SPI3W

Sample List

This chapter introduces the details of the SPI3W sample. The RTL87x2G provides the following samples for the SPI3W peripheral.

• Polling

Functional Overview

The 3-wire serial peripheral interface (SPI3W) supports single write and single/burst read. The host controller can use the SPI3W to write and read registers in the sensor, and to read out the motion information. The host controller always initiates communication; the sensor never initiates data transfers. SPI3W_CS, SPI3W_CLK and SPI3W_DATA may be driven directly by the host controller. SPI3W_DATA may also be driven by the sensor when data is read out from sensor registers.

Feature List

• Support 2-wire and 3-wire communication.

• Support single write.

• Support single/burst read.

• Programmable delay between write and read.

• Max SPI3W Clock 2MHz.

• Half duplex.

• Master only.

Data Format

The data contains two bytes, the first byte is the address bit, the highest bit is the read/write control bit, and the second byte is the data byte. As shown below:

3-Wire SPI Data Format

Write Operation

Write data from the controller to the sensor, only supports single write mode, as shown in the figure below. No delay needs to be added between the address byte and the data byte.

SPI3W write mode format

Read Operation

Single Read Mode

In single read mode, SPI3W will first send an address byte, after which the SPI3W_DATA line switches to receive mode to receive a data byte sent back by the counterpart sensor.

Between the first byte (address byte) and the second byte (data byte), a delay of T-Hold needs to be added. This delay is configured by SPI3W_InitTypeDef::SPI3W_ReadDelay.

SPI3W single read format

Burst Read Mode

In burst read mode, SPI can continuously read multiple bytes of data. SPI3W will first send an address byte, after which the SPI3W_DATA line switches to receive mode to receive multiple data bytes from the responding sensor.

Between the first byte (address byte) and the second byte (data byte 0), a delay of T-Hold needs to be added. This delay is configured by SPI3W_InitTypeDef::SPI3W_ReadDelay. There is no delay between the third byte (data byte 1) and the second byte (data byte 0) until the end of a burst read.

SPI3W burst read format

Polling Mode Flow

Polling Write Flow

The flow for writing data in SPI3W polling mode is as shown in the diagram:

SPI3W polling write flow

bool spi3wire_writebyte(uint8_t address, uint8_t data)
{
   uint32_t timeout = 0;

   /* Check 3wire spi busy or not */
   while (SPI3W_GetFlagStatus(SPI3W_FLAG_BUSY) == SET)
   {
      timeout++;
      if (timeout > 0x1ffff)
      {
            return false;
      }
   }
   /* Write data */
   SPI3W_StartWrite(address, data);

   timeout = 0;
   while (SPI3W_GetFlagStatus(SPI3W_FLAG_BUSY) == SET)
   {
      timeout++;
      if (timeout > 0x1ffff)
      {
            return false;
      }
   }
   return true;
}

Polling Read Flow

The flow for reading data in SPI3W polling mode is as shown in the diagram:

SPI3W polling read flow

uint8_t spi3wire_readbyte(uint8_t address)
{
   uint8_t reg_value = 0;
   uint32_t timeout = 0;

   /* Check spi busy or not */
   while (SPI3W_GetFlagStatus(SPI3W_FLAG_BUSY) == SET)
   {
      timeout++;
      if (timeout > 0x1ffff)
      {
            break;
      }
   }

   /* Clear Receive data length */
   SPI3W_ClearRxDataLen();

   SPI3W_StartRead(address, 1);

   timeout = 0;
   while (SPI3W_GetFlagStatus(SPI3W_FLAG_BUSY) == SET)
   {
      timeout++;
      if (timeout > 0x1ffff)
      {
            break;
      }
   }

   /* Get the length of received data */
   while (SPI3W_GetRxDataLen() == 0);
   /* Read data */
   SPI3W_ReadBuf(&reg_value, 1);

   return reg_value;
}