/**
@defgroup    boards_atmega328p Standalone ATmega328p
@ingroup     boards
@brief       Support for using the ATmega328p as standalone board

## Overview

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.

\htmlonly<style>div.image img[src="https://github.com/maribu/images/raw/master/ATmega328p.jpg"]{width:600px;}</style>\endhtmlonly
@image html "https://github.com/maribu/images/raw/master/ATmega328p.jpg" "ATmega328p DIP package on a breadboard"<br>

### MCU
| MCU           | ATmega328p                             |
|:------------- |:-------------------------------------- |
| Family        | AVR/ATmega                             |
| Vendor        | Microchip (previously Atmel)           |
| RAM           | 2Kb                                    |
| Flash         | 32Kb                                   |
| Frequency     | 8MHz (up to 20MHz with external clock) |
| Timers        | 3 (2x 8bit, 1x 16bit)                  |
| ADCs          | 6 analog input pins                    |
| UARTs         | 1                                      |
| SPIs          | 1                                      |
| I2Cs          | 1 (called TWI)                         |
| Vcc           | 2.7V - 5.5V (when clocked at 8MHz)     |
| Datasheet     | [Official datasheet](http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf) |

### Pinout

\htmlonly<style>div.image img[src="https://camo.githubusercontent.com/c55beef2f138da61fe671a1e4a307ff4ffbc318d/68747470733a2f2f692e696d6775722e636f6d2f715849456368542e6a7067"]{width:100%;}</style>\endhtmlonly
@image html "https://camo.githubusercontent.com/c55beef2f138da61fe671a1e4a307ff4ffbc318d/68747470733a2f2f692e696d6775722e636f6d2f715849456368542e6a7067" "Pinout of the ATmega328p"<br>

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 resulting in a
clock speed of 8MHz. By setting the `CKDIV8` fuse the clock divider can be
enabled to operate the ATmega328p at 1MHz.

This "board" is configured to use 8MHz as core clock, so that the ATmega328p
can be used without external circuitry and without any changes in the default
fuse configuration.

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 clockd 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 |
|:-------- |:-------------- |
| MISO     | 18/PB4/MISO    |
| VCC      | 7/VCC          |
| SCK      | 19/PB5/SCK     |
| MOSI     | 17/PB3/MOSI    |
| RESET    | 1/RESET        |
| 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.

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