Update rtc_api.c

Removed redundant RTC->CR define code + added RTC compensation code to calibrate RTC.

Author: star297

libraries/mbed/targets/hal/TARGET_Freescale/TARGET_KLXX/rtc_api.c

@@ -38,14 +38,32 @@ static void init(void) {
 void rtc_init(void) {
     init();
 
-    //Configure the TSR. default value: 1
+    // Configure the TSR. default value: 1
     RTC->TSR = 1;
-    
-    if (PinMap_RTC[0].pin == NC) {        //Use OSC32K
-        RTC->CR |= RTC_CR_OSCE_MASK;
-        //delay for OSCE stabilization
-        for(int i=0; i<0x1000; i++) __NOP();
-    }
+
+    // Configure Time Compensation Register to calibrate RTC accuracy
+
+    // dissable LRL lock
+    RTC->LR &= ~RTC_LR_LRL_MASK;
+    // RTC->TCR: RTC_TCR_CIR_MASK,RTC_TCR_CIR(x)=0,RTC_TCR_TCR(x)=0  Default no correction
+    RTC->TCR = RTC_TCR_CIR(0) | RTC_TCR_TCR(0);
+    /*
+        RTC_TCR_CIR(x) sets the compensation interval in seconds from 1 to 256.
+        0x05 will apply the compensation once every 4 seconds.
+
+        RTC_TCR_TCR(x) sets the Register Overflow
+        0x80 Time Prescaler Register overflows every 32896 clock cycles. (+128)
+        ... ... RTC runs slower
+        0xFF Time Prescaler Register overflows every 32769 clock cycles.
+        0x00 Time Prescaler Register overflows every 32768 clock cycles, Default.
+        0x01 Time Prescaler Register overflows every 32767 clock cycles.
+        ... ... RTC runs faster
+        0x7F Time Prescaler Register overflows every 32641 clock cycles. (-128)
+    */
+    // enable TCL lock
+    RTC->LR |= RTC_LR_TCL_MASK;
+    // enable LRL lock
+    RTC->LR |= RTC_LR_LRL_MASK;
 
     // enable counter
     RTC->SR |= RTC_SR_TCE_MASK;