Unable to get ADC results using accelerometer

[saiteja]

hi, i am working on ble_app_uart_simple_adc example which is referred from the link github.com/.../nrf51-ADC-examples i flashed the code successfully. and detecting the device using NRFTool box application. during the circuit setup i connected accelerometer(ADXL335) Z- output to the p0.01 of the nrf51 development kit and connected ground. after connecting to the device through nrftool box UART application. the log file showing unknown values(strange symbols). how to get accelerometer z output values using this ble_app_uart_simple_adc example code.

[Stefan Birnir Sverrisson]

Hi

The nRFtoolbox UART app is probably receiving the correct ADC values, but it displays the values as ascii characters. If you would instead send readable ascii characters, then the nrftoolbox UART app would be able to display it correctly. E.g. if you would write the adc_event_handler as follows:

static void adc_event_handler(nrf_drv_adc_evt_t const * p_event)
{
	uint8_t adc_result_temp[] = {'c','a','f','e'};
	
	if (p_event->type == NRF_DRV_ADC_EVT_DONE)
	{
		ble_nus_string_send(&m_nus, &adc_result_temp[0], 4);
	}
}

it would write "cafe" in nRFtoolbox. If you want to see the real ADC values however, it is better to connect with nRF Connect app, then you see the raw hexadecimal values instead of ascii characters.

[Stefan Birnir Sverrisson]

Yeah, the nRFtoolbox displays the characters in a strange way. Try to use the nRF Connect app instead in order to see the real ADC output in hexadecimal format. You can not see the real values on nRFToolbox UART app.

What the code is doing in the adc_event_handler is to split every 10-bit ADC output value into two 8-bit values before sending it over BLE. Since the ADC buffer size is 4, the adc_event_handler is called every 4 samples. This will result in 4 samples to be sent every time over BLE, i.e. 8 bytes. You can see the values of those 8 bytes if you connect with nRF Connect app. You can also see the ADC output values if you connect to the UART output as described in the documentation for the example.

When you disconnect the accelerometer from the ADC input pin the input pin will be floating, so the input voltage to the ADC input pin P0.01 can be whatever, and you will see whatever value on the ADC output. To test, you can try to get zero ADC output by shorting P0.01 to GND or get a positive ADC output by shorting P0.01 to VDD on the nRF51-DK board.