RCB128RFA1 & RCB_BB UART serial echo client

serial.c

#include <stdint.h>
#include <avr/io.h>
#include <avr/interrupt.h>

#define BAUD 9600

// doc8266: ATmega128RFA1 data sheet. Available at:
//
//   http://www.atmel.com/dyn/resources/prod_documents/doc8266.pdf

void clock_init()
{
    cli();

    // See doc8266 p154: 11.10 System Clock Prescaler

    CLKPR = (1 << CLKPCE);
    // CLKPS = 1: RC oscillator divided by 4
    CLKPR = (1 << CLKPS0);

    sei();
}

void usart_init()
{
    // See doc8266 p342: 23.3.1 Internal Clock Generation

    // Asynchronous normal mode (U2X1 = 0): (Fosc/(16 * Baud)) - 1
    UBRR1 = ((F_CPU/16)/BAUD) - 1;

    // Enable receive and transmit
    UCSR1B = (1 << RXEN1) | (1 << TXEN1);

    // Set 8 data bits (UCSZ12:10 = 011) and 2 stop bits (USBS1 = 1)
    UCSR1C = (1 << UCSZ11) | (1 << UCSZ10) | (1 << USBS1);
    
    // Configure the MAX3221E: PD7 = !FORCEOFF, PD6 = FORCEON, PD4 = !EN
    DDRD = (1 << PD7) | (1 << PD6) | (1 << PD4);

    // Force the MAX3221E on: !FORCEOFF = 1, FORCEON = 1, !EN = 0
    PORTD = (1 << PD7) | (1 << PD6);
}

uint8_t usart_recv()
{
    // See doc8266 p349: 23.7.1 Receiving Frames with 5 to 8 Data Bits

    // RXC1: USART Receive Complete
    while (!(UCSR1A & (1 << RXC1)))
        ;
    return UDR1;
}

void usart_send(uint8_t b)
{
    // See doc8266 p347: 23.6.1 Sending Frames with 5 to 8 Data Bits

    // UDRE1: USART Data Register Empty
    while (!(UCSR1A & (1 << UDRE1)))
        ;
    UDR1 = b;
}

int main(void)
{
    clock_init();
    usart_init();

    while(1)
        usart_send(usart_recv());
}

See also: my explanation/ramble about this code.