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Mobile, AL
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Hello all - I've got a project going and trying to understand the Arduino Duemilanove schematic - well, R2 and C13 anyway.  These are the parts shown in this schematic:
http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf

as coming from FT232RL pins #2 and #3 to the mega8 reset pin (why is that shown as "Atmega8 rather than ATmega168"? - wrong schematic I expect).  Looks like R2 is in the RTS (pin #3) and C13 is in the DTR (pin #2) lines.  C13 is a 100nF (.1uF) cap, and R2 is listed as "100_NM" - is that a 100 ohm resistor?  OR what value?  I've looked over my Duemilanove board and all I find are 102 (1K) and 1002 (10K) resistors.  Did I miss something?

Can anyone explain what these lines do?  I can see the RTS triggering the RESET pin (#1), but will the DTR line feed thru the capacitor?  And just what does the "RESET-EN" do on that line?

Will the Deumilanove with m168 chip be the same wiring as the schematic shown?

Thank you to all for any help,

Ken H>
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Quote
why is that shown as "Atmega8 rather than ATmega168

The ATmega8 was probably already in the Eagle libraries and has the same pinout as the ATmega168.

Quote
R2 is listed as "100_NM" - is that a 100 ohm resistor?

It's a non-placed part. It can optionally be placed to allow the RTS pin to reset the ATmega168.

Quote
will the DTR line feed thru the capacitor?

Yes. When the host computer opens the serial port, it asserts the DTR line, and the capacitor causes that change in level to cause a pulse to appear on the ATmega168's reset line.

Quote
And just what does the "RESET-EN" do on that line?

It allows you to cut the DTR trace to disable this "auto-reset" behavior, then if you change your mind in the future you can short the RESET-EN jumper to restore this behavior.

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Will the Deumilanove with m168 chip be the same wiring as the schematic shown?

Yes.

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Mobile, AL
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Thank you for the very informative post - it has answered a couple of my questions..... and gave rise to a couple more.

The Diecimila schematic shows the two parts in the RTS and DTR line both as "100_NM" resistors, while the http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf schematic shows the RTS as a "R2, 100_NM" resistor and "DTR as C13, 100nF".

Should I use two 100 ohm resistors?  OR one 100 ohm resistor and one 100nF capacitor?

I am building a board to emulate the Arduino with a ZIF socket mounted that will allow programming from the Arduino IDE program.  Most of my projects are mounting the programmed m168 chip on a separate PCB for a project (voltmeter, temperature controller, etc).  While working up the program I am constantly moving m168 chip from project PCB to the Arduino for programming.  Seems like it would be easier to move to a board with a ZIF socket to burn program - and save wear 'n tear on my Arduino board.  So, I got a FT232RL chip, laying out a PCB for toner transfer in Eagle to a ZIF socket.  I am duplicating the portion of the Arduino schematic to accomplish this.

Thank you again for your help -

Ken H>
 
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It should be enough just to use a 100nF capacitor on the DTR line. That is what the Uno and Duemilanove have. Remember, the "100_NM" resistor is "no-mount", that is it does not actually come attached to the board when you buy one.

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Mobile, AL
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Hey, thanks a lot for the help - I'm etching the board today and "might" have it working sometime this weekend.

Ken H>
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