brownout / watchdog reset on voltage drop

RE: brownout / watchdog reset on voltage drop

haakonsh — Mon, 06 Aug 2018 12:21:55 GMT

CBD is correct, the power supply does not fit the use-case. There are a few ways to mitigate the issue, like CBD has already mentioned.

I suggest you take a look at TI's TPS3898 and similar power supply supervisors/reset chips and calculate the required size of your capacitor bank.

RE: brownout / watchdog reset on voltage drop

cbd — Mon, 06 Aug 2018 07:39:16 GMT

You're really pushing the limits if your maximum power draw is 2mA. As noted in the specification and you've seen in practice you require much more than that when BLE is active, hence your capacitor bank.

The system will hold in reset until the internals have stabilised, but I would suggest that you look into a means of holding off powering up your 52832 until after the power bank caps have been fully charged.

Are you only enabling the SoftDevice once you've ascertained that your power supply is of sufficient voltage? With a direct coin cell connection I'd be wary of enabling it if the supply voltage was 2.5V or less as the dip in voltage when BLE transmits can reset the unit. You'll need to calculate/simulate your power cycles with your capacitor bank included.

Have you been using the power fail comparator to help detect insufficient power and disable the BLE?

It strikes me that your support circuitry will need to provide the "user intervention" in the case of brownout reset. Sadly such circuitry will eat into your power budget.

RE: brownout / watchdog reset on voltage drop

mrono — Mon, 06 Aug 2018 06:32:59 GMT

Note that I'm not the original poster, but I think this is an issue that is going go surprise a lot of people if it truly is designed the way you describe.

My requirements are simply that the device needs to recover *without user intervention* from a brown-out situation. Assuming of course that the cause of the brown-out goes away when the MCU resets, and Vdd rises back to an acceptable level.

RE: brownout / watchdog reset on voltage drop

rgrr2 — Fri, 03 Aug 2018 14:19:04 GMT

You are not serious about that? User intervention!? Aarrrggghhhh!

So our current limit provides enough power to keep the brownout detection alive and the nRF is stuck.

I doubt that brownout is implemented that way in e.g. MSP430.

Could someone please give me a pointer where this behaviour is described?

Thanks

Hardy

RE: brownout / watchdog reset on voltage drop

haakonsh — Fri, 03 Aug 2018 13:48:46 GMT

It's designed to require user intervention, like the change of a coin cell battery.

Do you mind sharing some more information on your power supply and your requirements?

RE: brownout / watchdog reset on voltage drop

mrono — Fri, 03 Aug 2018 09:52:48 GMT

[quote userid="13562" url="~/f/nordic-q-a/37058/brownout-watchdog-reset-on-voltage-drop/142583"]it holds the system in a reset state until power cycled.[/quote]

So if you ever do get a brown-out event, how do you recover? How is the power cycling going to happen without user intervention?

RE: brownout / watchdog reset on voltage drop

haakonsh — Fri, 03 Aug 2018 08:16:53 GMT

Brown-out reset.

The current consumption is probably equal to the CPU run current.

The Brown-out detection is a hard limit in any design, you just need to make sure you do not dip below 1.7V and that your dip is not more than 300mV OR you have a slow enough rise time to adhere to:

"A step increase in supply voltage of 300 mV or more, with rise time of 300 ms or less, within the valid supply range, may result in a system reset" as per Power-on reset spec.

RE: brownout / watchdog reset on voltage drop

AmbystomaLabs — Thu, 02 Aug 2018 15:38:16 GMT

Haakonsh can you please provide a link to the detailed spec describing this aspect of brownout reset? I don't recall reading about it and it does bring up some interesting design challenges.

RE: brownout / watchdog reset on voltage drop

rgrr2 — Thu, 02 Aug 2018 11:55:36 GMT

Ooops... what does that mean? The system is hold in a reset state until power cycled? Means that VDD must go down to 0V? How much current is consumed during this state?

RE: brownout / watchdog reset on voltage drop

haakonsh — Thu, 02 Aug 2018 11:21:26 GMT

I misspoke sorry, it does not short VDD to GND, it holds the system in a reset state until power cycled. This will drain the battery as mentioned previously.

RE: brownout / watchdog reset on voltage drop

rgrr2 — Thu, 02 Aug 2018 10:42:46 GMT

Is this "kill switch" somewhere documented? Do I understand it correctly, that the kill switch shortens VDD to Gnd until a certain voltage level is reached?

I will do my best to provide a VDD plot tomorrow.

RE: brownout / watchdog reset on voltage drop

haakonsh — Thu, 02 Aug 2018 10:28:37 GMT

When the brown/out detection is triggered the SoC will short VDD to GND in order to drain the battery. It is a sort of kill switch that prevents spurious operation whenever a coin cell battery has been discharged.

I suggest you increase the capacitance of the 4.7µF supply capcitor.

Do you have any scope of VDD to share?

RE: brownout / watchdog reset on voltage drop

rgrr2 — Wed, 01 Aug 2018 13:58:49 GMT

Thanks for the suggestions. I'm already using POF to prevent the situation. But this can't be the final thing, because if voltage drops to fast the device will be stuck again.

No, device is not powered by a coin cell. Actually it is a loop powered (4..20mA) field device, current budget for the BLE connectivity is max. 2mA which is really limited by hardware.

Concerning your suggestion with the intermediate start up state: I will check that tomorrow.

Any idea what could be the minimum code for a 1s low power wait?

Out of memory I think, it must contain something like

- NRF_POWER->TASKS_LOWPWR = 1;

- start LFCLK

- init RTC to generate an event after 1s

- while (rtc event did not happen) __WFE();

Hardy

RE: brownout / watchdog reset on voltage drop

AmbystomaLabs — Wed, 01 Aug 2018 12:49:19 GMT

The brownout reset likely is working as I don't think there is a way to prevent it.

What is probably going on is that once your battery voltage is low and the reset occurs there is insufficient battery power to boot and the device gets into some odd in between state. Booting takes quite a bit of power since it takes a little while and the processor runs continuously during this time.

You can set the POF for a higher voltage and this will give you an interrupt that you can use to prevent the problem. Thoughts would be terminate the connection, properly service your watchdog, etc.

Also, you should put code in early in main that looks at vdd_nrf and if it is low then you can put the processor to sleep for a minute while the battery voltage picks up. Then once it is high enough continue with the boot.

I'm assuming you are using a coin cell. This is a common problem with coin cells.