diff --git a/src/device/avr/avr_tiny85.go b/src/device/avr/avr_tiny85.go
new file mode 100644
index 00000000..645914e4
--- /dev/null
+++ b/src/device/avr/avr_tiny85.go
@@ -0,0 +1,16 @@
+//go:build avr && attiny85
+// +build avr,attiny85
+
+package avr
+
+// The attiny85 have only TIMSK when other AVR boards have TIMSK0 and TIMSK1, etc.
+// Create an alias of TIMSK, TIMSK_OCIE0A, TIMSK_OCIE0B and TIMSK_TOIE0 so we can use
+// common code to mask interrupts.
+
+var TIMSK0 = TIMSK
+
+const (
+	TIMSK0_OCIE0A = TIMSK_OCIE0A
+	TIMSK0_OCIE0B = TIMSK_OCIE0B
+	TIMSK0_TOIE0  = TIMSK_TOIE0
+)
diff --git a/src/machine/machine_atmega1280.go b/src/machine/machine_atmega1280.go
index cc63d1e0..62d58659 100644
--- a/src/machine/machine_atmega1280.go
+++ b/src/machine/machine_atmega1280.go
@@ -179,6 +179,9 @@ func (pwm PWM) Configure(config PWMConfig) error {
 			avr.TCCR0B.Set(prescaler)
 			// Set the PWM mode to fast PWM (mode = 3).
 			avr.TCCR0A.Set(avr.TCCR0A_WGM00 | avr.TCCR0A_WGM01)
+			// monotonic timer is using the same time as PWM:0
+			// we must adust internal settings of monotonic timer when PWM:0 settings changed
+			AdjustMonotonicTimer()
 		} else {
 			avr.TCCR2B.Set(prescaler)
 			// Set the PWM mode to fast PWM (mode = 3).
@@ -714,6 +717,9 @@ func (pwm PWM) Set(channel uint8, value uint32) {
 				avr.TCCR0A.SetBits(avr.TCCR0A_COM0B1)
 			}
 		}
+		// monotonic timer is using the same time as PWM:0
+		// we must adust internal settings of monotonic timer when PWM:0 settings changed
+		AdjustMonotonicTimer()
 	case 1:
 		mask := interrupt.Disable()
 		switch channel {
diff --git a/src/machine/machine_atmega328p.go b/src/machine/machine_atmega328p.go
index fe5ace95..390b69f4 100644
--- a/src/machine/machine_atmega328p.go
+++ b/src/machine/machine_atmega328p.go
@@ -69,6 +69,9 @@ func (pwm PWM) Configure(config PWMConfig) error {
 			avr.TCCR0B.Set(prescaler)
 			// Set the PWM mode to fast PWM (mode = 3).
 			avr.TCCR0A.Set(avr.TCCR0A_WGM00 | avr.TCCR0A_WGM01)
+			// monotonic timer is using the same time as PWM:0
+			// we must adust internal settings of monotonic timer when PWM:0 settings changed
+			AdjustMonotonicTimer()
 		} else {
 			avr.TCCR2B.Set(prescaler)
 			// Set the PWM mode to fast PWM (mode = 3).
@@ -395,6 +398,9 @@ func (pwm PWM) Set(channel uint8, value uint32) {
 				avr.TCCR0A.SetBits(avr.TCCR0A_COM0B1)
 			}
 		}
+		// monotonic timer is using the same time as PWM:0
+		// we must adust internal settings of monotonic timer when PWM:0 settings changed
+		AdjustMonotonicTimer()
 	case 1:
 		mask := interrupt.Disable()
 		switch channel {
diff --git a/src/machine/machine_avr.go b/src/machine/machine_avr.go
index 22863846..b791594f 100644
--- a/src/machine/machine_avr.go
+++ b/src/machine/machine_avr.go
@@ -4,6 +4,7 @@ package machine
 
 import (
 	"device/avr"
+	"runtime/interrupt"
 	"runtime/volatile"
 	"unsafe"
 )
@@ -142,3 +143,76 @@ func (a ADC) Get() uint16 {
 
