Menu won't reappear after calculator is power-cycled

[vladkorotnev]

The Arduino currently doesn't have any means to detect when the calculator is off to switch back to the menu disk image.

We somehow need to detect the voltage dropping from 5 to 3.3 volts and make it into a digital signal to reset the arduino or gracefully finish all the write operations and remount the pdp driver image.

[vladkorotnev]

I wonder if the atmega consumes much current while reset is low, so if we could use that voltage drop signal to keep the atmega in reset mode while the calculator is off.

[piotr433]

The microcontroller draws current when it is in the reset state (ca. 7mA measured on my ATMEGA328 board), so it would drain calculator batteries.

I propose powering the microcontroller through the following circuit built with junk box parts:

It disconnects the power when the input voltage drops below 3.5V. Loaded with a 330 ohm resistor it turns fully on at the input voltage greater than 4.2V. The threshold voltage is a sum of:
base-emitter voltage drop * (100k + 10k) / 100k = 0.75V
red LED voltage drop = 1.56V
green LED voltage drop = 1.73V
A transistor with a high current gain is preferred (I have used a BC558C).