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Standalone ATmega328p

Support for using the ATmega328p as standalone board. More...

Detailed Description

Support for using the ATmega328p as standalone board.


The ATmega328p is most popular in the Arduino UNO. However, the 28 PDIP package of the ATmega328p can easily be used without any "board": Just place it on a bread board, and connect a TTL adapter and an ISP and you're ready to go.

The ATmega328p has two internal oscillators, one clocked at 8MHz and one at 128kHz. By default the fuses of the ATmega328p are configured that the internal 8MHz oscillator can be used. This allows the ATmega328p to be operated without any external components at a supply voltage anywhere between 2.7V and 5.5V.

ATmega328p DIP package on a breadboard


MCU ATmega328p
Family AVR/ATmega
Vendor Microchip (previously Atmel)
Flash 32KiB
Frequency 1MHz/8MHz (up to 20MHz with external clock)
Timers 3 (2x 8bit, 1x 16bit)
ADCs 6 analog input pins
SPIs 1
I2Cs 1 (called TWI)
Vcc 2.7V - 5.5V (when clocked at 8MHz)
Datasheet Official datasheet


Pinout of the ATmega328p

All credit for above pinout image goes to https://github.com/MCUdude/MiniCore#pinout

Clock Frequency

The ATmega328p has two internal oscillators clocked at 8MHz and at 128kHz that allow it to be operated without any external clock source or crystal. By default the fuses are configured to use the internal 8MHz oscillator and a clock divider resulting in a clock speed of 1MHz. By clearing the CKDIV8 fuse the clock divider can be disabled to operate the ATmega328p at 8MHz without an external clock source. This can be done like this:

avrdude -c usbtiny -p m328p -B 32 -U lfuse:w:0xe2:m

(Replace usbtiny with the ISP programmer you are using. The -B 32 might be needed on some ISP programmers to communicate with slow ATmega MCUs. It will not be needed anymore after the clock device has been disabled.)

This "board" is configured to use 8MHz as core clock, so that the ATmega328p runs at the highest frequency possible without external clock sources.

By setting the environment variable ATMEGA328P_CLOCK to a custom frequency in Hz (e.g. 1000000 for 1MHz), this core clock can be changed easily. Refer to the datasheet on how to configure the ATmega328p to use an external crystal, an external clock source or the clock divider.

Relation Between Supply Voltage, Clock Frequency and Power Consumption

A higher supply voltage results in a higher current drawn. Thus, lower power consumption can be achieved by using a lower supply voltage. However, higher clock frequencies require higher supply voltages for reliable operation.

The lowest possible supply voltage at 8 MHz is 2.7V (with some safety margin), which results in an active supply current of less than 3 mA (about 8 mW power consumption) according to the datasheet. At 1 MHz core clock a supply voltage of 1.8V is possible resulting in an active supply current of less than 0.3 mA (about 0.5 mW power consumption). For more details, refer to the official datasheet.

Flashing the Device

In order to flash the ATmega328P without a bootloader, an ICSP programmer is needed. Connect the programmer as follows:

ISCP pin ATmega328p pin
Ground 22/GND

The tool avrdude needs to be installed. When using the usbtiny (or one of the super cheap clones) running

make BOARD=atmega328p flash

will take care of everything. To use the programmer <FOOBAR> instead, run

make BOARD=atmega328p PROGRAMMER=<FOOBAR> flash

Serial Terminal

Connect a TTL adapter with pins 2/RXD and 3/TXD an run

make BOARD=atmega328p term

Please note that the supply voltage should be compatible with the logic level of the TTL adapter. Usually everything between 3.3 V and 5 V should work.

On-Chip Debugging (OCD)

E.g. with the AVR Dragon and AVaRICE you can debug the ATmega328P using the debugWIRE interface. Compared to the ATmega MCUs with JTAG interface the debug facilities are however significantly reduced: Only a single hardware breakpoint and no watchpoints are supported. The hardware breakpoint is used for single-stepping. If you set breakpoints, the AVR Dragon will transparently replace the instruction to break upon with a break instruction. Once the breakpoint is hit, the break instruction is overwritten with the original instruction. Thus, every breakpoint hit cause two flash cycles to be performed, which not only results in a slow debugging experience, but also causes significant wear.

In order to enable debugWIRE run (replace <PROGRAMMER> with the programmer you use, e.g. with dragon_isp if you use the AVR Dragon):

avrdude -c <PROGRAMMER> -p m328p -U hfuse:w:0x99:m

You can disable it again via:

avrdude -c <PROGRAMMER> -p m328p -U hfuse:w:0xd9:m
As the reset pin is repurposed for debugWIRE, a regular ISP will not be able to disable the debugWIRE interface anymore. The AVR Dragon can temporary disable the debugWIRE interface via a debugWIRE command and avrdude will do so when debugWIRE is used. So only enable debugWIRE if you have an AVR DRagon or other means to disable debugWIRE later on again.

For debugging, the ATmega328P needs to be connected to the AVR Dragon in the very same way it needs to be connected for programming (see above). Once the MCU is connected and debugWIRE is enabled via the fuse settings, you can start debugging via:

make debug

If this fails after flashing the ATmega328P, it is like due to debugWIRE being temporary disabled by avrdude in order to use the ISP feature. Perform a cold boot and the ATmega328P will have debugWIRE enabled again.

If you are using a different debugger than the AVR Dragon, you have to export the AVR_DEBUGDEVICE environment variable to the required flag to pass to AVaRICE, e.g. when using the Atmel-ICE you have to export AVR_DEBUGDEVICE=--edbg. If the debug device is not connected via USB, you also need to export AVR_DEBUGINTERFACE to the correct value.


Don't expect having a working network stack due to very limited resources ;-)


file  board.h
 Board specific definitions for the standalone ATmega328p "board".
file  periph_conf.h
 Peripheral MCU configuration for the ATmega328p standalone "board".