teensy40: fix PIT clock, which actually uses 24 MHz OSC

see: https://forum.pjrc.com/threads/63979-What-peripherals-are-affected-by-the-undocumented-24-MHz-OSC-circuit-on-Teensy-4-0

src/runtime/runtime_mimxrt1062_clock.go

@@ -9,7 +9,7 @@ import (
 // Core, bus, and peripheral clock frequencies (Hz)
 const (
 	CORE_FREQ      = 600000000 // 600     MHz
-	OSC_FREQ       = 100000    // 100     kHz  (see note below)
+	SYSTICK_FREQ   = 100000    // 100     kHz  (see note below)
 	_                          // -----------
 	AHB_FREQ       = 600000000 // 600     MHz
 	CAN_FREQ       = 40000000  //  40     MHz
@@ -65,7 +65,7 @@ var (
 	}
 )
 
-// Note about OSC_FREQ from Teensyduino (cores/teensy4/startup.c):
+// Note about SYSTICK_FREQ from Teensyduino (cores/teensy4/startup.c):
 //
 // |  ARM SysTick is used for most Ardiuno timing functions, delay(), millis(),
 // |  micros().  SysTick can run from either the ARM core clock, or from an

src/runtime/runtime_mimxrt1062_time.go

@@ -11,15 +11,14 @@ import (
 
 type timeUnit int64
 
-const (
-	SYST_FREQ      = OSC_FREQ // HW divides 24 MHz XTALOSC down to 100 kHz
-	lastCycle      = SYST_FREQ/1000 - 1
-	microsPerCycle = 1000000 / SYST_FREQ
+const ( // HW divides 24 MHz XTALOSC down to 100 kHz
+	lastCycle      = SYSTICK_FREQ/1000 - 1
+	microsPerCycle = 1000000 / SYSTICK_FREQ
 )
 
 const (
-	PIT_FREQ          = OSC_FREQ // HW divides 24 MHz XTALOSC down to 100 kHz
-	pitMicrosPerCycle = 1000000 / PIT_FREQ
+	PIT_FREQ          = PERCLK_FREQ
+	pitCyclesPerMicro = PIT_FREQ / 1000000
 	pitSleepTimer     = 0 // x4 32-bit PIT timers [0..3]
 )
 
@@ -88,7 +87,7 @@ func sleepTicks(duration timeUnit) {
 		curr := ticks()
 		last := curr + duration
 		for curr < last {
-			cycles := timeUnit((last - curr) * pitMicrosPerCycle)
+			cycles := timeUnit((last - curr) / pitCyclesPerMicro)
 			if cycles > 0xFFFFFFFF {
 				cycles = 0xFFFFFFFF
 			}