nRF52832 with LIS2DH accelerometer not powering up from battery

RE: nRF52832 with LIS2DH accelerometer not powering up from battery

AmbystomaLabs — Mon, 25 Jun 2018 16:30:35 GMT

Trying to run in debug mode on a CR battery is just a mistake so there isn't any light to shed on it.

Depending on how you have debug mode configured the nRF can have SWD, RTT and UART software/hardware running. All these consume a lot of power. Even without your accelerometer, in debug mode an nRF either won't run or won't run for very long on a CR battery.

This still doesn't solve your problem of having too little local capacitance for a CR based design. Yes, you might get it to boot on a fresh battery. Will it boot and run on a 50% depleted battery? I doubt it.

Also you will forever be plagued by brownout resets of your device during TX/RX cycles.

You need more capacitance and do a proper current/power consumption audit. It's standard practice for any design.

Once you know your peak duty and peak current needs you can then go through and calculate capacitance requirements and keep the ripple from the CR battery in a reasonable range.

RE: nRF52832 with LIS2DH accelerometer not powering up from battery

Coz — Mon, 25 Jun 2018 15:33:27 GMT

So we've been checking everything on the board and also checking the power to the board with the scope, with and without the capacitor. This allowed us to check the difference in the signal when the board was successfully powering up and not, although we have not noticed anything as to why the board had not been powering up.

We've also been trying various things in the settings of the project to see whether that changes anything or not, like delaying the initialisation of the TWI driver etc. One of the other settings we changed was the programming of the board in debugging and release mode. We found out that when programming the board in debug mode the board would not power up with the batteries but if the board is programmed in release mode then the board powers up correctly and the LIS2 works just fine.

These results are repeatable every time although we are still baffled as to why this is working the way it is. Does anyone have any information on the matter and can shed some light to it? Any help greatly appreciated.

RE: nRF52832 with LIS2DH accelerometer not powering up from battery

AmbystomaLabs — Wed, 20 Jun 2018 20:17:58 GMT

You will never get it to work reliably with just a 4.7uF. Just look at any CR battery spec for confirmation.

There is plenty of material in the devzone on CR battery designs. Also, TI did a nice whitepaper on the subject which you can find on their website.

Playing around with your startup might get the board to boot as Turbo noted, but it will never give you a reliable product.

You need more capacitance on the board and pay close attention to your max TX/RX duty cycle as you write your code.

RE: nRF52832 with LIS2DH accelerometer not powering up from battery

Turbo J — Wed, 20 Jun 2018 16:13:35 GMT

[quote userid="27108" url="~/f/nordic-q-a/35595/nrf52832-with-lis2dh-accelerometer-not-powering-up-from-battery"] The board size is about 12mm[/quote]

Those batteries really don't like the operating current of an NRF52 due to thier high internal resistance.

You may want to look at the VCC voltage with an oscilloscope...

A few ideas to try:

Replacing the 4µ7 caps with the largest cap that fits in the footprint.

Delaying the initialization of LIS2DF and the TWI driver, using a low power (RTC based) timer, for a second or two after POR. This should raise the voltage of the battery that was drained during initialization of the NRF52 - but it also means you cannot simply use nrf_delay(), as this will keep the CPU running and current consumtion high.