Questions about delays

[cosmonavt]

Hello,

I was wondering that in order to make a delay of 1 second, why so we say _delay_ms(100); and not _delay_ms(1000); ?

Secondly, I want to make a delay of 24 hours using a tiny AVR. I need to measure these 24 hours very accurately. First off, I am using AVR Studio 5, how do I set the f_CPU value to 10MHZ? Secondly, _delay_ms has a max value of 262.14, how do I go about measuring 24 hours?

I thought of a way to use the time period over frequency formula. This would give me the number of oscillations required to reach 24 hours. Then I can use this value in a for loop to measure the delay.

The number would be very large of course. How do I go about this?

 

[misterT]

Hello,

I was wondering that in order to make a delay of 1 second, why so we say _delay_ms(100); and not _delay_ms(1000); ?

Secondly, I want to make a delay of 24 hours using a tiny AVR. I need to measure these 24 hours very accurately. First off, I am using AVR Studio 5, how do I set the f_CPU value to 10MHZ? Secondly, _delay_ms has a max value of 262.14, how do I go about measuring 24 hours?

I thought of a way to use the time period over frequency formula. This would give me the number of oscillations required to reach 24 hours. Then I can use this value in a for loop to measure the delay.

The number would be very large of course. How do I go about this?

First:
I would say _delay_ms(1000); to create 1 second delay.

Second:
To create 24 hour clock, you can setup a Timer/Counter to create interrupts at constant intervals. Use a 32 bit integer calculate the number of interrupts. If the interval is 10ms, then 24 hours is 8 640 000 "ticks". Google "avr timer interrupt tutorial".
How accurate is "very accurate". Do you mean something like a Chinese wristwatch, or more like an atomic clock?

 

[cosmonavt]

Well I guess the delay function is dependent on the frequency. In this video the guy says _delay_ms(100) to make a 1 sec delay.

 

[misterT]

Well I guess the delay function is dependent on the frequency. In this video the guy says _delay_ms(100) to make a 1 sec delay.

the delay function uses F_CPU value to calculate the delay cycles, so you have to set the F_CPU to correct value. The guy in the video probably has a wrong value in F_CPU.
If you have 10 Mhz clock, you would say:

#define F_CPU 10000000UL

https://www.nongnu.org/avr-libc/user-manual/group__util__delay.html

 

[cosmonavt]

OK my bad luck, the MCU im using is ATTiny13 which has only one 8 bit timer. Means its maximum count is 255. With a frequency of 10MHz, the time period is 1 x 10^-7 second. To get a delay of 24 hours, I need 8.64 x 10^11 cycles. Would I have to run the timer again and again? Is that possible? If I reduce the frequency, then I guess the accuracy will be affected. But since I need a 24 hour delay, how much accuracy would I need?

Anyways, dividing 8.64 x 10^11 by 255 gives 3388235294. Would this work?

for ( i = 0, i < 3388235294,  ) {
        ++i;
}

 

[3v0]

It looks like you have a prescaler that can be used with the timer.

TCCR0B – Timer/Counter Control Register B

I you were to select a perscaler of 16 the counter would count at 1/16 the system clock speed.

 

[cosmonavt]

Yes I have a prescaler. Isn't the largest value of the prescaler 1024? If I set F_CPU value to 400KHz, and prescaler to 1024, then I have a freq of 390.625. This means that it has a time period of 2.56*10^-3 which is a much greater value. How do I generate an hour long delay with this now?

 

[cosmonavt]

OK I generated something like this:

#define F_CPU 400UL
#include <avr/io.h>
#include <util/delay.h>

void timer0_init(){
	TCCR0B = (1<<CS00)|(0<<CS01)|(1<<CS02);
	TCNT0 = 0;
}

void main(){
	DDRC = 0x01;
	timer0_init();
	
	while(1){
		if (TCNT0 > 34560000){
			while(1)
			{
				PORTC = 0x01;
				_delay_ms(1000);
				PORTC = 0x00;
				_delay_ms(1000);
			}
		}	
	}		
}

When the counter reaches 34560000, the LED on Pin C0 should start blinking forever.

Is this code correct? The device has an 8 bit timer, the max it can count to is 255. What do I do about the value 34560000? Ive reduced the frequency to a mere 400 Hz to make the time period bigger, but I can't seem to get the counter value within the range for an 8bit timer.

BTW the prescaler is set to 1024.

 

[cosmonavt]

Does decreasing the frequency reduce the power consumption? For example, I set the frequency to a mere 400 Hz by

define F_CPU 400UL

Would it increase the amp hours of the battery?

 

[misterT]

F_CPU is not a setting. It just tells the compiler your MCU frequency.. it does not set it.

in other words:

#define F_CPU 400UL

does nothing to your MCU frequency.

But, decreasing the frequency does reduce power consumption.

 

[Pommie]

If you have your prescaler at 1024 and you allow the timer to overflow, it will happen every 1024*256 = 262144 cycles and will set the interrupt flag for the timer.

In your code do,

    if(InterruptFlag==1){
        InterruptFlag=0;
        count+=262144;
        if(count>10000000){    // = clock speed
             count-=10000000;
             seconds++;
             // here do if(seconds==60) etc
        }
    }

You now are counting seconds and so it is easy to extend to count minutes and hours.

Mike.

 

[cosmonavt]

How exactly do we set the frequency then?

 

[cosmonavt]

I see that ATTiny 13A has two selectable frequencies, one is 4.8 MHz, the other is 9.6 MHz. If I use teh lower frequency and the maximum prescaler (1024), then I get a frequency of 4687.5 and a time period of 2.13*10^-4.

24 hours will then take (24*60*60) / (2.13*10^-4) = 405,000,000 cycles.

Now isn't it possible for me to create a timer where the initial counter is first set to zero, then since I have an 8 bit timer, use a compare value of 250 (when it exceeds this, 250 cycles are completed) and use a for loop to loop this process 405,000,000/250 = 1620,000 times?

How do I set the frequency to 4.8MHz? There are two possible CKSEL values. 0b10 will give 9.6MHz while 0b01 will give 4.8 MHz. I can't seem to set this frequency...

 

[Pommie]

I assumed your frequency was set by your crystal.

The code above should count seconds with complete accuracy.

Edit, your original post stated 10MHz.

Mike.

 

[cosmonavt]

In the datasheet of ATTiny13, there are two frequencies mentioned. One is 4.8MHz and the other is 9.6 MHz. How exactly do I know which one is it running at? Or how do I modify the clock select fuses (CKSEL) which are needed to decide between 4.8MHz and 9.6MHz.

P.S. please throw some light on the code you posted above.

Thanks.