Change Pololu A-Star Micro to 8 MHz and 3.3V

Hi everyone,

after purchasing my first A-Star Micro I want to make it 3.3V IO compatible.
I already found a post on an Orangutan where the change seems easily made and try to build a little step-by-step thread here describing the required changes for A-Star Micro.

The first steps I took before changing any hardware on the board:

  1. Decrease brown-out voltage to 2.6V via AVR ISP programmer (like for Orangutan)
  2. Made no change on CKSEL fuses because they should work with 8-16 MHz resonator range.

Afterwards I replaced the 16 MHz crystal resonator with an 8 MHz resonator to allow 3.3V operation of the ATMega32U4. Power supply is still 5V up to here.
This leads to the USB malfunctioning. I suppose this is due to all bootloader code and clock divider settings of USB interface are built with 16 MHz.

Therefore, I compiled the Caterina bootloader from with F_CPU = 8000000 instead of 16000000 and VID=0x1FFB and PID=0x0101. Afterwards renamed Caterina.hex to Caterina-A-Star8Mhz.hex and programmed it to the board with AVR ISP.
Afterwards the USB is working again and I added the following lines to the boards.txt file in \Arduino15\packages\pololu-a-star\hardware\avr<version>:

[details=boards.txt] A-Star 32U4 8MHz

a-star32U4_8.bootloader.lock_bits=0xEF“Pololu A-Star 32U4”“Pololu Corporation”{build.usb_flags}[/details]
This contains the changed 8MHz CPU clock, the changed brown-out detection to 2.6V in the fuse settings, and the new built caterina/Caterina-A-Star8Mhz.hex, which has to be placed accordingly.

Now after choosing the “Pololu A-Star 32U4 8MHz” and the corresponding serial port, the board was directly programmable via Arduino IDE 1.8.1.

Now the Board is fully functional at 8 MHz but ist still using 5V internal supply voltage. Therefore, I replaced the voltage regulator with an LP2985-33, removed the transistor and bridged the VBUS_IF pad to the VIN pad to also get 3.3V supply via USB.
Afterwards the LEDs light up a little darker and the board is now on 3.3V including IO voltage
with 8MHz.

I hope that someone might find this little tutorial useful, but I don’t give any warranties.

Best regards,

P.S.: Here are the boards.txt file and the compiled Caterina (packed together with the blink example sketch like described here: for 8MHz:
boards.txt (3.3 KB)
Caterina-A-Star8Mhz.hex (22.2 KB)

1 Like

Hello, Chris.

Thank you for sharing how you made the A-Star 32U4 Micro run at 3.3 V.

The ATmega32U4 cannot operate at 16 MHz when its voltage is 3.3 V, but you did not need to change the 16 MHz resonator. Instead, you could have programmed the CKDIV8 fuse so that when the AVR starts up, the clock is divided down to 2 MHz. Then you could make the bootloader change the clock divider from 8 to 2 using the CLKPR register, resulting in a speed of 8 MHz.

I think you meant to write VBUSF, not VBUS_IF.

Since you have shorted VBUSF to VIN, you should be careful to never have USB plugged in at the same time as an external power supply on VIN.