This file just contains a few peripheral definitions.

The F_CPU macro defines the CPU clock frequency, to be used in delay loops, as well as in the UART baud rate calculation.

The macro UART_BAUD defines the RS-232 baud rate. Depending on the actual CPU frequency, only a limited range of baud rates can be supported.

The remaining macros customize the IO port and pins used for the HD44780 LCD driver. Each definition consists of a letter naming the port this pin is attached to, and a respective bit number. For accessing the data lines, only the first data line gets its own macro (line D4 on the HD44780, lines D0 through D3 are not used in 4-bit mode), all other data lines are expected to be in ascending order next to D4.