Update us_ticker.c

Fixed hardcoded MRT_Clock_MHz so that the code will also work at a SystemCoreClock different from 30MHz. Optimized time calculations for us_ticker_read. Same modifications as done previously for LPC812.

Author: Willem23

libraries/mbed/targets/hal/TARGET_NXP/TARGET_LPC82X/us_ticker.c

@@ -13,77 +13,91 @@
  * 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"
 
 static int us_ticker_inited = 0;
-static int ticker_expired = 0;
+int MRT_Clock_MHz;
+unsigned int ticker_fullcount_us;
+unsigned long int ticker_expired_count_us = 0;
 
 #define US_TICKER_TIMER_IRQn     MRT_IRQn
-#define MRT_CLOCK_MHZ            30
 
-void us_ticker_init(void)
-{
+void us_ticker_init(void) {
+    
     if (us_ticker_inited)
         return;
 
     us_ticker_inited = 1;
 
+    // Calculate MRT clock value (MRT has no prescaler)
+    MRT_Clock_MHz = (SystemCoreClock / 1000000);
+    // Calculate fullcounter value in us (MRT has 31 bits and clock is 30MHz)
+    ticker_fullcount_us = 0x80000000UL/MRT_Clock_MHz;
+
     // Enable the MRT clock
     LPC_SYSCON->SYSAHBCLKCTRL |= (1 << 10);
 
     // Clear peripheral reset the MRT
     LPC_SYSCON->PRESETCTRL |= (1 << 7);
 
-    // Force load interval value
+    // Force load interval value (Bit 0-30 is interval value, Bit 31 is Force Load bit)     
     LPC_MRT->INTVAL0 = 0xFFFFFFFFUL;
-    // Enable ch0 interrupt
-    LPC_MRT->CTRL0 = 1;
+    // Enable Ch0 interrupt, Mode 0 is Repeat Interrupt
+    LPC_MRT->CTRL0 = (0x0 << 1) | (0x1 << 0);
 
-    // Force load interval value
+    // Force load interval value (Bit 0-30 is interval value, Bit 31 is Force Load bit)     
     LPC_MRT->INTVAL1 = 0x80000000UL;
-    // Disable ch1 interrupt
-    LPC_MRT->CTRL1 = 0;
-
+    // Disable ch1 interrupt, Mode 0 is Repeat Interrupt
+    LPC_MRT->CTRL1 = (0x0 << 1) | (0x0 << 0);
+    
     // Set MRT interrupt vector
     NVIC_SetVector(US_TICKER_TIMER_IRQn, (uint32_t)us_ticker_irq_handler);
     NVIC_EnableIRQ(US_TICKER_TIMER_IRQn);
 }
 
-uint32_t us_ticker_read()
-{
+//TIMER0 is used for us ticker and timers (Timer, wait(), wait_us() etc)
+uint32_t us_ticker_read() {
+    
     if (!us_ticker_inited)
         us_ticker_init();
 
     // Generate ticker value
-    // MRT source clock is SystemCoreClock (30MHz) and 31-bit down count timer
-    // Calculate expected value using number of expired times
-    return (0x7FFFFFFFUL - LPC_MRT->TIMER0)/MRT_CLOCK_MHZ + (ticker_expired * (0x80000000UL/MRT_CLOCK_MHZ));
+    // MRT source clock is SystemCoreClock (30MHz) and MRT is a 31-bit countdown timer
+    // Calculate expected value using number of expired times to mimic a 32bit timer @ 1 MHz
+    return (0x7FFFFFFFUL - LPC_MRT->TIMER0)/MRT_Clock_MHz + ticker_expired_count_us;        
 }
 
-
-void us_ticker_set_interrupt(timestamp_t timestamp)
-{
-    // Force load interval value
-    LPC_MRT->INTVAL1 = (((timestamp - us_ticker_read()) * MRT_CLOCK_MHZ) | 0x80000000UL);
-
+//TIMER1 is used for Timestamped interrupts (Ticker(), Timeout())
+void us_ticker_set_interrupt(timestamp_t timestamp) {
+    
+    // MRT source clock is SystemCoreClock (30MHz) and MRT is a 31-bit countdown timer    
+    // Force load interval value (Bit 0-30 is interval value, Bit 31 is Force Load bit)
+    // Note: The MRT has less counter headroom available than the typical mbed 32bit timer @ 1 MHz.
+    //       The calculated counter interval until the next timestamp will be truncated and an
+    //       'early' interrupt will be generated in case the max required count interval exceeds
+    //       the available 31 bits space. However, the mbed us_ticker interrupt handler will 
+    //       check current time against the next scheduled timestamp and simply re-issue the
+    //       same interrupt again when needed. The calculated counter interval will now be smaller.
+    LPC_MRT->INTVAL1 = (((timestamp - us_ticker_read()) * MRT_Clock_MHz) | 0x80000000UL);
+    
     // Enable interrupt 
     LPC_MRT->CTRL1 |= 1;
 }
 
-void us_ticker_disable_interrupt()
-{
+//Disable Timestamped interrupts triggered by TIMER1
+void us_ticker_disable_interrupt() {
+    //Timer1 for Timestamped interrupts (31 bits downcounter @ SystemCoreClock)    
     LPC_MRT->CTRL1 &= ~1;
 }
 
-void us_ticker_clear_interrupt()
-{
+void us_ticker_clear_interrupt() {
+    
+    //Timer1 for Timestamped interrupts (31 bits downcounter @ SystemCoreClock)
     if (LPC_MRT->STAT1 & 1)
         LPC_MRT->STAT1 = 1;
 
+    //Timer0 for us counter (31 bits downcounter @ SystemCoreClock)
     if (LPC_MRT->STAT0 & 1) {
         LPC_MRT->STAT0 = 1;
-        ticker_expired++;
+          ticker_expired_count_us += ticker_fullcount_us;
     }
 }