TWI not working for HRMI using nRF52

Question

Hello,

I am trying to implement a Heart Rate Monitor Interface (HRMI) board with my nRF52, using the TWI_sensor example from SDK 12.2. I am unable to read any heart rate values and it appears the the SDA and SCL lines are not communicating whatsoever. This is because the data I am reading is garbage and will not change.

Here is my process:

Set the HRMI mode by writing <0x53><N> to the HRMI I2C address #define HRMI_I2C_ADDR 127 using HRMI_set_mode(), where <N> sets the mode to average mode when N=1:

 ret_code_t HRMI_set_mode() {
 ret_code_t ret_code;
 uint8_t reg[2] = {0x53, 0x01}; // <0x53><N> 
 ret_code = nrf_drv_twi_tx(&m_twi_master, HRMI_I2C_ADDR, reg, sizeof(reg), false);
 return ret_code;
 }

Request a set of heart rate samples by writing <0x47><N> to the HRMI I2C address (127) using start_HR(), where <N> is the number of samples to request:

 ret_code_t start_HR(uint8_t num_samples) 
 {
 ret_code_t ret_code;
 uint8_t reg[2] = {HRMI_GET_HR_DATA, num_samples}; // <0x47><N> 
 ret_code = nrf_drv_twi_tx(&m_twi_master, HRMI_I2C_ADDR, reg, sizeof(reg), false);
 return ret_code;
 }

Read the response by allocating an array for data returned over I2C using fetch_HR():

 ret_code_t fetch_HR(uint8_t * hr){
 ret_code_t ret_code;
 uint8_t returned_over_I2C[3]; //Array to hold returned data
 ret_code = nrf_drv_twi_rx(&m_twi_master, HRMI_I2C_ADDR, returned_over_I2C,  sizeof(returned_over_I2C));  
 nrf_delay_ms(10);
 	*hr = returned_over_I2C[2];
 	return ret_code;
 }

Here is my main function with the function calls to do this:

    int main(void)
    {	
    ret_code_t err_code;  
    
	uint8_t hr;
		
    err_code = twi_master_init();

    HRMI_set_mode(); // Set HRMI to average mode

    APP_ERROR_CHECK(err_code);
    
    while (true)
    {           
        start_HR(0x01); //Request N = 1 (0x01) HR samples
        fetch_HR(&hr); 
        SEGGER_RTT_printf(0, "Heart Rate: %d\n", hr););
	    } }

As for my circuitry, my SDA and SCL lines both have a 4.7kohm pull-up resistor bridged between 5V. On the HRMI board itself, I have not installed the jumper SJ1, and I have installed jumper OP0. No other jumpers are installed to ensure that the I2C address is in fact 127. This is explained in the HRMI datasheet.

I have checked off everything from the troubleshooting section for I2C in the HRMI datasheet (p.33). I am confident my TWI setup is correct because I am able to get sensor data over I2C concurrently from an accelerometer when I run my code (not shown).

When I read all the values from my returned_over_I2C array, the data appears as <0x00><0x36><0x244>, where <0x244> should be the HR1 value read. Again, these values appear even when I disconnect the SDA and SCL lines from the HRMI board.

Does anyone have any insight into this?

Thank you,

Levi