uart.c

/* uart.cc - UART initialization & communication */
/* Copyright (C) 2013 Goswin von Brederlow <goswin-v-b@web.de>

   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

/* Reference material:
 * http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
 * Chapter 13: UART
 */

#include <stdint.h>
#include <stdbool.h>

#include "mmio.h"
#include "uart.h"

enum {
    // The GPIO registers base address.
    GPIO_BASE = 0xE0200000,

    // The offsets for reach register.

    // Controls actuation of pull up/down to ALL GPIO pins.
    GPPUD = (GPIO_BASE + 0x94),

    // Controls actuation of pull up/down for specific GPIO pin.
    GPPUDCLK0 = (GPIO_BASE + 0x98),

    // The base address for UART.
    UART0_BASE = 0xE0201000,

    // The offsets for reach register for the UART.
    UART0_DR     = (UART0_BASE + 0x00),
    UART0_RSRECR = (UART0_BASE + 0x04),
    UART0_FR     = (UART0_BASE + 0x18),
    UART0_ILPR   = (UART0_BASE + 0x20),
    UART0_IBRD   = (UART0_BASE + 0x24),
    UART0_FBRD   = (UART0_BASE + 0x28),
    UART0_LCRH   = (UART0_BASE + 0x2C),
    UART0_CR     = (UART0_BASE + 0x30),
    UART0_IFLS   = (UART0_BASE + 0x34),
    UART0_IMSC   = (UART0_BASE + 0x38),
    UART0_RIS    = (UART0_BASE + 0x3C),
    UART0_MIS    = (UART0_BASE + 0x40),
    UART0_ICR    = (UART0_BASE + 0x44),
    UART0_DMACR  = (UART0_BASE + 0x48),
    UART0_ITCR   = (UART0_BASE + 0x80),
    UART0_ITIP   = (UART0_BASE + 0x84),
    UART0_ITOP   = (UART0_BASE + 0x88),
    UART0_TDR    = (UART0_BASE + 0x8C),
};

/*
 * delay function
 * int32_t delay: number of cycles to delay
 *
 * This just loops <delay> times in a way that the compiler
 * wont optimize away.
 */
void delay(int32_t count) {
    asm volatile("1: subs %[count], %[count], #1; bne 1b"
		 : : [count]"r"(count));
}
    
/*
 * Initialize UART0.
 */
void uart_init(void) {
    // Disable UART0.
    mmio_write(UART0_CR, 0x00000000);
    // Setup the GPIO pin 14 && 15.
    
    // Disable pull up/down for all GPIO pins & delay for 150 cycles.
    mmio_write(GPPUD, 0x00000000);
    delay(150);

    // Disable pull up/down for pin 14,15 & delay for 150 cycles.
    mmio_write(GPPUDCLK0, (1 << 14) | (1 << 15));
    delay(150);

    // Write 0 to GPPUDCLK0 to make it take effect.
    mmio_write(GPPUDCLK0, 0x00000000);
    
    // Clear pending interrupts.
    mmio_write(UART0_ICR, 0x7FF);

    // Set integer & fractional part of baud rate.
    // Divider = UART_CLOCK/(16 * Baud)
    // Fraction part register = (Fractional part * 64) + 0.5
    // UART_CLOCK = 3000000; Baud = 115200.

    // Divider = 3000000/(16 * 115200) = 1.627 = ~1.
    // Fractional part register = (.627 * 64) + 0.5 = 40.6 = ~40.
    mmio_write(UART0_IBRD, 1);
    mmio_write(UART0_FBRD, 40);

    // Enable FIFO & 8 bit data transmissio (1 stop bit, no parity).
    mmio_write(UART0_LCRH, (1 << 4) | (1 << 5) | (1 << 6));

    // Mask all interrupts.
    mmio_write(UART0_IMSC, (1 << 1) | (1 << 4) | (1 << 5) |
		(1 << 6) | (1 << 7) | (1 << 8) |
		(1 << 9) | (1 << 10));

    // Enable UART0, receive & transfer part of UART.
    mmio_write(UART0_CR, (1 << 0) | (1 << 8) | (1 << 9));
}

/*
 * Transmit a character via UART0.
 * int c: character to send.
 */
int putchar(int c) {
    // wait for UART to become ready to transmit
    while(true) {
	if (!(mmio_read(UART0_FR) & (1 << 5))) {
	    break;
	}
    }
    mmio_write(UART0_DR, c);
    return c;
}

/*
 * Receive a byte via UART0.
 *
 * Returns:
 * uint8_t: byte received.
 */
char getc(void) {
    // wait for UART to have recieved something
    while(true) {
	if (!(mmio_read(UART0_FR) & (1 << 4))) {
	    break;
	}
    }
    return mmio_read(UART0_DR);
}

/*
 * print a string to the UART one character at a time
 * const char *str: 0-terminated string
 */
int puts(const char *str) {
    while(*str) {
	putchar(*str++);
    }
    return 0;
}

// boot.S
void __attribute__((noreturn)) abort(void);

void __attribute__((noreturn)) panic(const char *msg) {
    puts(msg);
    delay(100000000);
    abort();
}