nRF52 RTC to wake up time

[Niall]

My application requires ultra-low power consumption, hence the nRF52810 will operate in low power - RTC running from LFRC clock, full RAM retention (1.5uA current spec'd). The RTC will periodically wake up the device to take SPI data. Can someone please advise what the time from wake up from this low power state to initiating an SPI transaction might be.

What I am struggling to understand is the time taken from enabling the HFINT (3us start up) to HFCLK for SPI being enabled and SPI transaction commencing. Is there a base power up time required for the nRF52 to go from operating in a low current state to running the SPI port? Power management or system clocks being enabled for example.

The system power I am trying to account for will impact the frequency of the sensor measurement on the SPI line. The longer each SPI transaction takes the more power I will be forced to burn. Any advice would be much appreciated.

Regards

Niall

[hobby16]

Use SPI interrupt to wake up instead of RTC interrupt.

Better, use the SPI's DMA to store incoming data in RAM (no CPU usage needed so even more power saving) and wake up only when the RAM buffer zone is full.

[Niall]

Hi, thanks for the response. Unfortunately this isn't an option for me because the ADC I am using must be triggered (and configured) each time the system wakes.

[Niall]

Hi, thanks for the response. Unfortunately this isn't an option for me because the ADC I am using must be triggered (and configured) each time the system wakes.

[Jared]

Hi,

You could use PPI and connect it to an RTC Compare event, and then set the TASK as a start SPI TASKS_START. The operation should use 1 µs to start the transmission once the TASK is triggered, and 0.0625 µs to a PPI event from low power mode. So a total of 1.0625 µs. 

Best regards

Jared