SD Write Problem

[bryanhsieh]

Hi Everyone,

I am trying to record data from ADC to a SD card every 500ms using nrf52832. I am following the fatfs example. Before I had a problem whereby every approximately 10s I would lose a few samples from the ADC as shown by the figure below. I thought the problem was due to the SD card write latency as every 10s I observed an increase in current that usually lasted for around a few hundred milliseconds. After reading around I found that instead of opening and closing the file everytime I wanted to record data onto the SD card I should use f_sync() and this somehow fixed the problem.

But then I still observe this regular increase in write duration, so I was wondering if this is caused by the SD card write latency or is it due to software bugs. And how does the fatfs module handle this (ie when calling f_write) if it's due to SD write latency.

Thanks for the help. Bryan

White lines showing loss of ADC samples x-axis = sample number y-axis = voltage in uV

Significant increase in write duration for the fifth peak

White lines showing loss of ADC samples x-axis = sample number y-axis = voltage in uV

---

Update: I tested with another SD card and the problem with the missing samples came back. So f_sync did not solve the problem.

---

Update: 13/11/17

Code For writing to the SD card added.

In Main

while(1)
		{
			if( sampling_done == true)
			{
				sdWrite(data);
			}
			power_manage();
		}

In SAADC callback

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
		
		if(p_event->type == NRF_DRV_SAADC_EVT_DONE)
		{			
				
					ret_code_t err_code;
					
					err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, NUM_OF_CHANNELS);
					APP_ERROR_CHECK(err_code);
					
					int16_t temp[4]={0};
					int8_t notification[8]={0};
					temp[0] = ADC_result_conversion(p_event->data.done.p_buffer[0]);
					temp[1] = ADC_result_conversion(p_event->data.done.p_buffer[1]);
					temp[2] = ADC_result_conversion(p_event->data.done.p_buffer[2]);
					temp[3] = ADC_result_conversion(p_event->data.done.p_buffer[3]);

			
					NRF_LOG_INFO("ADC event number: %d\r\n", (int)event_cnt);
					NRF_LOG_INFO("CH1: %d.%02d \r\n",	NRF_LOG_FLOAT(temp[0]));
					NRF_LOG_INFO("CH2: %d.%02d \r\n",	NRF_LOG_FLOAT(temp[1]));
					NRF_LOG_INFO("CH3: %d.%02d \r\n", NRF_LOG_FLOAT(temp[2]));
					NRF_LOG_INFO("CH4: %d.%02d \r\n", NRF_LOG_FLOAT(temp[3]));
									
					NRF_LOG_INFO("\n");
					NRF_LOG_FLUSH();
					memcpy( &data[0]+NUM_OF_CHANNELS*event_cnt, &temp[0], NUM_OF_CHANNELS*2);
					memcpy( &notification[0], &temp[0] , NUM_OF_CHANNELS*2);
					event_cnt++;
					
					if( event_cnt >= MAX_NUM_SAMPLES)
					{
						//sdWrite(data);
						sampling_done = true;
						event_cnt = 0;
					}
				
					ble_cus_custom_value_update(&m_cus, notification);
					sample_cnt++;		                                                            
    }
}

In sdWrite function

static void sdWrite(int16_t data[])
{
		
		FRESULT ff_result;
	  uint32_t bytes_written;
		char buffer[3000] = {0};
		int n=0;
		int counter=0;
	
   
		while(counter < MAX_NUM_SAMPLES)
		{
			n += sprintf(&buffer[n], "%d %d %d %d\r\n", data[NUM_OF_CHANNELS*counter], data[NUM_OF_CHANNELS*counter+1], data[NUM_OF_CHANNELS*counter+2], data[NUM_OF_CHANNELS*counter+3]); //format the data to ASCII
			counter++;
		}
			ff_result = f_write(&file, buffer , n , (UINT *) &bytes_written);
			if (ff_result != FR_OK)
			{
					NRF_LOG_INFO("Write failed\r\n.");
			}
			else
			{
				    //NRF_LOG_INFO("%d bytes written.\r\n", bytes_written);
			}
		 NRF_LOG_FLUSH();
		 f_sync(&file);
		
}

[Jørgen Holmefjord]

What do you mean when you say you lose samples? Are they not written to SD card, or not sampled? Are you seeing any errors when writing to the card?