nRF52832 SAADC Occasionally returns 0xFFFF

[CurtisHx]

I'm having a problem with the nRF52 SAADC. It's configured for

• One shot operation on Channel 0 using pin Ain4.
• Gain of 1/6
• Tacq set to 40us
• 8 bit resolution
• Internal reference
• Single ended (PSELN connected to ground)
• No oversample
• burst disabled.
• Sample rate controlled from task

Here's the contents of my "read" function:

uint16_t sample;
NRF_SAADC->RESULT.PTR = uint32_t(&sample);
NRF_SAADC->RESULT.MAXCNT = sizeof(sample);

NRF_SAADC->EVENTS_STARTED = 0;
NRF_SAADC->TASKS_START = 1;
while(NRF_SAADC->EVENTS_STARTED ==0){nop();}

NRF_SAADC->EVENTS_DONE = 0;
NRF_SAADC->TASKS_SAMPLE = 1;
while(NRF_SAADC->EVENTS_DONE ==0){nop();}

NRF_SAADC->EVENTS_STOPPED = 0;
NRF_SAADC->TASKS_STOP = 1;
while(NRF_SAADC->EVENTS_STOPPED ==0){nop();}

I have the Ain4 pin connected to ground through a 200 ohm resistor. Is it possible for the ADC to return negative when it's configured for single ended and the analog input goes lower than ground?

[Wojtek]

I think 0xFFFF = -1 in int16_t. So, it is almost 0.

[CurtisHx]

@Wojtek Yes, 0xFFFF is -1 in int16_t, which makes the read voltage very, very close to 0 (-14mV with my configuration). I'm not used to an ADC returning negative, and I'm trying to figure out if I'm doing something wrong or this particular ADC will return negative.

[Jørgen Holmefjord]

Hi,

It is normal that the SAADC can return slightly negative values. See this thread.

Best regards,

Jørgen

[Siddharth rana]

Hello jorgen.

Right now I am facing the same issue .I get 0xfff value in buffer can u help me out

[Jørgen Holmefjord]

I'm not sure what you want help with. As described in the answers here it is normal that the SAADC can return negative values. Store it in a int16_t variable and set negative values to 0 if you do not want them in your application.