Purpose: USART frame modes, channel numbers and pin multiplexing.

USART0: PE0/PE1/PE2; USART1: PD2/PD3/PD5; USART2: PH0/PH1/PH2; USART3: PJ0/PJ1/PJ2. pins uses encoded logical pin values.

#include <atmega2560/USART.h>
 
namespace mcu = atmega2560;
 
static_assert(mcu::usart::module_count == 4, "ATmega2560 USART definition");

Hardware controllers

USART	RXD	TXD	XCK
0	PE0	PE1	PE2
1	PD2	PD3	PD5
2	PH0	PH1	PH2
3	PJ0	PJ1	PJ2

Per-instance pins are available under usart::usart0 through usart::usart3; the usart::pins namespace provides compact names for all channels.

These are hardware definitions, not runtime register access or initialization.