Arduino flasher 3.3v

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This explains how to:

  • Easily lower the voltage of an arduino
  • Use that arduino to flash a coreboot image on a GM45 Thinkpad with a SOIC16 chip

It requires:

  • An AVR Arduino at 5v
  • An USB<->Serial adapter capable of providing enough current to power up:
    • The arduino
    • The flash chip
    • The circuits around the flash chip

It was tested with:

  • An "nano version 3.3"
  • A Sparkfun "FTDI Basic 3v3" (Uses an FTDI FT232R)


In the Atmega328/P datasheet, the "32.3. Speed Grades" chapter describes (pages 368 and 369) the link between maximum frequency of the microcontroller and the voltage. At 3.3v, the maximum frequency is 12Mhz.


Build the code and flash it

First download frser-duino:

git clone --recursive
cd frser-duino

Then modify the F_CPU value in main.h to be 12Mhz instead of 16Mhz: F_CPU will look like that:

#define F_CPU 16000000UL

Change it to:

#define F_CPU 12000000UL
  • Flash the Arduino with frser-duino
# make ftdi
# make flash-ftdi

Build the programmer

  • Connect the FTDI adapter to RX and TX of the arduino
  • Connect the 3V3 of the FTDI adapter to the 5V pin of the Arduino
  • Connect the GND of the FTDI adapter to the GDN of the arduino.
Arduino USB<->Serial port adapter
5v 3.3v
  • If you use a clip (Like a SOIC-8 or SOIC16 Pomona clip), connect it to the arduino
  • Connect the chip to the clip, or if you don't use a clip, the chip to the arduino
  • Connect an USB cable between the USB<->Serial adapter and the computer. Never connect the cable between the Arduino USB port and the computer while the flasher is connected to a flash chip. That would result in the I/O voltage being 5V instead of 3.3V.


Run flashrom like that:

flashrom -p serprog:dev=/dev/ttyUSB0:2000000

With some strange macronix flash chip (that are present in the Thinkpad X200) you might need to add spispeed=100k like that:

flashrom -p serprog:dev=/dev/ttyUSB0:2000000,spispeed=100k


Thanks a lot to SwiftGeek on IRC (#libreboot on Freenode) for finding the first workaround to make it detect flash chips at lower voltage. This project would not have been possible without that.