File content as of revision 308:59cc78a25982:

/* mbed Microcontroller Library
 * Copyright (c) 2006-2013 ARM Limited
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include <stddef.h>
#include "us_ticker_api.h"
#include "PeripheralNames.h"
#include "clk_freqs.h"

#define PIT_TIMER           PIT->CHANNEL[0]
#define PIT_TIMER_IRQ       PIT0_IRQn
#define PIT_TICKER          PIT->CHANNEL[1]
#define PIT_TICKER_IRQ      PIT1_IRQn

static void timer_init(void);
static void ticker_init(void);


static int us_ticker_inited = 0;
static uint32_t clk_mhz;

void us_ticker_init(void) {
    if (us_ticker_inited)
        return;
    us_ticker_inited = 1;
    
    SIM->SCGC6 |= SIM_SCGC6_PIT_MASK;   // Clock PIT
    PIT->MCR = 0;                       // Enable PIT
    
    clk_mhz = bus_frequency() / 1000000;

    timer_init();
    ticker_init();
}

/******************************************************************************
 * Timer for us timing.
 *
 * The K20D5M does not have a prescaler on its PIT timer nor the option
 * to chain timers, which is why a software timer is required to get 32-bit
 * word length.
 ******************************************************************************/
static volatile uint32_t msb_counter = 0;
static uint32_t timer_ldval = 0;

static void timer_isr(void) {
    msb_counter++;
    PIT_TIMER.TFLG = 1;
}

static void timer_init(void) {  
    //CLZ counts the leading zeros, returning number of bits not used by clk_mhz
    timer_ldval = clk_mhz << __CLZ(clk_mhz);

    PIT_TIMER.LDVAL = timer_ldval;  // 1us
    PIT_TIMER.TCTRL |= PIT_TCTRL_TIE_MASK;
    PIT_TIMER.TCTRL |= PIT_TCTRL_TEN_MASK;  // Start timer 0

    NVIC_SetVector(PIT_TIMER_IRQ, (uint32_t)timer_isr);
    NVIC_EnableIRQ(PIT_TIMER_IRQ);
}

uint32_t us_ticker_read() {
    if (!us_ticker_inited)
        us_ticker_init();
        
    uint32_t retval;
    __disable_irq(); 
    retval = (timer_ldval - PIT_TIMER.CVAL) / clk_mhz; //Hardware bits
    retval |= msb_counter << __CLZ(clk_mhz);           //Software bits
    
    if (PIT_TIMER.TFLG == 1) {                         //If overflow bit is set, force it to be handled
        timer_isr();                                   //Handle IRQ, read again to make sure software/hardware bits are synced
        NVIC_ClearPendingIRQ(PIT_TIMER_IRQ);
        return us_ticker_read();
    }

    __enable_irq();
    return retval;
}

/******************************************************************************
 * Timer Event
 *
 * It schedules interrupts at given (32bit)us interval of time.
 * It is implemented using PIT channel 1, since no prescaler is available,
 * some bits are implemented in software.
 ******************************************************************************/
static void ticker_isr(void);

static void ticker_init(void) {
    /* Set interrupt handler */
    NVIC_SetVector(PIT_TICKER_IRQ, (uint32_t)ticker_isr);
    NVIC_EnableIRQ(PIT_TICKER_IRQ);
}

void us_ticker_disable_interrupt(void) {
    PIT_TICKER.TCTRL &= ~PIT_TCTRL_TIE_MASK;
}

void us_ticker_clear_interrupt(void) {
    // we already clear interrupt in lptmr_isr
}

static uint32_t us_ticker_int_counter = 0;

inline static void ticker_set(uint32_t count) {
    PIT_TICKER.TCTRL = 0;
    PIT_TICKER.LDVAL = count;
    PIT_TICKER.TCTRL = PIT_TCTRL_TIE_MASK | PIT_TCTRL_TEN_MASK;
}

static void ticker_isr(void) {
    // Clear IRQ flag
    PIT_TICKER.TFLG = 1;

    if (us_ticker_int_counter > 0) {
        ticker_set(0xFFFFFFFF);
        us_ticker_int_counter--;
    } else {
        // This function is going to disable the interrupts if there are
        // no other events in the queue
        us_ticker_irq_handler();
    }
}

void us_ticker_set_interrupt(timestamp_t timestamp) {
    int delta = (int)((uint32_t)timestamp - us_ticker_read());
    if (delta <= 0) {
        // This event was in the past:
        us_ticker_irq_handler();
        return;
    }

    //Calculate how much falls outside the 32-bit after multiplying with clk_mhz
    //We shift twice 16-bit to keep everything within the 32-bit variable
    us_ticker_int_counter = (uint32_t)(delta >> 16);
    us_ticker_int_counter *= clk_mhz;
    us_ticker_int_counter >>= 16;
    
    uint32_t us_ticker_int_remainder = (uint32_t)delta * clk_mhz;
    if (us_ticker_int_remainder == 0) {
        ticker_set(0xFFFFFFFF);
        us_ticker_int_counter--;
    } else {
        ticker_set(us_ticker_int_remainder);
    }
}