qmk-firmware/tmk_core/common/uart.c

// TODO: Teensy support(ATMega32u4/AT90USB128)
// Fixed for Arduino Duemilanove ATmega168p by Jun Wako
/* UART Example for Teensy USB Development Board
 * http://www.pjrc.com/teensy/
 * Copyright (c) 2009 PJRC.COM, LLC
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

// Version 1.0: Initial Release
// Version 1.1: Add support for Teensy 2.0, minor optimizations

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

#include "uart.h"

// These buffers may be any size from 2 to 256 bytes.
#define RX_BUFFER_SIZE 64
#define TX_BUFFER_SIZE 40

static volatile uint8_t tx_buffer[TX_BUFFER_SIZE];
static volatile uint8_t tx_buffer_head;
static volatile uint8_t tx_buffer_tail;
static volatile uint8_t rx_buffer[RX_BUFFER_SIZE];
static volatile uint8_t rx_buffer_head;
static volatile uint8_t rx_buffer_tail;

// Initialize the UART
void uart_init(uint32_t baud) {
    cli();
    UBRR0          = (F_CPU / 4 / baud - 1) / 2;
    UCSR0A         = (1 << U2X0);
    UCSR0B         = (1 << RXEN0) | (1 << TXEN0) | (1 << RXCIE0);
    UCSR0C         = (1 << UCSZ01) | (1 << UCSZ00);
    tx_buffer_head = tx_buffer_tail = 0;
    rx_buffer_head = rx_buffer_tail = 0;
    sei();
}

// Transmit a byte
void uart_putchar(uint8_t c) {
    uint8_t i;

    i = tx_buffer_head + 1;
    if (i >= TX_BUFFER_SIZE) i = 0;
    while (tx_buffer_tail == i)
        ;  // wait until space in buffer
    // cli();
    tx_buffer[i]   = c;
    tx_buffer_head = i;
    UCSR0B         = (1 << RXEN0) | (1 << TXEN0) | (1 << RXCIE0) | (1 << UDRIE0);
    // sei();
}

// Receive a byte
uint8_t uart_getchar(void) {
    uint8_t c, i;

    while (rx_buffer_head == rx_buffer_tail)
        ;  // wait for character
    i = rx_buffer_tail + 1;
    if (i >= RX_BUFFER_SIZE) i = 0;
    c              = rx_buffer[i];
    rx_buffer_tail = i;
    return c;
}

// Return the number of bytes waiting in the receive buffer.
// Call this before uart_getchar() to check if it will need
// to wait for a byte to arrive.
uint8_t uart_available(void) {
    uint8_t head, tail;

    head = rx_buffer_head;
    tail = rx_buffer_tail;
    if (head >= tail) return head - tail;
    return RX_BUFFER_SIZE + head - tail;
}

// Transmit Interrupt
ISR(USART_UDRE_vect) {
    uint8_t i;

    if (tx_buffer_head == tx_buffer_tail) {
        // buffer is empty, disable transmit interrupt
        UCSR0B = (1 << RXEN0) | (1 << TXEN0) | (1 << RXCIE0);
    } else {
        i = tx_buffer_tail + 1;
        if (i >= TX_BUFFER_SIZE) i = 0;
        UDR0           = tx_buffer[i];
        tx_buffer_tail = i;
    }
}

// Receive Interrupt
ISR(USART_RX_vect) {
    uint8_t c, i;

    c = UDR0;
    i = rx_buffer_head + 1;
    if (i >= RX_BUFFER_SIZE) i = 0;
    if (i != rx_buffer_tail) {
        rx_buffer[i]   = c;
        rx_buffer_head = i;
    }
}