HEADS UP: This page is very much a draft, and it's definitely not easy to follow at this point.

Also, my experience developing for Atmel AVR parts is done using AVR-GCC and/or Atmel Studio, and I haven't really picked up the proper 'Arduino way' of doing things. There's probably much more easy/elegant methods of doing the bootloader installation than I've explained here.

This is a wiki, so feel free to expand on this article or make changes as you see fit.


  • Introduction
    • Why overclock?
    • Is this reliable/safe?
    • Tools required
  • Crystal selection and replacement
    • Choosing a new crystal
    • Replacing the crystal
  • Updating bootloader
    • Downloading and compiling Optiboot
    • Backing up and flashing new software
  • Marlin firmware changes
    • platformio.ini
    • macros.h
    • pins_ANET_10.h
    • speed_lookuptable.h
  • Compile and install new Marlin firmware

Why overclock?

I overclocked my printer for fun, and to learn more about the guts of Marlin.

Additionally, my build now has a stiff AM8-inspired frame, higher stepper motor currents, 0.5mm nozzle, etc… and is capable of printing with decent quality at speeds well above 100mm/s. I reached the point where turning up the speed further results in the printer stuttering when going around corners, because the planner couldn't keep up.

The 12864 LCD that I have on the printer doesn't help the situation either; it's well understood that having a graphic LCD on the printer slows down the printer, as sending pixels serially to the display takes away time from the planner. I could go back to the original character LCD display, but having a knob spoils me, and the Anet V1.0 card doesn't have sufficient I/O pins to hook up both a character LCD and a knob.

I could just buy a proper 32 bit board, that would fix this problem entirely, but that's no fun :)

Is this reliable/safe?

According to its datasheet, the ATMega1284P processor used on the Anet V1.0 motherboard is rated for 20MHz operation at the operating point of 125 degrees C and +4.5V. If you run the processor at a higher voltage (like the regulated +5.0V that the Anet motherboard provides) and a more reasonable operating temperature, the processor should be capable of running at a higher frequency while remaining stable. Overclocking a microcontroller isn't much different from overclocking a desktop CPU in this regard.

If you're mounting your printer in an enclosure for printing ABS or whatever, this means you'll be running the processor in a much hotter ambient environment than normal. Overclocking the processor beyond 20MHz probably isn't a good idea in this case.

As far as safety goes, 3D printers by nature aren't safe, especially cheap printers like the A8. Even without this modification, there's a chance your printer could malfunction and cause damage or injury, and doing this modification does not make the printer any safer. If the processor running on the printer crashes outright, Marlin uses the watchdog timer in the processor and should reboot. But it's possible that silent corruption of the processor's state could occur, and Marlin does not have any safeguards against this. I can therefore make no guarantees of safety, and I'm not responsible for any bad consequences if your printer does something it shouldn't.

Tools required

To change the crystal, you will need soldering iron, solder (lead or lead free is fine), solder wick, and a suitable place to work. Desoldering HC49S packages is a pain in the ass, having two irons or a hot air rework station greatly helps with getting the old crystal off.

Having a power supply to power the control board outside of the printer during reflashing is very convenient, this lets you make sure that everything works before you reassemble your printer. The Anet board will run from a 9V to 15V supply.

Doing this modification requires updating the bootloader on the ATMega1284P chip, so you'll need an ISP capable programmer to do this. I suggest getting a USB ISP programmer which is supported by the “avrdude” application.

Choosing a new crystal

The Anet V1.0 board requires a “HC-49S” surface mount crystal. Choose one with a 12pf load capacitance, as this best matches the load capacitance provided on the Anet board.

Here are some suitable crystal frequencies to choose from:

Frequency % above stock % overclock Notes
16MHz 0% 0% Stock crystal frequency on the Anet V1.0 board.
20MHz +25% 0% Maximum clock frequency officially supported by the Mega1284P.
24MHz +50% +20% What I successfully used on my own board.
Can be salvaged from some USB devices (eg, high speed hubs)
25MHz +56.25% +25% Slightly faster, might be OK.
Significant baud rate error (see below)
Can be salvaged from devices with Ethernet (old motherboards, network cards, etc)
27MHz +68.8% +35% May be unstable.
significant baud rate error (see below)
Can be salvaged from video equipment (DVD players, etc)

Note: If you're using the USB interface of the printer, using Octoprint for example, the crystal you choose affects how accurate the baud rate of the Mega1284P's UART will be at a given baud rate. For reliability reasons this should not exceed +-5%; ideally this value is less than +-2.5%. If you don't plan on using the printer's USB connection, this isn't an issue.

Crystal freq (below), baud (right) 115200 230400 250000 460800 500000 921600 1000000 1500000
16MHz 2.1% -3.5% 0% 8.5% 0% 8.5% 0% 33%
20MHz -1.4% -1.4% 0% 8.5% 0% -9.6% -16.7% -16.7%
24MHz 0.16% 0.16% 0% -7.0% 0% 8.5% 0% 0%
25MHz 0.5% -3.1% -3.8% -3.1% 4.2% 13% 4.2% 4.2%
27MHz 1% -2.3% -3.6% 4.6% -3.6% 8.4% 12.5% 12.5%

To calculate for different clock frequencies and baud rates, use the following math:

  • UART divisor = crystal frequency / 8 / Desired baud rate
  • Actual baud rate = crystal frequency / 8 / rounded divisor value
  • Actual baud rate and desired baud rate should be +-5%, preferably +-2.5%

Replacing the crystal

Before replacing the crystal, I'd suggest doing the first step in “Backing up existing firmware” below. This proves that your ISP tools work - if for some reason you can't get it to work, you can just put the card back in your printer and try again later.

