Elegoo Nano timer prescaler problem

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I am trying to use Timer2 on an Elegoo Nano to produce a "tick" interrupt every 10uS. My plan was to set Timer2 to CTC mode with a /8 prescaler (to get a timer clock of 2MHz from the 16MHz chip clock) and a compare value of 19 (to reset the timer every 20 counts) which should trigger the compareA ISR at 100KHz (every 10us). The problem is that only prescale values of 64 or greater work as expected. Setting the prescaler to any value lower than 64 actually results in the "tick" interval getting much longer (10 times the interval or more). I have carefully studied the ATmega628 datasheet to see if I am configuring every register correctly, but I don't see anything wrong with what I am doing. Here is my code...

void initTimer() _BV( CS21 );

 // ---Enable Timer2 CTC interrupts for compare A match
 TIMSK2 

SIGNAL( TIMER2_COMPA_vect ) 

 /*
 ** This should execute every 10uS, but for prescaler values
 ** below 64 the interval is many times longer than expected.
 */

Can anyone point out my mistake?

arduino-nano interrupt timers

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asked 2 hours ago

PhoenixRevealed

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I am trying to use Timer2 on an Elegoo Nano to produce a "tick" interrupt every 10uS. My plan was to set Timer2 to CTC mode with a /8 prescaler (to get a timer clock of 2MHz from the 16MHz chip clock) and a compare value of 19 (to reset the timer every 20 counts) which should trigger the compareA ISR at 100KHz (every 10us). The problem is that only prescale values of 64 or greater work as expected. Setting the prescaler to any value lower than 64 actually results in the "tick" interval getting much longer (10 times the interval or more). I have carefully studied the ATmega628 datasheet to see if I am configuring every register correctly, but I don't see anything wrong with what I am doing. Here is my code...

void initTimer() _BV( CS21 );

 // ---Enable Timer2 CTC interrupts for compare A match
 TIMSK2 

SIGNAL( TIMER2_COMPA_vect ) 

 /*
 ** This should execute every 10uS, but for prescaler values
 ** below 64 the interval is many times longer than expected.
 */

Can anyone point out my mistake?

arduino-nano interrupt timers

share|improve this question

asked 2 hours ago

PhoenixRevealed

61

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PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

up vote
1
down vote

favorite

up vote
1
down vote

favorite

I am trying to use Timer2 on an Elegoo Nano to produce a "tick" interrupt every 10uS. My plan was to set Timer2 to CTC mode with a /8 prescaler (to get a timer clock of 2MHz from the 16MHz chip clock) and a compare value of 19 (to reset the timer every 20 counts) which should trigger the compareA ISR at 100KHz (every 10us). The problem is that only prescale values of 64 or greater work as expected. Setting the prescaler to any value lower than 64 actually results in the "tick" interval getting much longer (10 times the interval or more). I have carefully studied the ATmega628 datasheet to see if I am configuring every register correctly, but I don't see anything wrong with what I am doing. Here is my code...

void initTimer() _BV( CS21 );

 // ---Enable Timer2 CTC interrupts for compare A match
 TIMSK2 

SIGNAL( TIMER2_COMPA_vect ) 

 /*
 ** This should execute every 10uS, but for prescaler values
 ** below 64 the interval is many times longer than expected.
 */

Can anyone point out my mistake?

arduino-nano interrupt timers

share|improve this question

asked 2 hours ago

PhoenixRevealed

61

New contributor

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

I am trying to use Timer2 on an Elegoo Nano to produce a "tick" interrupt every 10uS. My plan was to set Timer2 to CTC mode with a /8 prescaler (to get a timer clock of 2MHz from the 16MHz chip clock) and a compare value of 19 (to reset the timer every 20 counts) which should trigger the compareA ISR at 100KHz (every 10us). The problem is that only prescale values of 64 or greater work as expected. Setting the prescaler to any value lower than 64 actually results in the "tick" interval getting much longer (10 times the interval or more). I have carefully studied the ATmega628 datasheet to see if I am configuring every register correctly, but I don't see anything wrong with what I am doing. Here is my code...

void initTimer() _BV( CS21 );

 // ---Enable Timer2 CTC interrupts for compare A match
 TIMSK2 

SIGNAL( TIMER2_COMPA_vect ) 

