lpc1768 and beetle: fix us ticker set interrupt for timestamp in the past

If the event is in the past, set pending interrupt instead of directly using
timestamp.

Author: 0xc0170

targets/TARGET_ARM_SSG/TARGET_BEETLE/us_ticker.c

@@ -99,7 +99,7 @@ void us_ticker_set_interrupt(timestamp_t timestamp) {
     /* Check if the event was in the past */
     if (delta <= 0) {
         /* This event was in the past */
-        Timer_SetInterrupt(TIMER0, 0);
+        NVIC_SetPendingIRQ(Timer_GetIRQn(TIMER0));
         return;
     }
 

targets/TARGET_NXP/TARGET_LPC176X/us_ticker.c

@@ -49,10 +49,17 @@ uint32_t us_ticker_read() {
 }
 
 void us_ticker_set_interrupt(timestamp_t timestamp) {
-    // set match value
-    US_TICKER_TIMER->MR0 = (uint32_t)timestamp;
-    // enable match interrupt
-    US_TICKER_TIMER->MCR |= 1;
+    int current_time = us_ticker_read();
+    int delta = (int)(timestamp - current_time);
+    if (delta <= 0) {
+        NVIC_SetPendingIRQ(US_TICKER_TIMER_IRQn);
+    } else {
+        // set match value
+        US_TICKER_TIMER->MR0 = (uint32_t)timestamp;
+        // enable match interrupt
+        US_TICKER_TIMER->MCR |= 1;
+    }
+
 }
 
 void us_ticker_disable_interrupt(void) {