diff --git a/lambda-test/display-test.c b/lambda-test/display-test.c
index 1d62d72..311cc3b 100644
--- a/lambda-test/display-test.c
+++ b/lambda-test/display-test.c
@@ -27,7 +27,7 @@
 
 static bool testCycle(void) {
 	cancelAlert(true);
-	position = displayPosCurrent;;
+	position = displayPosCurrent;
 	updatePending = false;
 
 	assertTrue(position == displayPosCurrent);
@@ -37,7 +37,7 @@
 	assertTrue(position == displayPosMax);
 	assertTrue(updatePending);
 	assertTrue(beepCount == 1);
-	assertTrue(beepLength == 2);
+	assertTrue(beepLength == 1);
 
 	cycleDisplay();
 	assertTrue(position == displayPosHeater);
@@ -67,8 +67,6 @@
 }
 
 static bool testUpdateMeas(void) {
-	updatePending = false;
-
 	// initial measurements
 	assertTrue(measLatest.tempI == 0);
 	assertTrue(measLatest.tempO == 0);
@@ -90,7 +88,6 @@
 	assertTrue(measMax.tempI == 1);
 	assertTrue(measMax.tempO == 2);
 	assertTrue(measMax.lambda == 3);
-	assertTrue(updatePending);
 
 	Measurement meas2 = {10, 20, 30, 40};
 	updateMeas(meas2);
diff --git a/lambda-test/interrupts-test.c b/lambda-test/interrupts-test.c
index 76c54c3..1ed25af 100644
--- a/lambda-test/interrupts-test.c
+++ b/lambda-test/interrupts-test.c
@@ -46,11 +46,10 @@
 
 	// ADC interrupt enabled
 	assertTrue(bit_is_set(ADCSRA, ADIE));
-	// PC interrupts enabled
-	// assertTrue(bit_is_set(PCICR, PCIE0));
-	// assertTrue(bit_is_set(PCMSK0, PB0));
-	// enable timer 0 overflow interrupt
-	assertTrue(bit_is_set(TIMSK0, TOIE0));
+
+	// timer0 compare match A interrupt enabled
+	assertTrue(bit_is_set(TIMSK0, OCIE0A));
+
 	// USART RX complete interrupt 0 enabled
 	assertTrue(bit_is_set(UCSR0B, RXCIE0));
 
@@ -63,17 +62,20 @@
 static bool testInitTimers(void) {
 	initTimers();
 
-	// timer0 clock prescaler /64 = 15.625 kHz overflowing every 16.2 ms
-	uint8_t prescalerBy64 = (1 << CS00) | (1 << CS01);
-	assertTrue((TCCR0B & prescalerBy64) == prescalerBy64);
+	// timer0 clear timer on compare match mode, TOP OCR0A
+	assertTrue(bit_is_set(TCCR0A, WGM01));
+	// timer0 clock prescaler/256
+	uint8_t timer0Prescaler = (1 << CS02);
+	assertTrue((TCCR0B & timer0Prescaler) == timer0Prescaler);
+	// timer0 compare match
+	assertTrue(OCR0A == 122);
 
-	// timer1 Clear Timer on Compare Match mode, TOP OCR1A
+	// timer1 clear timer on compare match mode, TOP OCR1A
 	assertTrue(bit_is_set(TCCR1B, WGM12));
 	// timer1 clock prescaler/8
-	assertTrue(bit_is_set(TCCR1B, CS11));
-	// toggles PB1 at 7.8 kHz generating a 3.9 kHz beep
-	// assertTrue(OCR1A == 15);
-	// 2 kHz is less noisy on the small piezo beeper
+	uint8_t timer1Prescaler = (1 << CS11);
+	assertTrue((TCCR1B & timer1Prescaler) == timer1Prescaler);
+	// timer1 compare match
 	assertTrue(OCR1A == 31);
 
 	return true;
@@ -81,16 +83,14 @@
 
 static bool testTime(void) {
 	resetTime();
-	assertTrue(getInts() == 0);
 	assertTrue(getTime() == 0);
 
 	_delay_ms(1000);
 
-	assertTrue(getInts() >= INTS_PER_SEC);
 	assertTrue(getTime() == 1);
 
