Non-obvious power problem

[John]

Hello,

This is less a question and more of a survey. We've recently completed development on a project using the nRF51822 but have run into an odd power problem. We built and tested the first 500 or so units. Our design has the unit in System Off mode while they are in stock or not in use by the customer. However, after a couple of weeks 5 units in stock were found to have completely depleted 2 AAA batteries.

We've gone through all of the usual suspects such as floating pins, misuse of peripherals and most anything we could search for in this forum. So, I thought I would post and see if anyone had experiences with odd power bugs using the nRF51 that they would share to give me additional ideas to try.

We are using the nRF51 as our main processor on this product, and it is running the FreeRTOS port using tickless idle. When we are in our "storage mode" as we call our system off behavior while in stock, the board typically burns well under 1uA. While in stock the units are boxed and the only GPIO that could wake the device back up is a button push which should be impossible.

Thanks in advance for any ideas or just good old war stories.

Regards, John

[Stefan Birnir Sverrisson]

Hi John

Are you aware of the behavior of the debug pins?

[John]

Thanks Stefan, I was not aware of that. I'll have the HW guys check on that. For completeness sake, I'll attach the schematic (it's not a complicated board by any means).

Also I wanted to note that this is the first time we have used System Off instead of the System On Idle mode for low power. Additionally it is the first time we have used the sd_power_pof_* API's with the threshold set to 2.1V. I'm not sure if there are any rookie mistakes we could be making with either of those functionalities.

[Stefan Birnir Sverrisson]

In system off, it is much simpler to get low current consumption, as entering system off disables all peripherals and the only way to wake up from system off is with a reset, assert on sense enabled pin, or with low power comparator. In the nRF51 Series Reference manual 3.0, section 11.1.2, it says:

If the device is in System OFF mode, entering into debug interface mode will generate a wakeup.

So if you are in system off and there is noise on SWDCLK pin, it can cause the chip to wake up and enter into debug mode with ~1mA current consumption. That is the most likely explanation I would think, as the battery is depleted in two weeks. So I would suggest to put 470ohm external pulldown resistor on SWDCLK pin

[John]

Once again you have proven why you are over 14,000 reputation points. We ran units through the final production steps until we detected one that was in a high power state. We were able to remove the unit from its case very carefully and get it on a multi meter to measure power. When it was supposed to be in system idle mode it was burning 1.2mA, an hour later when it went into system off mode, power consumption jumped to 4-5mA. Clearly it was running with debugging enabled.

One question I have been asked and have answered no to with 99% confidence is that the only way into debug mode is via the pins. There is no way to enter debug mode programatically in the FW?

[Stefan Birnir Sverrisson]

That is correct. I don't know any other method to enter debug mode than is described in section 11.1.2 in the nRF51 Series Reference manual 3.0, i.e. via the debug pins.