Unplug your printer, disconnect all wiring, and remove the motherboard from your printer.

Desolder the existing crystal. Hot air and a pair of tweezers makes this very easy, as does using two soldering irons. But if you have just one iron, the best method is to apply two large blobs of solder to the joints at each end of the crystal, and move your iron back and forth between the two sides to try to get both sides simultaneously melted, very gently pulling up on the crystal when you do this. Eventually it will come loose, or you'll be able to get one end lifted up enough to clean under it with solder wick.

Clean up the crystal footprint pads on the PCB with solder wick, and apply a thin layer of fresh solder. Install the new crystal and solder both ends.

Once the crystal is changed, put the control board back in your printer and connect ONLY the power input to the board; leave everything else disconnected for now. Connect your ISP programmer to the board.

Downloading and compiling Optiboot

If you're using a 24MHz crystal, just copy this paste and save it as a .hex file:

You'll need to download and compile Optiboot first. More information can be found on the Optiboot github page:

Use the following options to compile it for the Anet V1.0 board. Change the “24000000” value to the frequency of the crystal you're using in Hz - don't forget the L on the end.

make AVR_FREQ=24000000L BAUD_RATE=57600 LED=A4 UART=0 atmega1284p

Save the .hex file for later.

Backing up existing firmware


avrdude -p atmega1284p -c <your programmer> -U flash:r:anet10_backup.hex:hex -U eeprom:r:anet10_backup.eep:hex

Flashing new firmware

Use AVRDUDE to set the fuse/lockbit values on the processor, then program in the new bootloader:

avrdude -p atmega1284p -c <your programmer> -U hfuse:w:0xDE:m lfuse:w:0xf7:m efuse:w:0xfc:m lock:w:0x2f:m
avrdude -p atmega1284p -c <your programmer> -U flash:w:optiboot_atmega1284p.hex

The fuse values given move the bootloader start address to 0xFE00/0x1FC00 (word/byte) which frees up 3K of application space from the bootloader section. The lock bits prevent the bootloader from overwriting itself and bricking the board, if you send in a code image which is big enough to expand into the bootloader space.

Once the board is programmed, connect the board to your PC via its USB connection, and attempt to “ping” the new bootloader with the following avrdude command. Change “COM1” to the serial port corresponding to the printer on your computer.

avrdude -c arduino -p m1284p -P COM1 -b 57600 -v

If everything goes well, you should see a screen that looks like this, indicating that the bootloader is up and running.

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e9705
avrdude: safemode: lfuse reads as 0
avrdude: safemode: hfuse reads as 0
avrdude: safemode: efuse reads as 0

avrdude: safemode: lfuse reads as 0
avrdude: safemode: hfuse reads as 0
avrdude: safemode: efuse reads as 0
avrdude: safemode: Fuses OK (E:00, H:00, L:00)

avrdude done.  Thank you.

A number of files in Marlin have to be modified for operation at frequencies other than 16 or 20MHz. This is in addition to the changes to the Configuration.h and Configuration_adv.h files.


Find the following section of this file:

platform = atmelavr
framework = arduino
board = sanguino_atmega1284p
build_flags = ${common.build_flags}
upload_speed = 57600
lib_deps = ${common.lib_deps}

Immediately after this section, add the following line. This tells the compiler what the new CPU frequency is. If you're using a crystal other than 24MHz, change the “24000000” value to the new crystal frequency in Hz. Don't forget the “L”.

board_f_cpu = 24000000L


Find the following section:


Immediately after these lines, add the following:


Next find the following section:

  #define DELAY_1US DELAY_20_NOP

Add the two additional lines shown (the #elif line and the line after), with the “24” value altered to correspond to the crystal frequency you've chosen. Here is the setting for 24MHz - Note that “DELAY_20_NOP” and “DELAY_4_NOP” add up to 24. If you're using a frequency that isn't a multiple of 1MHz, then round up (eg, use 25 for a 24.576MHz crystal)

  #define DELAY_1US DELAY_20_NOP


If you're using the stock A8 display, you can ignore this section. If you're using the full graphics display from the A6, this is required or else the display on the printer will be garbled.

Find the following three lines, these control how fast data is written to the LCD:

#define ST7920_DELAY_1 DELAY_0_NOP
#define ST7920_DELAY_2 DELAY_1_NOP
#define ST7920_DELAY_3 DELAY_2_NOP

Change them to the following values for 24MHz operation. If your display becomes glitchy or garbled, try adding 1 to the ST7920_DELAY_2 and ST7920_DELAY_3 settings.

#define ST7920_DELAY_1 DELAY_1_NOP
#define ST7920_DELAY_2 DELAY_3_NOP
#define ST7920_DELAY_3 DELAY_7_NOP


Marlin only comes with two lookup tables provided in this file, for operation at 16MHz and 20MHz. For other clock frequencies, a new stepper speed lookup table has to be added.

Here is a replacement for the file which has the 24MHz table added:

To generate a table for a different frequency, Marlin provides a Python script in the buildroot/share/scripts which can be used to generate a new table. For example, to generate a table for 25MHz operation, use the following:

python --cpu-freq 25 > new_table.h

If you're using a crystal frequency which isn't a multiple of 1MHz, you'll need to edit the script to remove the “* 1000000”, and call the script with –cpu-freq <hz> instead.

There are many guides online describing how to compile and install Marlin on an Anet A8. Find one and link it here :)

  • reprap/anet/anet_mainboard_overclocking.txt
  • Last modified: 2018/12/02 02:27
  • (external edit)