Brown-out and POR not 100% failsafe?

[M6]

I've noticed that our custom board sometimes does not work (e.g. execute the code) when the VCC drops to about 0.3V- 0.5V and comes back on again to 3.3V.

For testing I've made a simple program: It makes one GPIO an output and then toggles this GPIO in a while loop. This program is flashed at address 0, the soft device is not used.

I measure the VCC and toggling of the GPIO with a scope. See attached graph. Upper trace is the toggling of the GPIO. Lower trace is the VCC. After the second VCC dropout the GPIO isn't toggled anymore.

Nothing is attached on the GPIO and SWDIO/NRESET pins. The SWCLK is pulled down with a 12K resistor. Grounding SWCLK did not make a difference.

Device used is a NRF51822. I've tested this with the C0, G2 and H0 revisions.

Is this known behaviour or am I missing something?

[kchuva]

I have the same problem and the same behaviour on my nRF51822 H0 revision. S110 SoftDevice is enabled. When the VCC drops to about 0.5 V and slow comes back to 3.0 V, the brown-out reset is not happenning. So, I can't establish connection to BLE device.

If VCC return to 3.0 V rapidly, than brown-out reset device successfully and connection to BLE device is established.

I don't think when this happens the flash is corrupted, because after full power reset the firmware is started correctly.

Does brown-out must always reset the MCU when VCC drops below 1.7 V?

Are there any known problems with reset?

Can the slew rate of the voltage or the time spent at low levels affect on the reset system?

[Kenneth]

Hi, we need more information that would allow us to find exact cause, please create a case on My Page https://www.nordicsemi.com/eng/nordic/mypage and add the following details:

• Schematic and layout.
• A description of test setup.
• Frequency of occurrence and build code.
• Analog capture of VDD, nRESET, DEC1 and DEC2.

[Reinier Gerritsen]

Same issue here, when the power supply drops to the 0.5V region and then back to 3.3V, a power-on reset is not always initiated. When the device is in this faulty state, the power must first drop below 0.4V in order for the device to start normally again. NRF51822QFAAH0 (1544MG), no soft device.

[kchuva]

Hi, Reinier, can you use a 1Mohm resistor between SWDIO/nRESET and GND? That will increase current consumption with VDD/1Mohm. This way solved our problem

[Reinier Gerritsen]

Thanks Konstantin! I replaced the 12K pull-up for a 1M pull-down. As far as I can see this solves the problem. The increased power supply is no problem at all, we don't use batteries in this application. I got the same advice from M6 (the original poster) who happens to be a former co-worker!