 /*
 ** This should execute every 10uS, but for prescaler values
 ** below 64 the interval is many times longer than expected.
 */

Can anyone point out my mistake?

arduino-nano interrupt timers

arduino-nano interrupt timers

share|improve this question

asked 2 hours ago

PhoenixRevealed

61

New contributor

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 2 hours ago

PhoenixRevealed

61

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PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

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share|improve this question

asked 2 hours ago

PhoenixRevealed

61

New contributor

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 hours ago

PhoenixRevealed

61

asked 2 hours ago

PhoenixRevealed

61

61

New contributor

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

New contributor

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

PhoenixRevealed is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

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Can anyone point out my mistake?

Your mistake is in thinking that it's possible to do more than you can within a 10uS period.

At 16MHz each instruction takes 63ns (for single-clock instructions). 10nS gives you a maximum of (10,000 / 63) 156 assembly instructions. Into that you have to fit the preamble and postamble to store the registers to the stack and retrieve them again.

The remaining clock cycles (maybe no more than 100) won't give you much time to do complex things. And given that many of the Arduino API calls are quite heavyweight, even just something as "simple" as a digitalWrite() can overflow that many times over.

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answered 1 hour ago

Majenko♦

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1 Answer
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1 Answer
1

active

oldest

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votes

up vote
2
down vote

Can anyone point out my mistake?

Your mistake is in thinking that it's possible to do more than you can within a 10uS period.

At 16MHz each instruction takes 63ns (for single-clock instructions). 10nS gives you a maximum of (10,000 / 63) 156 assembly instructions. Into that you have to fit the preamble and postamble to store the registers to the stack and retrieve them again.

The remaining clock cycles (maybe no more than 100) won't give you much time to do complex things. And given that many of the Arduino API calls are quite heavyweight, even just something as "simple" as a digitalWrite() can overflow that many times over.

share|improve this answer

answered 1 hour ago

Majenko♦

62.5k42873

add a comment | 

up vote
2
down vote

Can anyone point out my mistake?

Your mistake is in thinking that it's possible to do more than you can within a 10uS period.

At 16MHz each instruction takes 63ns (for single-clock instructions). 10nS gives you a maximum of (10,000 / 63) 156 assembly instructions. Into that you have to fit the preamble and postamble to store the registers to the stack and retrieve them again.

The remaining clock cycles (maybe no more than 100) won't give you much time to do complex things. And given that many of the Arduino API calls are quite heavyweight, even just something as "simple" as a digitalWrite() can overflow that many times over.

share|improve this answer

answered 1 hour ago

Majenko♦

62.5k42873

add a comment | 

up vote
2
down vote

up vote
2
down vote

Can anyone point out my mistake?

Your mistake is in thinking that it's possible to do more than you can within a 10uS period.

At 16MHz each instruction takes 63ns (for single-clock instructions). 10nS gives you a maximum of (10,000 / 63) 156 assembly instructions. Into that you have to fit the preamble and postamble to store the registers to the stack and retrieve them again.

The remaining clock cycles (maybe no more than 100) won't give you much time to do complex things. And given that many of the Arduino API calls are quite heavyweight, even just something as "simple" as a digitalWrite() can overflow that many times over.

share|improve this answer

answered 1 hour ago

Majenko♦

62.5k42873

Can anyone point out my mistake?

Your mistake is in thinking that it's possible to do more than you can within a 10uS period.

At 16MHz each instruction takes 63ns (for single-clock instructions). 10nS gives you a maximum of (10,000 / 63) 156 assembly instructions. Into that you have to fit the preamble and postamble to store the registers to the stack and retrieve them again.

The remaining clock cycles (maybe no more than 100) won't give you much time to do complex things. And given that many of the Arduino API calls are quite heavyweight, even just something as "simple" as a digitalWrite() can overflow that many times over.

share|improve this answer

answered 1 hour ago

Majenko♦

62.5k42873

share|improve this answer

share|improve this answer

answered 1 hour ago

Majenko♦

62.5k42873

answered 1 hour ago

Majenko♦

62.5k42873

answered 1 hour ago

Majenko♦

62.5k42873

62.5k42873

add a comment | 

add a comment | 

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