 	// add 1:11:10
-	addInts(4270UL * INTS_PER_SEC);
+	addTime(4270UL);
 
 	// HHHHH:MM:SS
 	char str[12];
diff --git a/lambda-test/release-test.c b/lambda-test/release-test.c
index b93e8b4..d29edd3 100644
--- a/lambda-test/release-test.c
+++ b/lambda-test/release-test.c
@@ -31,9 +31,6 @@
 	assertTrue(PD3 == LCD_DB6);
 	assertTrue(PD2 == LCD_DB7);
 
-	// average value
-	assertTrue(61 == INTS_PER_SEC);
-
 	// nominal value
 	assertTrue(100 == SHUNT_MILLIOHMS);
 
diff --git a/lambda-test/rules-test.c b/lambda-test/rules-test.c
index 147660d..fba85dc 100644
--- a/lambda-test/rules-test.c
+++ b/lambda-test/rules-test.c
@@ -349,7 +349,7 @@
 	setHeaterOn(true);
 
 	// more than 3 mins
-	addInts(INTS_PER_SEC * 181);
+	addTime(181);
 
 	meas.current = 5000;
 	reason(meas);
@@ -370,7 +370,7 @@
 	setHeaterOn(true);
 
 	// more than 3 hours below 400°C
-	addInts(INTS_PER_SEC * 10800UL);
+	addTime(10800UL);
 
 	meas.tempI = 300;
 	meas.current = 1300;
diff --git a/lambda/adc.c b/lambda/adc.c
index 526732a..de4841f 100644
--- a/lambda/adc.c
+++ b/lambda/adc.c
@@ -24,8 +24,7 @@
 	// disable digital input on the ADC inputs
 	// http://www.openmusiclabs.com/learning/digital/atmega-adc/
 	DIDR0 = 0b00111111;
-	// ADC clock prescaler/8
-	ADCSRA |= (1 << ADPS1) | (1 << ADPS0);
+	ADCSRA |= ADC_PRESCALE;
 	// enable ADC
 	ADCSRA |= (1 << ADEN);
 }
diff --git a/lambda/adc.h b/lambda/adc.h
index 15fb225..3e8aac3 100644
--- a/lambda/adc.h
+++ b/lambda/adc.h
@@ -16,6 +16,14 @@
 #define ADC_NONLIN_0 -8
 #define ADC_NONLIN_AREF 12
 
+#if F_CPU == 1000000
+	// ADC clock prescaler/8 = 125kHz @ 1MHz
+	#define ADC_PRESCALE (1 << ADPS1) | (1 << ADPS0)
+#elif F_CPU == 8000000
+	// ADC clock prescaler/64 = 125kHz @ 8MHz
+	#define ADC_PRESCALE (1 << ADPS2) | (1 << ADPS1)
+#endif
+
 /**
  * Sets up reference voltage and clock prescaler of the ADC and enables it.
  */
diff --git a/lambda/command.c b/lambda/command.c
index bc2eab3..7c2f589 100644
--- a/lambda/command.c
+++ b/lambda/command.c
@@ -44,7 +44,7 @@
 		resetDisplay();
 		resetRules();
 		setHeaterOn(true);
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("sd")) == 0) {
 		// simulation disable
@@ -52,27 +52,27 @@
 		resetDisplay();
 		resetRules();
 		simulation = false;
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("le")) == 0) {
 		// logging enable
 		logging = true;
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("ld")) == 0) {
 		// logging disable
 		logging = false;
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("he")) == 0) {
 		// oxygen sensor heater enable
 		setHeaterOn(true);
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("hd")) == 0) {
 		// oxygen sensor heater disable
 		setHeaterOn(false);
-		beep(1, 2, 31);
+		beep(1, 1, 31);
 	}
 	else if (strcmp_P(fields[0], PSTR("cm")) == 0) {
 		// cycle menu
@@ -82,7 +82,7 @@
 		// test alert
 		char buf[16];
 		strcpy_P(buf, PSTR("Beep Beep Beep!"));
-		alert(3, 10, 15, buf, fields[1], false);
+		alert(3, 5, 15, buf, fields[1], false);
 	}
 	else if (strcmp_P(fields[0], PSTR("tb")) == 0) {
 		// test beep
@@ -97,15 +97,16 @@
 		beep(1, length, tone);
 	}
 	else if (simulation) {
-		// add one second per measurement to the timebase,
+		// add one second per measurement to the time,
 		// assuming one measurement was logged per second
-		addInts(INTS_PER_SEC);
+		addTime(1);
 		Measurement meas = readMeas(fields, fieldCount);
 		if (getHeaterState() == heaterStateOff ||
 				getHeaterState() == heaterStateReady) {
 			updateMeas(meas);
 		}
 		reason(meas);
+		setUpdatePending();
 	}
 
