I'm using an ethernet to serial TTYL adapter (datasheet: http://www.moxa.ru/files/manuals_nport/miineport_e2_e3_users_manual_v6.pdf) to communicate with my Arduino Mega remotely. To be able to upload code through this adapter, I wired in a 0.1uF cap from radio shack onto the DTR pin of my adapter and connected this to the Mega's reset pin, as suggested in various guides. However, this does not seem to be working, as when I open the serial monitor, the Mega does not reset. I tried measuring the voltage of the DTR pin with a voltmeter and found it was 3.4 volts when high, and 0.1 volts when low after the serial monitor was opened. I don't have an oscilloscope so I can't see what's happening on the reset pin end of the capacitor when it is plugged in.
Here's my thinking atm: the pull-up resistor puts the reset pin at 5 volts, and then when the DTR is driven low (by a margin of 3.3 volts), this puts the reset pin at 1.7 volts. This is probably not considered a logic low by the atmega2560. What would be the easiest way to fix this? The only thing I can think of (as someone who isn't an electrical engineer) is to use another resistor with much lower resistance to pull the reset pin high to the Mega's 3.3 volt output and use a capacitor with correspondingly higher capacitance. Would this work without damaging any components? Any better suggestions?
Edit: On second thought, I might be better off adding a resistor to ground to divide the arduino's 5 volt voltage and set the pin at ~3-3.5 volts. Agree?
It is a known problem.
You can lower the voltage of the reset pin. There is a 10k resistor to pull the reset high.
Schematic is here: http://arduino.cc/en/Main/arduinoBoardMega2560
You can add a resistor from RESET to GND to lower the reset a little.
I have been looking at the datasheet, but I don't know what value that resistor should be. I think the reset threshold voltage is 2.1V when the ATmega2560 is running at 5V.
Perhaps a value of 47k to 100k. That will lower the reset voltage to 4.1V to 4.5V.
Okay thanks! I have some 55k resistors which should do the trick by bringing the reset pin down to 4.23 volts (if not, I also have 21k resistors to bring it down to 3.39 volts).
ummonk:
I wired in a 0.1uF cap from radio shack onto the DTR pin of my adapter and connected this to the Mega's reset pin, as suggested in various guides. However, this does not seem to be working, as when I open the serial monitor, the Mega does not reset.
It's my understanding that THAT is exactly what this mod if for.
Under normal circumstances, without any mods, opening the serial monitor DOES cause a reset.
The reset is still connected via a 100nF capacitor to the usb-serial chip. You might have to disable that.
There is a "RESET-EN" to scratch away that connection.
Okay returned to work. I cut the trace and it worked (without needing any extra resistor)! Duh. I was wondering why this circuit was supposed to work even when the built-in cap was connected, but didn't see anything in the guides I read about that...
Also, the resistors (from radio-shack) are rated at 56k and 22k - however when I measured them they seemed to consistently have ~55k and ~21k ohms resistance.
ummonk:
I wired in a 0.1uF cap from radio shack onto the DTR pin of my adapter and connected this to the Mega's reset pin, as suggested in various guides. However, this does not seem to be working, as when I open the serial monitor, the Mega does not reset.
It's my understanding that THAT is exactly what this mod if for.
Under normal circumstances, without any mods, opening the serial monitor DOES cause a reset.
The mod is to cause it to reset only momentarily (since the DTR pin outputs low the whole time the serial monitor is being used, we can't connect it directly to RESET; we must place a cap in between instead).