Increase in power consumption after brown-out detection (BOD)

Question

I'm working on a low-power energy harvesting project, where I'm considering using an ATtiny85 to perform some minor tasks. The ATtiny is put in Power Down mode most of the time and does not consume much power in this mode.

Since the power source is unstable, and/or could be absent from time to time, I've been enabling the internal brown-out detection (BOD) by setting the fuses. In this way, the ATtiny is reset when the voltage gets too low. This is to ensure that the microcontroller starts working properly again when sufficient power returns. This also seems to work, so that the chip starts running again when the voltage is above a certain threshold.

However, when the voltage drops below the BOD level, I notice a considerably larger power surge through the ATtiny, even when all peripherals are disconnected. It seems to me that some sort of gate is opened inside the chip when the BOD fires, and the current is limited by some sort of internal resistance. When the voltage increases somewhat above the BOD level, the large power surge stops.

I've been searching through the documentation/datasheet and forums online to find out why this happens but can't seem to find some sort of explanation or ways to prevent this from happening. Since this is a low-power energy harvesting project, it is important that the internal protection from low voltages does not contribute to the problem it detects.

Any useful input would be appreciated.

Answer 1

When the Vcc level is below the BOD activation voltage, the chip is held in a reset state...

...which uses significantly more current than the chip does in sleep state...

You are probably better off disabling the BOD and instead using an external Voltage Supervisor chip to turn the power on to the ATTINY when there is sufficient voltage and turn it off when there is not. There are many of these chips and they can use less than a microwatt when quiescent.