Merge pull request #330 from Defragster/cmicros

CCmicros - micros version using Cycle counter

teensy4/EventResponder.cpp

 
 // TODO: this doesn't work for IMXRT - no longer using predefined names
 extern "C" volatile uint32_t systick_millis_count;
+extern "C" volatile uint32_t systick_cycle_count;
+extern "C" uint32_t systick_safe_read; // micros() synchronization
 extern "C" void systick_isr(void)
 {
+	systick_cycle_count = ARM_DWT_CYCCNT;
 	systick_millis_count++;
 	MillisTimer::runFromTimer();
 }

teensy4/delay.c

 #include "core_pins.h"
+#include "arm_math.h"	// micros() synchronization
 
 //volatile uint32_t F_CPU = 396000000;
 //volatile uint32_t F_BUS = 132000000;
 volatile uint32_t systick_millis_count = 0;
+volatile uint32_t systick_cycle_count = 0;
+uint32_t systick_safe_read;	 // micros() synchronization
 
 // page 411 says "24 MHz XTALOSC can be the external clock source of SYSTICK"
 // Testing shows the frequency is actually 100 kHz - but how?  Did NXP really
 	// TODO...
 }
 
+uint32_t micros(void)
+{
+	uint32_t ccdelta, usec, 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;
+	usec = 1000*smc + (ccdelta/(F_CPU_ACTUAL/1000000));
+	return usec;
+}
+
+#if 0 // kept to compare test to cycle count micro()
 uint32_t micros(void)
 {
 	uint32_t msec, tick, elapsed, istatus, usec;
 	prev_usec = usec;
 	return usec;
 }
-
+#endif

teensy4/startup.c

 // the ARM clock to run at different speeds.
 #define SYSTICK_EXT_FREQ 100000
 
+extern volatile uint32_t systick_cycle_count;
 static void configure_systick(void)
 {
 	_VectorsRam[14] = pendablesrvreq_isr;
 	SCB_SHPR3 = 0x20000000;  // Systick = priority 32
 	ARM_DEMCR |= ARM_DEMCR_TRCENA;
 	ARM_DWT_CTRL |= ARM_DWT_CTRL_CYCCNTENA; // turn on cycle counter
+	systick_cycle_count = ARM_DWT_CYCCNT; // compiled 0, corrected w/1st systick
 }
 
 
 //  R2
 //  R1
 //  R0
+__attribute__((weak))
 void unused_interrupt_vector(void)
 {
 	// TODO: polling Serial to complete buffered transmits