Optimize micros()

teensy4/clockspeed.c

 
 	F_CPU_ACTUAL = frequency;
 	F_BUS_ACTUAL = frequency / div_ipg;
+	scale_cpu_cycles_to_microseconds = 0xFFFFFFFFu / (uint32_t)(frequency / 1000000u);
 
 	printf("New Frequency: ARM=%u, IPG=%u\n", frequency, frequency / div_ipg);
 

teensy4/core_pins.h

 
 extern volatile uint32_t F_CPU_ACTUAL;
 extern volatile uint32_t F_BUS_ACTUAL;
+extern volatile uint32_t scale_cpu_cycles_to_microseconds;
 extern volatile uint32_t systick_millis_count;
 
 static inline uint32_t millis(void) __attribute__((always_inline, unused));

teensy4/delay.c

 //volatile uint32_t F_BUS = 132000000;
 volatile uint32_t systick_millis_count = 0;
 volatile uint32_t systick_cycle_count = 0;
+volatile uint32_t scale_cpu_cycles_to_microseconds = 0;
 uint32_t systick_safe_read;	 // micros() synchronization
 
 // page 411 says "24 MHz XTALOSC can be the external clock source of SYSTICK"
 
 uint32_t micros(void)
 {
-	uint32_t ccdelta, usec, smc, scc;
+	uint32_t smc, scc;
 	do {
 		__LDREXW(&systick_safe_read);
 		smc = systick_millis_count;
 		scc = systick_cycle_count;
 	} while ( __STREXW(1, &systick_safe_read));
-	ccdelta = ARM_DWT_CYCCNT - scc;
-	uint32_t frac = ccdelta/(F_CPU_ACTUAL/1000000);
-	if (frac > 999) frac = 999;
-	usec = 1000*smc + frac;
+	uint32_t cyccnt = ARM_DWT_CYCCNT;
+	asm volatile("" : : : "memory");
+	uint32_t ccdelta = cyccnt - scc;
+	uint32_t frac = ((uint64_t)ccdelta * scale_cpu_cycles_to_microseconds) >> 32;
+	if (frac > 1000) frac = 1000;
+	uint32_t usec = 1000*smc + frac;
 	return usec;
 }