LM75B sensor mode transition failure

Question

Hi, I'm using NXP LM75B temprature sensor based on nRF SDK's example code. 
 I would like to save power on this sensor. 
 I have prepared the code as follows: 
 /* Mode for LM75B. */
#define LM75B_MODE_NORMAL 0U
#define LM75B_MODE_SHUTDOWN 1U // Add 
 /**
 * @brief Function for setting sleep mode on LM75B temperature sensor.
 */
void LM75B_sdn_mode(void)
{
 ret_code_t err_code;

 /* Writing to LM75B_REG_CONF "1" set temperature sensor in SHUTDOWN mode. */
 uint8_t reg[2] = {LM75B_REG_CONF, LM75B_MODE_SHUTDOWN};
 err_code = nrf_drv_twi_tx(&m_twi, LM75B_ADDR, reg, sizeof(reg), false);
 APP_ERROR_CHECK(err_code);
 while (m_xfer_done == false);

 /* Writing to pointer byte. */
 reg[0] = LM75B_REG_TEMP;
 m_xfer_done = false;
 err_code = nrf_drv_twi_tx(&m_twi, LM75B_ADDR, reg, 1, false);
 APP_ERROR_CHECK(err_code);
 while (m_xfer_done == false);
} 
 Following main() is work fine: 
 /**@brief Function for application main entry.
 */
int main(void)
{	
 uint32_t err_code;
 LmHandlerErrorStatus_t lm_Error_code;
	
 // Initialize logs.
 log_init();
 NRF_LOG_INFO("LoRaWan end-device started.");
	
 // Initialize clocks & power
 nrf_drv_clock_init();
 nrf_drv_clock_lfclk_request(NULL);
 nrf_pwr_mgmt_init();

 // Enable nRF52 DCDC
 NRF_POWER->DCDCEN = 1;

 // Initialize Scheduler and timer
 timers_init();

 // Initialize NVM
 NvmDataMgmtInit();
 
 // Initialize TWI
 twi_init();

 // Initialize LM75B
 LM75B_set_mode();
 nrf_delay_ms(200);

 // Initialize LoRa chip.
 err_code = lora_hardware_init();
 APP_ERROR_CHECK(err_code);

 // Initialize transmission periodicity variable
 TxPeriodicity = APP_TX_DUTYCYCLE + randr(-APP_TX_DUTYCYCLE_RND, APP_TX_DUTYCYCLE_RND);
	
 // Initialize LoRaWan
 lm_Error_code = LmHandlerInit(&LmHandlerCallbacks, &LmHandlerParams);
 APP_ERROR_CHECK_BOOL(lm_Error_code == LORAMAC_HANDLER_SUCCESS);

 // Set system maximum tolerated rx error in milliseconds
 LmHandlerSetSystemMaxRxError(20);

 // The LoRa-Alliance Compliance protocol package should always be initialized and activated.
 LmHandlerPackageRegister(PACKAGE_ID_COMPLIANCE, &LmhpComplianceParams);

 // Start Join procedure
 LmHandlerJoin();

 StartTxProcess(LORAMAC_HANDLER_TX_ON_TIMER);
 
 // Enter main loop.
 for (;;)
 {
 // Process the LoRaMac events
 LmHandlerProcess();

 // Process application uplinks management
 UplinkProcess();

 // Process events managed by the scheduler
 app_sched_execute();

 if (IsMacProcessPending == 1)
 {
 // Clear flag and prevent MCU to go into low power modes.
 IsMacProcessPending = 0;
 }
 else
 {
 if (NRF_LOG_PROCESS() == false)
 {
 nrf_pwr_mgmt_run();
 }	
 }
 }
} 
 And the following code is also work fine (This is only a function test. No sensor is used.): 
 // Initialize LM75B
// LM75B_set_mode();
// nrf_delay_ms(200);
 LM75B_sdn_mode();
 nrf_delay_ms(200); 
 But the following code stops with some error. (Normal mode -> Shutdown mode transition test.): 
 // Initialize LM75B
 LM75B_set_mode();
 nrf_delay_ms(200);
 LM75B_sdn_mode();
 nrf_delay_ms(200); 
 I don't think the problem is caused by AD conversion timing since I put in 200mSec weighting. 
 I believe I am making a basic oversight. Do you know what the problem is? 
 thanks, 
 tyro

user14926 · Accepted Answer

Hi, 
 Try to modify LM75B_sdn_mode() by adding the following line before the first call to nrf_drv_twi_tx(): 
 m_xfer_done = false; 
 If you have not changed LM75B_set_mode() from the default example, this flag is not cleared at the end of the function. When you call nrf_drv_twi_tx() in your new function and go to the while-loop to wait for the flag, the flag is already set and the function will not wait for the transfer to complete. When you call nrf_drv_twi_tx() the second time in this function, the first transfer is not completed, and the driver will report an error. 
 Best regards, Jørgen

LM75B sensor mode transition failure

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