Useage of HFCLK at SD110 startup

[victor]

Hello,

We are developing an application in which we have an NRF51822-CFAC with sd110 v7.0.0 together with very small energy allowances. Our problem is that we cannot get the NRF51822 to boot up and enable the softdevice without consuming the entire energy buffer that we have. When investigating what seems to be the culprit of the energy consumption we see that the NRF51822 enables the external 16 MHz clock for approximately 20 ms, during which the energy buffer is drained. My question is therefore if there is any way of starting up the softdevice without having it enabling the 16 MHz clock for such a long period of time?

Thankful for any advice.

[Stefan Birnir Sverrisson]

Hi

The softdevice itself should not take much time to enable. It can be however time consuming to enable the low frequency 32kHz external crystal clock source. When initializing the softdevice, you specify the 32kHz clock source to be either external crystal or the internal RC. Try to use the RC instead of the crystal in order to decrease the softdevice initialization time. If you want to use an external 32kHz crystal, you could start with manually enabling the crystal and go to sleep while you wait for it to start. Then you wake up again and initialize the softdevice.

[butch]

An old thread but ... the answer describes the 32kHz clock while the question was about the 16MHz clock. The radio needs the accurate 16MHz crystal clock. It does take about 0.5mSec to start (another thread says the SoftDevice starts it 1.5mSec before it is needed.) Which is less than the 20mSec reported in the original question. Another thread says the clock draws 25uA in standby, and 470uA when active (used by the radio.) The other thread doesn't say how much current the clock uses while starting (the active current?) You need these numbers to calculate a theoretical power budget, for the state: coming out of sleep and starting the HF clock so the radio can use it. As Stefan said, you can sleep the mcu during this state (because there is an interrupt on HFCLKSTARTED event.) You could read code to see whether SoftDevice sleeps, or spins the mcu (drawing 4mA) during this state?

[butch]

The nRF52 product spec under Clock/Electrical specifications does give the typical startup current and a typical startup time for the HFXO.