 	return uint16(avr.ADCL.Get()) | uint16(avr.ADCH.Get())<<8
 }
+
+var Ticks int64     // nanoseconds since start
+var tickNanos int64 // nanoseconds per each tick
+
+func InitMonotonicTimer() {
+	tickNanos = 0
+	Ticks = 0
+
+	interrupt.New(avr.IRQ_TIMER0_OVF, func(i interrupt.Interrupt) {
+		Ticks += tickNanos
+	})
+
+	// initial initialization of the Timer0
+	// - Mask interrupt
+	avr.TIMSK0.ClearBits(avr.TIMSK0_TOIE0 | avr.TIMSK0_OCIE0A | avr.TIMSK0_OCIE0B)
+
+	// - Write new values to TCNT2, OCR2x, and TCCR2x.
+	avr.TCNT0.Set(0)
+	avr.OCR0A.Set(0xff)
+	// - Set mode 3
+	avr.TCCR0A.Set(avr.TCCR0A_WGM00 | avr.TCCR0A_WGM01)
+	// - Set prescaler 1
+	avr.TCCR0B.Set(avr.TCCR0B_CS00)
+
+	AdjustMonotonicTimer()
+
+	// - Unmask interrupt
+	avr.TIMSK0.SetBits(avr.TIMSK0_TOIE0)
+}
+
+func AdjustMonotonicTimer() {
+	// adjust the tickNanos
+	tickNanos = currentTickNanos()
+}
+
+func currentTickNanos() int64 {
+	// this time depends on clk_IO, prescale, mode and OCR0A
+	// assuming the clock source is CPU clock
+	prescaler := int64(avr.TCCR0B.Get() & 0x7)
+	clock := (int64(1e12) / prescaler) / int64(CPUFrequency())
+	mode := avr.TCCR0A.Get() & 0x7
+
+	/*
+	 Mode WGM02 WGM01 WGM00 Timer/Counter       TOP  Update of  TOV Flag
+	                        Mode of Operation        OCRx at    Set on
+	 0    0     0     0     Normal              0xFF Immediate  MAX
+	 1    0     0     1     PWM, Phase Correct  0xFF TOP        BOTTOM
+	 2    0     1     0     CTC                 OCRA Immediate  MAX
+	 3    0     1     1     Fast PWM            0xFF BOTTOM     MAX
+	 5    1     0     1     PWM, Phase Correct  OCRA TOP        BOTTOM
+	 7    1     1     1     Fast PWM            OCRA BOTTOM     TOP
+	*/
+	switch mode {
+	case 0, 3:
+		// normal & fast PWM
+		// TOV0 Interrupt when moving from MAX (0xff) to 0x00
+		return clock * 256 / 1000
+	case 1:
+		// Phase Correct PWM
+		// TOV0 Interrupt when moving from MAX (0xff) to 0x00
+		return clock * 256 * 2 / 1000
+	case 2, 7:
+		// CTC & fast PWM
+		// TOV0 Interrupt when moving from MAX (OCRA) to 0x00
+		return clock * int64(avr.OCR0A.Get()) / 1000
+	case 5:
+		// Phase Correct PWM
+		// TOV0 Interrupt when moving from MAX (OCRA) to 0x00
+		return clock * int64(avr.OCR0A.Get()) * 2 / 1000
+	}
+
+	return clock / 1000 // for unknown
+}
diff --git a/src/runtime/runtime_avr.go b/src/runtime/runtime_avr.go
index 492390c4..2937efc8 100644
--- a/src/runtime/runtime_avr.go
+++ b/src/runtime/runtime_avr.go
@@ -4,14 +4,15 @@ package runtime
 
 import (
 	"device/avr"
+	"machine"
+	"runtime/interrupt"
 	"unsafe"
 )
 
 const BOARD = "arduino"
 
-type timeUnit uint32
-
-var currentTime timeUnit
+// timeUnit in nanoseconds
+type timeUnit int64
 
 // Watchdog timer periods. These can be off by a large margin (hence the jump
 // between 64ms and 125ms which is not an exact double), so don't rely on this
@@ -27,6 +28,10 @@ const (
 	WDT_PERIOD_2S
 )
 
+const timerRecalibrateInterval = 6e7 // 1 minute
+
+var nextTimerRecalibrate timeUnit
+
 //go:extern _sbss
 var _sbss [0]byte
 
@@ -56,32 +61,49 @@ func postinit() {
 
 func init() {
 	initUART()
+	machine.InitMonotonicTimer()
+	nextTimerRecalibrate = ticks() + timerRecalibrateInterval
 }
 
-const tickNanos = 1024 * 16384 // roughly 16ms in nanoseconds
-
 func ticksToNanoseconds(ticks timeUnit) int64 {
-	return int64(ticks) * tickNanos
+	return int64(ticks)
 }
 
 func nanosecondsToTicks(ns int64) timeUnit {
-	return timeUnit(ns / tickNanos)
+	return timeUnit(ns)
 }
 
-// Sleep this number of ticks of 16ms.
-//
-// TODO: not very accurate. Improve accuracy by calibrating on startup and every
-// once in a while.
+// Sleep this number of ticks of nanoseconds.
 func sleepTicks(d timeUnit) {
-	currentTime += d
-	for d != 0 {
-		sleepWDT(WDT_PERIOD_16MS)
-		d -= 1
+	waitTill := ticks() + d
+	// recalibrate if we have some time (>100ms) and it was a while when we did it last time.
+	if d > 100000 {
+		now := waitTill - d
+		if nextTimerRecalibrate < now {
+			nextTimerRecalibrate = now + timerRecalibrateInterval
+			machine.AdjustMonotonicTimer()
+		}
+	}
+	for {
+		// wait for interrupt
+		avr.Asm("sleep")
+		if waitTill <= ticks() {
+			// done waiting
+			return
+		}
+		if hasScheduler {
+			// The interrupt may have awoken a goroutine, so bail out early.
+			return
+		}
 	}
 }
 
-func ticks() timeUnit {
-	return currentTime
+// ticks return time since start in nanoseconds
+func ticks() (ticks timeUnit) {
+	state := interrupt.Disable()
+	ticks = timeUnit(machine.Ticks)
+	interrupt.Restore(state)
+	return
 }
 
 func exit(code int) {