[HAL][K20XX]

Fixed for the K20D50M a possible race condition in the software part of
the timer.
The Teensy has the ability to chain timers, so similar to the KSDK ones
we switched it to full hardware timers for both Ticker and regular
Timer.

libraries/mbed/targets/hal/TARGET_Freescale/TARGET_K20XX/TARGET_K20D50M/us_ticker.c

@@ -55,8 +55,10 @@ static volatile uint32_t msb_counter = 0;
 static uint32_t timer_ldval = 0;
 
 static void timer_isr(void) {
-    msb_counter++;
-    PIT_TIMER.TFLG = 1;
+    if (PIT_TIMER.TFLG == 1) {
+        msb_counter++;
+        PIT_TIMER.TFLG = 1;
+    }
 }
 
 static void timer_init(void) {  

libraries/mbed/targets/hal/TARGET_Freescale/TARGET_K20XX/TARGET_TEENSY3_1/us_ticker.c

@@ -0,0 +1,84 @@
+/* mbed Microcontroller Library
+ * Copyright (c) 2006-2015 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"
+
+static int us_ticker_inited = 0;
+
+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
+    
+    //Timer on PIT0+1, ticker on PIT 2+3
+    //Init timer
+    PIT->CHANNEL[1].LDVAL = 0xFFFFFFFF;
+    PIT->CHANNEL[1].TCTRL = PIT_TCTRL_CHN_MASK | PIT_TCTRL_TEN_MASK;   // Start timer 1, chained to timer 0
+    
+    // Use channel 0 as a prescaler for channel 1
+    uint32_t ldval = (bus_frequency() + 500000) / 1000000 - 1;
+    PIT->CHANNEL[0].LDVAL = ldval;
+    PIT->CHANNEL[0].TCTRL = PIT_TCTRL_TEN_MASK;    // Start timer
+    
+    //Init ticker
+    PIT->CHANNEL[2].LDVAL = ldval;
+    PIT->CHANNEL[2].TCTRL = PIT_TCTRL_TEN_MASK;    // Start timer 2 as prescaler
+    
+    NVIC_SetVector(PIT3_IRQn, (uint32_t)us_ticker_irq_handler);
+    NVIC_EnableIRQ(PIT3_IRQn);
+}
+
+/******************************************************************************
+ * Timer for us timing.
+ ******************************************************************************/
+
+uint32_t us_ticker_read() {
+    if (!us_ticker_inited)
+        us_ticker_init();
+   
+   return ~(PIT->CHANNEL[1].CVAL);
+}
+
+/******************************************************************************
+ * Ticker Event
+ ******************************************************************************/
+
+void us_ticker_disable_interrupt(void) {
+    PIT->CHANNEL[3].TCTRL &= ~PIT_TCTRL_TIE_MASK;
+}
+
+void us_ticker_clear_interrupt(void) {
+    PIT->CHANNEL[3].TFLG = 1;
+}
+
+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;
+    }
+    
+    PIT->CHANNEL[3].TCTRL = 0;
+    PIT->CHANNEL[3].LDVAL = delta;
+    PIT->CHANNEL[3].TCTRL = PIT_TCTRL_TIE_MASK | PIT_TCTRL_TEN_MASK | PIT_TCTRL_CHN_MASK;
+    
+}