Hey I’m writing my first tutorial 🙂
During my recent OLED testing I wrote the software using Arduino Pro Mini 3.3 – the only Arduino board with 3.3V I/O (Lilipad may be the other one but not breadboard friendly). Originally I was using the SPI interface and everything works fine. But when I’m trying to test the I2C interface I suddenly realize the I2C lines are not on the breadboard pins! Instead they are on top of the board and I have to use jumper wires to connect them, not as neat as I would like ;p
Looking into my inventory I found an Arduino Nano with all the necessary pins. But it uses 5V I/O only. So how to modify an Arduino Nano to 3.3V? Adafruit already has a tutorial about it. But judging from the photos the modification is on a Uno. Uno uses different USB chip from Nano’s FT232RL, and hence we need some more steps.
So here is what we need:
- Arduino Nano (of course)
- A 3.3V regulator in SOT-223 package. Please note the official Nano design uses UA78M05 regulator, which has a different pin configuration than the popular LM1117-3.3. The one I found compatible is Micrel’s MIC37100-3.3WS, Digikey link here.
- An Atmel ISP programmer, such as AVR JTAGICE mkII, or usbasp, or even a second Arduino as ISP.
- A good magnifier!
Schematic below is the official Arduino Nano design with necessary modifications marked in red:
- Replace the original voltage regulator.
- Locate D1 from the board, cut the trace at cathode side and connect cathode to VIN of the regulator. Alternatively you can also remove D1 completely, but by doing so the Nano board is unable to be powered from USB, You’ll need to provide your own source voltage from Pin 1.
- Cut the trace at FT232RL pin 4 (VCCIO), and connect pin 4 to pin 17 (3V3OUT). This enables FT232RL to use 3.3V logic.
- Cut the trace at FT232RL pin 20 (VCC) and connect pin 20 to D1 anode (VUSB).
I would suggest to trace your board carefully before doing step 3 and 4. In my case the board is a Chinese
enhanced modified version and all the power pins are connected by vias under FT232RL. So I had to desolder the whole chip to break the links. Just to share my modified board:
Till here the hardware modification is done.
However, if you carefully read the 660-page ATmega328 datasheet (which you should), at the bottom of first page:
Yes the chip is only specified at 10MHz with 3.3V supply. So unless you want to overclock your 3.3V Arduino at 16MHz, you should try to reduce the clock frequency.
Although the simplest way to modify clock frequency is to replace the 16MHz crystal with a 8MHz one, I’m out of luck because my board is using a tiny resonate instead of standard HC-49S crystal.
So the other way is modify the Arduino bootloader, configure CLKPR register to divide the clock by 2. To do this, follow the steps below:
- Go to “c:arduino-1.0.3hardwarearduinobootloaders”, make a copy of “atmega” folder, I name my new folder “atmegad2”
- Inside “atmegad2”, edit “ATmegaBOOT_168.c”, look for the main() function and add the following
// Disable interrupts
ch = SREG;
// Enable clock change
CLKPR = _BV(CLKPCE);
// Change clock division
CLKPR = 0x1;
// Enough time for new clock to be stable?
SREG = ch;
- Edit “Makefile”, add a new build target (I added right below “atmega328_pro8_isp: isp”)
atmega328_pro16div2: TARGET = atmega328_pro_16MHz_Div2
atmega328_pro16div2: MCU_TARGET = atmega328p
atmega328_pro16div2: CFLAGS += ‘-DMAX_TIME_COUNT=F_CPU>>4’ ‘-DNUM_LED_FLASHES=1’ -DBAUD_RATE=57600 -DDOUBLE_SPEED -DCLKDIV2
atmega328_pro16div2: AVR_FREQ = 8000000L
atmega328_pro16div2: LDSECTION = –section-start=.text=0x7800
- Create a batch file “env.bat”, content below. This is to set correct building environment for the boot loader. You do not need a separate installation of WinAVR as the current version of WinAVR has some definition problem with Arduino bootloader source. The WinAVR within Arduino distribution is just fine.
- Open a command prompt, cd into “c:arduino-1.0.3hardwarearduinobootloadersatmegad2”, execute the follow:
avr-gcc -g -Wall -O2 -mmcu=atmega328p -DF_CPU=8000000L ‘-DMAX_TIME_COUNT=F_CPU>>4’ ‘-DNUM_LED_FLASHES=1’ -DBAUD_RATE=57600 -DDOUBLE_SPEED -DCLKDIV2 -c -o ATmegaBOOT_168.o ATmegaBOOT_168.c
avr-gcc -g -Wall -O2 -mmcu=atmega328p -DF_CPU=8000000L ‘-DMAX_TIME_COUNT=F_CPU>>4’ ‘-DNUM_LED_FLASHES=1’ -DBAUD_RATE=57600 -DDOUBLE_SPEED -DCLKDIV2 -Wl,–section-start=.text=0x7800 -o ATmegaBOOT_168_atmega328_pro_16MHzDiv2.elf ATmegaBOOT_168.o
avr-objcopy -j .text -j .data -O ihex ATmegaBOOT_168_atmega328_pro_16MHzDiv2.elf ATmegaBOOT_168_atmega328_pro_16MHzDiv2.hex
rm ATmegaBOOT_168_atmega328_pro_16MHzDiv2.elf ATmegaBOOT_168.o
When the building ended, you should have “ATmegaBOOT_168_atmega328_pro_16MHzDiv2.hex” in the folder.
- Use you ISP programmer, burn “ATmegaBOOT_168_atmega328_pro_16MHzDiv2.hex” into ATmega328p chip. You do not need to reprogram the fuses but just in case you need, the fuse settings are:
HFUSE = DA
LFUSE = FF
EFUSE = 05
Now you can use your Arduino Nano just as an Arduino Pro Mini 3.3/8Mhz, write some blink LED program to test the new clock. Have fun with 3.3V devices!