 	updateDisplayIfPending();
diff --git a/lambda/display.c b/lambda/display.c
index b932ad2..a3c12bc 100644
--- a/lambda/display.c
+++ b/lambda/display.c
@@ -99,7 +99,7 @@
 		position = displayPosCurrent;
 	}
 	updatePending = true;
-	beep(1, 2, 31);
+	beep(1, 1, 31);
 }
 
 void updateMeas(Measurement const meas) {
@@ -108,8 +108,6 @@
 	measMax.tempI = MAX(measMax.tempI, meas.tempI);
 	measMax.tempO = MAX(measMax.tempO, meas.tempO);
 	measMax.lambda = MIN(measMax.lambda, meas.lambda);
-
-	updatePending = true;
 }
 
 void resetDisplay(void) {
@@ -159,3 +157,7 @@
 	strncat(lastLine1, line1, sizeof(lastLine1) - 1);
 	setText(line0, line1);
 }
+
+void setUpdatePending(void) {
+	updatePending = true;
+}
diff --git a/lambda/display.h b/lambda/display.h
index efaf506..4a51fb0 100644
--- a/lambda/display.h
+++ b/lambda/display.h
@@ -57,4 +57,10 @@
  */
 void displayText(const char* line0, const char* line1);
 
+/**
+ * Causes the display to be updated next time
+ * updateDisplayIfPending() is called.
+ */
+void setUpdatePending(void);
+
 #endif /* DISPLAY_H_ */
diff --git a/lambda/interrupts.c b/lambda/interrupts.c
index 6779e27..95ce891 100644
--- a/lambda/interrupts.c
+++ b/lambda/interrupts.c
@@ -24,37 +24,35 @@
 static volatile uint32_t ints = 0;
 
 /**
- * Called about every 16.4 ms.
+ * Called about every 1/32 seconds.
  */
-ISR(TIMER0_OVF_vect) {
+ISR(TIMER0_COMPA_vect) {
 	if (! isSimulation()) {
 		ints++;
 	}
 	oscillateBeep();
-	if (bit_is_clear(PINB, PB0) && ! buttonPressed) {
+	if (bit_is_clear(PIN, PIN_BUTTON) && ! buttonPressed) {
 		buttonPressed = true;
 		cycleDisplay();
-	} else if (bit_is_set(PINB, PB0)) {
+	} else if (bit_is_set(PIN, PIN_BUTTON)) {
 		buttonPressed = false;
 	}
 }
 
-uint32_t getInts(void) {
-	uint32_t atomicInts;
-	ATOMIC_BLOCK(ATOMIC_FORCEON) {
-		atomicInts = ints;
-	}
-	return atomicInts;
-}
-
-void addInts(uint32_t const add) {
-	ATOMIC_BLOCK(ATOMIC_FORCEON) {
-		ints += add;
-	}
-}
-
 uint32_t getTime(void) {
-	return getInts() / INTS_PER_SEC;
+	uint32_t time;
+	ATOMIC_BLOCK(ATOMIC_FORCEON) {
+		// 32 ints per second
+		time = ints >> 5;
+	}
+
+	return time;
+}
+
+void addTime(uint32_t const time) {
+	ATOMIC_BLOCK(ATOMIC_FORCEON) {
+		ints += (time << 5);
+	}
 }
 
 void formatTime(char* const str, size_t const size) {
@@ -92,29 +90,24 @@
 	// enable ADC interrupt
 	ADCSRA |= (1 << ADIE);
 
-	// enable timer 0 overflow interrupt
-	TIMSK0 |= (1 << TOIE0);
+	// enable timer0 compare match A interrupt
+	TIMSK0 |= (1 << OCIE0A);
 
