reading transactions after it sends the CMD8