Power Cycling Safeguard

Question

Details: 
 nRF52832 
 SDK 14.1 with matching SD 
 Compiling with GCC 
 
 I am trying to solve the following problem: 
 My device is powered by a coin cell (Manganese dioxide). When these cells reach their capacity their internal resistance increases. 
 If my device attempts a Bluetooth communication this may cause the battery voltage to dip below the brownout voltage - triggering a system reset. 
 It is then possible that during startup the device draws too much current and dies again, thereby creating a loop of power cycling which will continue to drain the battery until dead, but also potentially corrupt flash memory in the process. 
 
 My current methodology is to somehow keep a count of the number of resets that gets set to zero after startup, if the device resets 5 times without finishing the startup process, kill the startup and go into system off mode. 
 Unfortunately, no registers are retained on a brownout reset, so I cannot use them to store information on the reset. 
 I could use flash, but there is a risk of corrupting all of the flash data in the startup sequence as the power could die during initialisation or mid-write. 
 The solution that seems to be the most elegant is to simply store a variable in RAM, and create a "noinit" section as described here: noinit example 
 This seems to work for watchdog resets and software resets, but doesn't seem to work for pin resets or brownout resets. On a pin reset (easier to do), the values on reset are pretty consistent, which leads me to believe that some other part of the program that I'm not aware of it writing to that memory? 
 
 Are there any other methodologies I could try here? What is the Nordic best practice around this?

Kenneth · Accepted Answer

I would have measured the battery voltage after reset (for instance over a period of 10ms), and if it's below for instance 2.5V go directly to sleep and potentially blink an led at slow interval to inform user that battery is end of life. I don't think adding counter etc. is a good idea after reset, since you won't have any knowledge of time and also that you may have poor connection of battery if user shake the product or while the battery is inserted in the holder etc. which may give odd results I think. You might find this useful: http://www.low-powerdesign.com/121312-article-extending-battery-life.htm 
 Best regards, Kenneth

hmolesworth · Answer

You are broadly correct; the trick is to not use the coin cell to power the advertising power burst. The common solution is to use about 100uF low-ESR capacitance across the coin cell such that the coin cell basically trickle-charges the capacitance and the capacitors actually act as the burst power source. Tantalum or electrolytics are no use, they have too much leakage but ceramics work well. If you have a miniature device with no physical space then split the 100uF into 3 x 33uF or similar connected in parallel as they require less headroom. 
 To measure the residual battery capacity, turn on some internal peripheral or LED which has a (small) known current requirement; measure battery voltage before and during and the difference - or better the decaying voltage - will allow an estimate of capacity for a given temperature to allow a better choice of when to finally give up advertising attempts due to battery exhausted (at that temperature). 
 If you already have such capacitance (10uF is not enough) the only other option is to use a boost convertor to trickle charge a much higher voltage capacitor, then use that power instead (remember V^2) but of course now you are losing efficiency over the battery lifetime. You can also try using the internal DC-DC instead of the internal LDO to see if you get better results.

Power Cycling Safeguard

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