uart.c

//
// uart.c -- Serial I/O
//
//  Copyright (c) 2012-2013 Andrew Payne <andy@payne.org>
//

#include <freedom.h>
#include "common.h"

// Circular buffers for transmit and receive
#define BUFLEN 128

static uint8_t _tx_buffer[sizeof(RingBuffer) + BUFLEN] __attribute__ ((aligned(4)));
static uint8_t _rx_buffer[sizeof(RingBuffer) + BUFLEN] __attribute__ ((aligned(4)));

static RingBuffer *const tx_buffer = (RingBuffer *) &_tx_buffer;
static RingBuffer *const rx_buffer = (RingBuffer *) &_rx_buffer;

void UART0_IRQHandler()
{
    int status;
    
    status = UART0_S1;
    
    // If transmit data register empty, and data in the transmit buffer,
    // send it.  If it leaves the buffer empty, disable the transmit interrupt.
    if ((status & UART_S1_TDRE_MASK) && !buf_isempty(tx_buffer)) {
        UART0_D = buf_get_byte(tx_buffer);
        if(buf_isempty(tx_buffer))
            UART0_C2 &= ~UART_C2_TIE_MASK;
    }
    
    // If there is received data, read it into the receive buffer.  If the
    // buffer is full, disable the receive interrupt.
    if ((status & UART_S1_RDRF_MASK) && !buf_isfull(rx_buffer)) {
        buf_put_byte(rx_buffer, UART0_D);
        if(buf_isfull(rx_buffer))
            UART0_C2 &= ~UART_C2_RIE_MASK;
    }
}

int uart_write(char *p, int len)
{
    int i;
    
    for(i=0; i<len; i++) {
        while(buf_isfull(tx_buffer))        // Spin wait while full
            ;
        buf_put_byte(tx_buffer, *p++);
        UART0_C2 |= UART_C2_TIE_MASK;           // Turn on Tx interrupts
    }
    return len;
}

// A blocking write, useful for error/crash/debug reporting
int uart_write_err(char *p, int len)
{
    int i;
    
    __disable_irq();
    for(i=0; i<len; i++) {
        while((UART0_S1 & UART_S1_TDRE_MASK) == 0)  // Wait until transmit buffer empty
            ;
        
        UART0_D = *p++;                     // Send char
    }
    __enable_irq();
    return len;
}

int uart_read(char *p, int len)
{
    int i = len;

    while(i > 0) {
        while(buf_isempty(rx_buffer))           // Spin wait
            ;

        *p++ = buf_get_byte(rx_buffer);
        UART0_C2 |= UART_C2_RIE_MASK;           // Turn on Rx interrupt
        i--;
    }
    return len - i;
}

//
// uart_init() -- Initialize debug / OpenSDA UART0
//
//      The OpenSDA UART RX/TX is connected to pins 27/28, PTA1/PTA2 (ALT2)
//
void uart_init(int baud_rate)
{
    SIM_SCGC5 |= SIM_SCGC5_PORTA_MASK;
        
    // Turn on clock to UART0 module and select 48Mhz clock (FLL/PLL source)
    SIM_SCGC4 |= SIM_SCGC4_UART0_MASK;
    SIM_SOPT2 &= ~SIM_SOPT2_UART0SRC_MASK;
    SIM_SOPT2 |= SIM_SOPT2_UART0SRC(1);                 // FLL/PLL source

    // Select "Alt 2" usage to enable UART0 on pins
    PORTA_PCR1 = PORT_PCR_MUX(2);
    PORTA_PCR2 = PORT_PCR_MUX(2);

    UART0_C2 = 0;
    UART0_C1 = 0;
    UART0_C3 = 0;
    UART0_S2 = 0;     

    // Set the baud rate divisor
    #define OVER_SAMPLE 16
    uint16_t divisor = (CORE_CLOCK / OVER_SAMPLE) / baud_rate;
    UART0_C4 = UARTLP_C4_OSR(OVER_SAMPLE - 1);
    UART0_BDH = (divisor >> 8) & UARTLP_BDH_SBR_MASK;
    UART0_BDL = (divisor & UARTLP_BDL_SBR_MASK);

    // Initialize transmit and receive circular buffers
    buf_reset(tx_buffer, BUFLEN);
    buf_reset(rx_buffer, BUFLEN);

    // Enable the transmitter, receiver, and receive interrupts
    UART0_C2 = UARTLP_C2_RE_MASK | UARTLP_C2_TE_MASK | UART_C2_RIE_MASK;
    enable_irq(INT_UART0);
}