How to use 3-wire SPI on BMP3 sensor

[ArDes]

Hi everyone,

For my master's thesis, I designed a PCB with Bosch BMP384 sensors. In my previous version, I utilized 4-wire SPI for communication. However, in the newest version, I decided to switch to 3-wire SPI. I am using an Arduino to communicate with the sensors, and I found a GitHub repository from Bosch that explains how to write code for sensor communication (https://github.com/boschsensortec/BMP3_SensorAPI). The GitHub repository only provides code examples for 4-wire SPI and does not cover 3-wire SPI.

The sensors uses 4-wire SPI by default,  to use 3-wire SPI, bit 0 of register 0x1A needs to be changed from 0 to 1. I have tried multiple approaches, but I am unable to make it work. Does someone have an example of how to set up 3-wire SPI communication with a Bosch sensor?

Thank you in advance.

[BSTRobin]

Hi Christoforos,

In addition to setting IF-CONF to 3-wire SPI, you need to ensure that the schematic is connected according to the 3-wire SPI.

The remaining work requires the host driver to access BMP384 through a 3-wire SPI.

[ArDes]

Hi, 

The schematic of my PCB is indeed the same as in the picture. However, my struggle lies in the code. I don't know how to program the code to set up the 3-wire communication.

[ArDes]

This is the code I made to write on register 0x1A and to read on register 0x00 to see what the chip_id is. The chip id should be 0x50.When I read the register, I'm getting either 255 or 0 as the output, indicating a mistake in my code. Does someone know what I did wrong? 

#include <SPI.h>

const int ssPin = 6; // Set the SS (Slave Select) pin

#define BMP3_REG_IF_CONF UINT8_C(0x1A)

#define BMP3_REG_CHIP_ID UINT8_C(0x00)

uint32_t clockFrequency = 100000;

void get_reg(uint8_t reg_addr, uint8_t *reg_data, uint32_t len, uint8_t dummy_byte);

void read(uint8_t regAddress, uint8_t* dataBuffer, uint32_t numBytes, int pin);

void write(uint8_t regAddress, uint8_t* dataBuffer, uint32_t numBytes, int pin);

void set_reg(uint8_t *reg_addr, const uint8_t *reg_data, uint32_t len);

void setup() {

Serial.println("SPI SETUP BEGIN");

pinMode(ssPin, OUTPUT); // Set the SS pin as an output

SPI.begin(); // Initialize the SPI library

Serial.begin(9600); // Initialize serial communication for debugging

Serial.println("SPI SETUP DONE");

}

void loop() {

Serial.println("Start writing");

int8_t rslt;

uint8_t reg_addr1 = BMP3_REG_IF_CONF;

/* bit i want to change*/

uint8_t op_mode_reg_val = 1;

/* Write the power mode in the register */

set_reg(&reg_addr1, &op_mode_reg_val, 1);

Serial.println("end writing");

Serial.println("wait 5 sec");

delay(5000);

Serial.println("start reading");

uint8_t reg_addr2;

uint8_t reg = 0;

uint8_t dummy_byte = 1;

uint32_t len = 1;

reg_addr2 = BMP3_REG_CHIP_ID;

get_reg(reg_addr2, &reg, 1, dummy_byte);

// Print the value of reg

Serial.print("Register value: ");

Serial.println(reg);

Serial.println("end reading");

delay(5000);

}

void get_reg(uint8_t reg_addr, uint8_t *reg_data, uint32_t len, uint8_t dummy_byte){

uint32_t idx;

uint32_t temp_len = len + dummy_byte;

uint8_t temp_buff[len + dummy_byte];

reg_addr = reg_addr | 0x80;

read(reg_addr, temp_buff, temp_len, ssPin);

for (idx = 0; idx < len; idx++)

{

reg_data[idx] = temp_buff[idx + dummy_byte];

}

}

void read(uint8_t regAddress, uint8_t* dataBuffer, uint32_t numBytes, int pin){

SPI.beginTransaction(SPISettings(clockFrequency, MSBFIRST, SPI_MODE0));

digitalWrite(pin, LOW);

SPI.transfer(regAddress | 0x80);

// Read all requested bytes

for(uint32_t i = 0; i < numBytes; i++)

{

dataBuffer[i] = SPI.transfer(0);

}

// End transmission

digitalWrite(pin, HIGH);

SPI.endTransaction();

}

void set_reg(uint8_t *reg_addr, const uint8_t *reg_data, uint32_t len)

{

int8_t rslt;

uint8_t temp_buff[len * 2];

uint32_t temp_len;

uint8_t reg_addr_cnt;

temp_buff[0] = reg_data[0];

for (reg_addr_cnt = 0; reg_addr_cnt < len; reg_addr_cnt++)

{

reg_addr[reg_addr_cnt] = reg_addr[reg_addr_cnt] & 0x7F;

}

temp_len = len;

write(reg_addr[0], temp_buff, temp_len, ssPin);

}

void write(uint8_t regAddress, uint8_t* dataBuffer, uint32_t numBytes, int pin){

SPI.beginTransaction(SPISettings(clockFrequency, MSBFIRST, SPI_MODE0));

digitalWrite(pin, LOW);

SPI.transfer(regAddress);

/* Write all the data */

for(uint32_t i = 0; i < numBytes; i++)

{

SPI.transfer(dataBuffer[i]);

}

// End transmission

digitalWrite(pin, HIGH);

SPI.endTransaction();

}