 	// enable USART RX complete interrupt 0
 	UCSR0B |= (1 << RXCIE0);
-	// enable data register empty interrupt 0
-	// UCSR0B |= (1 << UDRIE0);
 
 	// enable global interrupts
 	sei();
 }
 
 void initTimers(void) {
-	// timer in normal mode is default
-	// timer0 clock prescaler/64 = 15.625 kHz overflowing every 16.4 ms
-	TCCR0B |= (1 << CS01) | (1 << CS00);
+	// timer0 clear timer on compare match mode, TOP OCR0A
+	TCCR0A |= (1 << WGM01);
+	TCCR0B |= TIMER0_PRESCALE;
+	OCR0A = TIMER0_COMP_MATCH;
 
-	// timer1 Clear Timer on Compare Match mode, TOP OCR1A
+	// timer1 clear timer on compare match mode, TOP OCR1A
 	TCCR1B |= (1 << WGM12);
-	// timer1 clock prescaler/8
-	TCCR1B |= (1 << CS11);
-	// timer1 Compare Match at 7.8 kHz generating a 3.9 kHz beep
-	// OCR1A = 15;
-	// 2 kHz is less noisy on the small piezo beeper
-	OCR1A = 31;
+	TCCR1B |= TIMER1_PRESCALE;
+	OCR1A = TIMER1_COMP_MATCH;
 }
diff --git a/lambda/interrupts.h b/lambda/interrupts.h
index c148fbb..436faa1 100644
--- a/lambda/interrupts.h
+++ b/lambda/interrupts.h
@@ -11,20 +11,29 @@
 #ifndef INTERRUPTS_H_
 #define INTERRUPTS_H_
 
-// 61 +/-1 depending on internal oscillator?
-#define INTS_PER_SEC 61
-
-/**
- * Returns the count of interrupts occurring about every 16.4 ms
- * since last reset.
- */
-uint32_t getInts(void);
-
-/**
- * Adds the given number of interrupts to the interrupt counter
- * used as timebase. Useful only for simulation.
- */
-void addInts(uint32_t add);
+#if F_CPU == 1000000
+	// timer0 clock prescaler/256 = 3.906 kHz @ 1 MHz
+	#define TIMER0_PRESCALE (1 << CS02)
+	// timer0 compare match about every 31.25 ms = 1000 ms >> 5
+	#define TIMER0_COMP_MATCH 122
+	// timer1 clock prescaler/8 = 125 kHz @ 1MHz
+	#define TIMER1_PRESCALE (1 << CS11)
+	// timer1 compare match at 7.8 kHz generating a 3.9 kHz beep
+	// #define TIMER1_CTC_TOP 15
+	// 2 kHz is less noisy on the small piezo beeper
+	#define TIMER1_COMP_MATCH 31
+#elif F_CPU == 8000000
+	// timer0 clock prescaler/1024 = 7.812 kHz @ 8 MHz
+	#define TIMER0_PRESCALE (1 << CS02) | (1 << CS00)
+	// timer0 compare match about every 31.25 ms = 1000 ms >> 5
+	#define TIMER0_COMP_MATCH 244
+	// timer1 clock prescaler/64 = 125 kHz @ 8MHz
+	#define TIMER1_PRESCALE (1 << CS11) | (1 << CS10)
+	// timer1 compare match at 7.8 kHz generating a 3.9 kHz beep
+	// #define TIMER1_CTC_TOP 15
+	// 2 kHz is less noisy on the small piezo beeper
+	#define TIMER1_COMP_MATCH 31
+#endif
 
 /**
  * Returns the time in about seconds since last reset.
@@ -32,6 +41,11 @@
 uint32_t getTime(void);
 
