ESP SPI Slave HD (Half Duplex) Mode Protocol

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 consists 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 Supported Commands.

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

    For some commands (WRBUF, RDBUF), this phase specifies the address of 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 turn around 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 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 use to send the data. As a result, the SPI clock cycles required for the same amount of data will be less than in 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’s the modes supported by 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 is determined by the command sent by the master (See Supported Commands), except from the QPI mode.

QPI Mode

The QPI mode is a special state of the SPI Slave. The master can send 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 the QPI mode. To exit form the QPI mode, master can send EXQPI command.

Supported Commands

Note

The command name are 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 large buffer as the size of data provided by the slave. After the master finish reading/writing a buffer, it has to send 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 are 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