Inaccurate documentation for Serial TTL?

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1

Hello,

The docs on the Arduino Duemilanove hardware at http://www.arduino.cc/en/Main/ArduinoBoardDuemilanove mention:

The ATmega168 and ATmega328 provide UART TTL (5V) serial communication, which is available on digital pins 0 (RX) and 1 (TX).

If my oscilloscope does not deceive me, the Voltage of the hardware serial port is actually 3.3V. This difference is important for building the right circuit to adjust the port levels for external serial devices (12V). Anyone can review confirm this?

2

The levels you see will work just fine with a TTL/RS232 adapter such as MAX232 and variants, and also with RS422/485 adapters. They expect a swing from ~<0.7V to ~>1.8V or so. 3.3V is plently for a high.
If using discrete transistors such as BC546/547 as found on the Serial Arduino (can be found in the Hardware section on the Main Site), 3.3V will also work fine.

3

michelem:
Hello,

The docs on the Arduino Duemilanove hardware at http://www.arduino.cc/en/Main/ArduinoBoardDuemilanove mention:

The ATmega168 and ATmega328 provide UART TTL (5V) serial communication, which is available on digital pins 0 (RX) and 1 (TX).

If my oscilloscope does not deceive me, the Voltage of the hardware serial port is actually 3.3V. This difference is important for building the right circuit to adjust the port levels for external serial devices (12V). Anyone can review confirm this?

I can't confirm, as I lack a scope, but I would be very interested in the answer! I did take a spin through the datasheet, but didn't find anything pertinent.

There was another thread maybe a couple weeks back that discussed level-shifting the TTL signals for XBees. If they're 3.3V to begin with, then this would be unnecessary!

4

If my oscilloscope does not deceive me, the Voltage of the hardware serial port is actually 3.3V.

Well something is deceiving you because it is a normal TTL level signal on every arduino I have ever used.
Have you checked the +5V line? What is the voltage of that?
Where are you raking the measurements?
Is it the TX or RX lines you are talking about.
There is a 1K series resistor in line between the arduino and the USB bridge chip.

5

Ah, well if it sounds too good to be true...