3.3V Arduino

In some cases the Atmega328 can be run at 16MHz and 3.3V, but this is usually unreliable. 16MHz is about a 20% overclock. The bigger problem is the error rate for the serial port which is almost one bit error per byte at higher bit rates.

According to the Atmega328 data sheet, the maximum frequency is dependent on VCC. As shown in Figure 29-1, the Maximum Frequency vs. VCC curve is linear between 1.8V < VCC < 2.7V and between 2.7V < VCC < 4.5V. Using the values from the graph of 2.7V @ 10MHz, 4.5V @ 20MHz, and interpolating for 3.3V, gives a maximum frequency of 13.333MHz.


According to WormFood's AVR Baud Rate Calculator many clock rates have very high serial error rates. The table below shows the error rates for 115200 baud for a range of processor frequencies. Notice that for 8MHz the error rate is 7.8% which in practice is an error in every 2 or 3 characters. For 16MHz the error rate is 3.7%, which is better than the rate for 8MHz, but is still high. Also note that the frequencies 9.216MHz/11.0592MHz/14.7456MHz all have zero percent error rates.

Crystal Frequency (MHz)	8	9.216	11.0592	14.7456	16	16.8	18.432	20
% error rate	7.8	0.0	0.0	0.0	3.7	1.3	0.0	1.4

Based on WormFood's tables, the highest frequency that is both accurate for serial baud rates and under the 13.333MHz speed limit is 11.0592MHz.

Building 9.216MHz/11.0592MHz/14.7456MHz bootloaders

In order to use alternate Atmega328 speeds, a new bootloader must be created to use the new speed.

The first clues on how to use frequencies other than 8MHz/16MHz with the Arduino came from posts in the Arduino forum:
ATmegaBOOT_168.c bootloader does not compile with newer AVR tool
SOLVED! ATMega328P at 14.7456 MHz
Extra credit needs to go to user "avenue33" for his help in clarifying the changes needed.

The Optiboot bootloader was chosen because it is the standard bootloader used in the current UNO boards as well as it is much smaller than the older boot loader and leaves more space for program code.

A new Arduino board type needs to be created for each of the new speeds. Add the new Arduino speeds to the list of boards using the original 16MHz UNO board settings as a template.

The following lines need to be added to the file: "Arduino 1.0.5\hardware\arduino\boards.txt"

Building Optiboot bootloaders

Before building custom versions of the Optiboot bootloader, a patch to the source code is needed along with creating an entry for each new frequency in the Makefile.

Edit the file to look like this:
"\Applications\Arduino.app\Contents\Resources\Java\hardware\arduino\bootloaders\atmega\ATmegaBOOT_168.c" around line 583


Next, add the following lines to add new board types in the makefile:
\Arduino 1.0.5\hardware\arduino\bootloaders\optiboot\Makefile


while in the folder: \Arduino 1.0.5\hardware\arduino\bootloaders\optiboot\
Run the omake.bat file from the command line


which produces these hex files:


A set of precompiled alternate frequency bootloaders can be found on the downloads page.

Using the new alternate Atmega328 speeds with the Arduino software

Once the boards.txt file has been uploaded and the Arduino application has been restarted, the 3 new boards will be available under Tools->Boards.


When using some versions of the Arduino envrionment, there are some changes needed to the sketches to allow for the new frequencies. The changes do not appear to be required when running version 1.05.

Fixing the time based functions

The function "delay" may not properly handle the new frequencies. Instead it is safer to use the "delay_ms" and "delay_us" functions. They are not defined by default in the Arduino environment, so they must specified with the include file and then use the _delay_ms() call instead of the delay() call.


Alternately, the functions can be specified with the following two lines:


The millis, micros, delay, delayMicroseconds in wiring.c may have problems with the new F_CPU value.

Here's an example of the Blink code with the delay_ms function used.

Fixing the baud rates

The AVR registers controlling the serial port are configured based on the crystal frequency (F_CPU). When the definition for F_CPU is not passed to the serial port initialization code via the boards.txt file, it needs to be included in the main code.

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