 /**
+ * Adds the given number of seconds. Useful only for simulation.
+ */
+void addTime(uint32_t time);
+
+/**
  * Writes the time since last reset in format HHHHH:MM:SS to the given string.
  */
 void formatTime(char* str, size_t size);
diff --git a/lambda/lambda.c b/lambda/lambda.c
index 48eb8e6..873537c 100644
--- a/lambda/lambda.c
+++ b/lambda/lambda.c
@@ -23,6 +23,7 @@
 #include <avr/io.h>
 #include <avr/interrupt.h>
 #include <avr/pgmspace.h>
+#include <avr/power.h>
 #include "usart.h"
 #include "lcdroutines.h"
 #include "adc.h"
@@ -39,6 +40,12 @@
  * runs commands sent via USART.
  */
 int main(void) {
+
+	// assuming 1 MHz default
+	if (F_CPU == 8000000UL) {
+		clock_prescale_set(clock_div_1);
+	}
+
 	initUSART();
 	lcd_init();
 	setupPorts();
@@ -47,24 +54,25 @@
 	initInterrupts();
 	initTimers();
 
-	alert_P(1, 2, 31, PSTR(MSG_WELCOME), PSTR(""), false);
+	alert_P(1, 1, 31, PSTR(MSG_WELCOME), PSTR(""), false);
 	// spend some time on being polite
 	while (getTime() < 3) {}
 	setHeaterOn(true);
 
-	uint32_t ints = 0;
+	uint32_t time = 0;
 	Measurement meas;
 
 	// main loop
 	while (true) {
-		if (! isSimulation() && getInts() >= ints + INTS_PER_SEC) {
-			ints = getInts();
+		if (! isSimulation() && getTime() > time) {
+			time = getTime();
 			meas = measure();
 			updateMeas(meas);
 			reason(meas);
 			if (isLogging()) {
 				logMeas(meas);
 			}
+			setUpdatePending();
 		}
 		if (! isSimulation()) {
 			updateDisplayIfPending();
diff --git a/lambda/pins.h b/lambda/pins.h
index f307075..244c81d 100644
--- a/lambda/pins.h
+++ b/lambda/pins.h
@@ -21,7 +21,9 @@
 /** ADC pin for the LSM 11 oxygen sensor heater current */
 #define ADC_HEATER PC3
 
-/** Port for menu button, beeper and oxygen sensor heater */
+/** Pin input register for menu button */
+#define PIN PINB
+/** Pin output register for menu button, beeper and oxygen sensor heater */
 #define PORT PORTB
 /** DDR for menu button, beeper and oxygen sensor heater */
 #define DDR DDRB
diff --git a/lambda/rules.c b/lambda/rules.c
index 2fa7f47..c083b08 100644
--- a/lambda/rules.c
+++ b/lambda/rules.c
@@ -13,7 +13,7 @@
 #include "messages.h"
 
 #define BEEPS 30
-#define LENGTH 20
+#define LENGTH 10
 #define TONE 31
 
 uint8_t age = 0;
@@ -126,7 +126,7 @@
 	}
 	if (meas.current <= milliAmpsReady && meas.current > milliAmpsDisconn) {
 		setHeaterState(heaterStateReady);
-		alert_P(3, 10, TONE, PSTR(MSG_HEATER_READY_0),
+		alert_P(3, 5, TONE, PSTR(MSG_HEATER_READY_0),
 				PSTR(MSG_HEATER_READY_1), false);
 	}
 }
@@ -158,7 +158,7 @@
 		Measurement const meas) {
 	if (isHeaterOn() && getTime() >= 10800 && meas.tempI < 400) {
 		setHeaterOn(false);
-		alert_P(3, 10, TONE, PSTR(MSG_HEATER_OFF_0),
+		alert_P(3, 5, TONE, PSTR(MSG_HEATER_OFF_0),
 				PSTR(MSG_HEATER_OFF_1), false);
 	}
 }
diff --git a/lambda/sensors.c b/lambda/sensors.c
index dee5abd..d87240c 100644
--- a/lambda/sensors.c
+++ b/lambda/sensors.c
@@ -195,7 +195,7 @@
 	if (on) {
 		PORTB |= (1 << PB2);
 		heaterState = heaterStateUp;
-		alert_P(1, 2, 31, PSTR(MSG_HEATER_UP_0), PSTR(MSG_HEATER_UP_1), true);
+		alert_P(1, 1, 31, PSTR(MSG_HEATER_UP_0), PSTR(MSG_HEATER_UP_1), true);
 	} else {
 		PORTB &= ~(1 << PB2);
 		heaterState = heaterStateOff;