configuring multiple saadc channels

[Masoumeh]

Hi,

I want to configure 2 channels of saadc in differential mode as follows:

void saadc_init(void) //ok
{
    ret_code_t err_code;

	nrf_saadc_channel_config_t channel_config1 =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_DIFFERENTIAL(NRF_SAADC_INPUT_AIN4,NRF_SAADC_INPUT_AIN6);
	nrf_saadc_channel_config_t channel_config2 =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_DIFFERENTIAL(NRF_SAADC_INPUT_AIN5,NRF_SAADC_INPUT_AIN7);

    err_code = nrf_drv_saadc_init(NULL, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_config1);
    APP_ERROR_CHECK(err_code);
		err_code = nrf_drv_saadc_channel_init(1, &channel_config2);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);
		
		nrf_saadc_continuous_mode_enable(800);
	

}

But when I received data, it seems that only the channel that its channel index is zero is enabled. may you help me to find the issue and configure this 2 channel in scan mode? i am working based on the saadc example in sk17. 

[Jørgen Holmefjord]

Hi,

It looks like you are using the internal timer to enable continuous sample mode, by calling the nrf_saadc_continuous_mode_enable() function. As written in the SAADC peripheral documentation:

"The SAMPLERATE timer mode cannot be combined with SCAN mode, and only one channel can be enabled in this mode."

You need to use an external TIMER/RTC to trigger the sampling when using SCAN mode.

Best regards,
Jørgen

[Jørgen Holmefjord]

There is a SAADC example in the SDK that shows how to use a TIMER to sample the SAADC at a regular interval. This can easily be extended with multiple channels.

If you want low-power SAADC, there are some examples available in this GitHub repository.

If you want to use the legacy SAADC driver API with low power and multiple channels, see the example in this post.

[Masoumeh]

Yes, I convert them to voltage but the program is ok, I tested it. I reduce the SR but the problem is still there. Also my acquisition time is 10us. the interesting thing is that, when I imply an sin input to the second channel, its function is ok and I will have the correct answer. 

Another problem I have is that I have some kind of noise on my signal which includes some harmonic in 2k,4k,6k,..... I think it comes from the power switching between LDO and DCDC. Do you know how often do MCU switch between them? or what is the frequency of this task? 

[Jørgen Holmefjord]

The switching between LDO and DCDC is handled automatically by the chip depending on the load: "Automatic switching between LDO and DC/DC regulator based on load to maximize efficiency"

The DCDC switching itself happens at 8MHz, as mentioned in this post. It is possible that it could be harmonics to this, but I would need to verify that with our developers. Have you verified that the harmonics disappear if you do not enable the DCDC regulator?

[Masoumeh]

Hi, I still have this problem. I wanted to ask that how do samples save in buffers of ADC? I put every 12 bit of my data in 1.5 bytes and put all data together in a buffer and send it by ble(Line 154 to 172 ).  I wanna see if I understand the concept of adc savind format . 

[Jørgen Holmefjord]

Hi,

As mentioned in the SAADC peripheral documentation:

"Samples stored as 16-bit 2’s complement values for differential and single-ended sampling"

and:

"Results are stored in little-endian byte order in Data RAM. Every sample will be sign extended to 16 bit before stored in the Result buffer."

You may face issues if you use 12 bit resolution only send the lower 12 bits of the sample, for instance will the samples "-1" and "4095" have the same binary value (0b1111111111111111 - 0b0000111111111111).

Best regards,
Jørgen

[Masoumeh]

Thanks, it helped.