Slow conversion of external ADC with SoftDevice

[Filya]

Hi

There was a problem using the external ADC  AD7792 in a project with SoftDevice. The problem is that when the ADC is processed in a project with SoftDevice, the waiting time for the end of the ADC conversion significantly increases - 10.5 seconds. The delay occurs at the moment when the ADC must respond to the completion of the conversion (low level RDY).

If use a project without SoftDevice (from the examples of "peripheral"), then the conversion is completed faster - 4.5 seconds.
I also tried to comment out the initialization of everything connected with bluetooth, leaving only the timer and ADC processing functions, but it did not give any results. Therefore, I can not understand why in projects where bluetooth is used (from the "ble_peripheral" examples), such a delay occurs.

To control the ADC, I use the usual pin switching. I do not use SPI module.

Maybe someone came across a similar or there are suggestions, what could be the reason ? 

Chip: SKB501 (analog nRF52840);

SDK: nRF5_SDK_15.2.0;

Softdevice: s132_nrf52_6.1.0_softdevice;

Thank you in advance.

[Edvin]

To control the ADC, I use the usual pin switching. I do not use SPI module.

 Can you explain what you mean by this? How do you control the ADC, and how do you get the data?

[awneil]

Post a schematic of how you have the ADC connected to the nRF52840.

The ADC neither knows nor cares whether you have a softdevice (it doesn't even know what microcontroller you're using) - it just responds to the signals on its interface pins as specified in its datasheet.

So use an oscilloscope or logic analyser on the interface lines to see what is happening in the two cases ...

[Filya]

I control the ADC by personally switching pins using the nrf_gpio_pin_set / nrf_gpio_pin_clear commands (for SCLK and DIN pins). And with the help of nrf_gpio_pin_read (DOUT/RDY pin), I read the data from the ADC.

[awneil]

Filya said:

I control the ADC by personally switching pins

commonly known as  "bit-banging",  which is obviously going to be affected by the CPU load - such as the SoftDevice.

So this is to be expected.

Should be obvious from observing the signals on an oscilloscope.

The reason microcontrollers have hardware for stuff like this is precisely so that it does not require CPU resources!

[Edvin]

Did you figure out why it takes more time when you use the softdevice? Does it take more time before you send the SPI cmd? Does it take more time before you get the reply from the AD7792?

Can you try to use a logic analyser on the SPI pins and see what's going on?