Arduino Mega reset circuit questions

Forum 2005-2010 (read only) Hardware Development
1

I'm trying to understand the Arduino Mega reset circuit.

At power on, C11 charges through R5. Presumably the DTR line on the FT232 is high, so there is no voltage across C12.

When the DTR line is brought low, C11 and C12 are now effectively in parallel, and since they are of the same value, the voltage at the microcontroller reset line should drop to 2.5 volts. Is that enough to actually reset the microcontroller? (I've noticed that resetting the arduino mega through the DTR line doesn't always seem to work...).

C11 and C12 quickly charge back up to 5 volts through R5. The DTR line is then brought back up high by the FT232. Surely this means that C12 is going to dump some of its charge into C11, and bring the voltage at the reset line to (I think) 7.5 volts?! The excess voltage at the reset line discharges through R5, bringing everything back to the starting point.

Is that the way it works?

Finally, what is the point of RESET-EN? It is by-passed by the connection just below it on the circuit diagram!

Thanks.

2

The circuit was slightly modified after the pdf was generated: C11 is actually 22 picofarads. The reference design (eagle files) are correct.

3

Finally, what is the point of RESET-EN? It is by-passed by the connection just below it on the circuit diagram!

Allows you to disable the auto-reset function by cutting that trace below. If you do that, RESET-EN allows you to re-enable it.
Neccesary depending on what your sketch does at power-up - mainly if it relies on serial data.

Can also use apparently a 100Ohm resistor from 5V to the reset line.

Refer "Automatic Software Reset" towards the bottom of http://arduino.cc/en/Main/ArduinoBoardMega