AVR microcontroller implementing Arduino Nano.
Same pin on the edge of the board as Arduino Nano, with (from left to right):
Port Mini USB: not directly plugged to the Atmega328P but rather to the chip at the back.
Atmega328P MCU: same as Arduino Nano.
Reset button: triggering a reset signal: connects the RST pin to GND while pressed.
Status LEDs: TX and RX for (USART? SPI?) data transmission, POW constantly on, and L blinking during a reset.
SPI port: 6 pins for SPI debugging (not mandatory if using USB),
AMS1117: voltage regulator.
CH340G: USB to UART signal converter.
The pins of the MCU are not numbered like the pins at the side of the board, even though most of the time, it is a plain wire in-between: