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Timer Prescaler problem

I am working on a project which requires use of 16bit timer . I am using Arduino Uno (Atmega 328p) board and Timer_1 (16 bit timer ) in Overflow mode. i want to use 1024 prescaler settings.

According to the datasheet of the controller i have to set [b]CS10 and CS12[/b] bits to use 1024 prescaler but the problem is OverFlow interrupt routine is never fired when 1024 or 256 prescaler is selected .

The Following code works just fine with Prescaler set to 8 or 64.

#include <inttypes.h> 
volatile uint8_t sCount = 0;
void setup()
{
 Timer_one_16_init(); // configure the timer 
 pinMode(13,OUTPUT);
}
void loop()
{
 // do nothing 
}
void Timer_one_16_init(){
 TCCR1A = 0;
 // set the pre-scaler to 1024 (slowest)
 TCCR1B |= (1<<CS12)|(1<<CS10);
 // set the bottom / starting value
 TCNT1 = 0x00;
 // set Overflow interrupt
 TIMSK1 |= (1<<TOIE1);
 // enable global interrupt 
 sei();
}
ISR(TIMER1_OVF_vect){
 sCount++;
 if (sCount == 60)
 {
 digitalWrite(13,!digitalRead(13));
 sCount = 0;
 }
}

i have no idea why this is happening. i have done everything which is given in the datasheet .

Am i doing something wrong ?

Thanks in advance

Answer*

You must write the clock-select bits to the correct value. The simplest
way to do it is to write the proper configuration into the `TCCR1B`
register:

 TCCR1B = (1<<CS12)|(1<<CS10);

In your program, you used `|=` instead of `=`. This doesn't write the
bits appropriately: it only sets (writes to 1) the bits `CS12` and
`CS10` but doesn't clear `CS11`. Since the Arduino core library sets the
prescaler to 64 (`CS11` and `CS10` set), you end up with all the
clock-select bits sets, in which case the timer is clocked by an
external signal on the T1 pin. Not what you want.

------------------------------------------------------------------------

**Edit**: Judging from the comments, it appears that at least dannyf did
not understand my answer, I am therefore adding a clarification. There
are two scenarios when you may want to tweak the control registers of a
peripheral:

1. The peripheral is normally controlled by the Arduino core library,
 and you want to use it through that library. Typical case: you want
 to use a timer to `analogWrite()`, or the ADC to `analogRead()`, but
 you want to clock the peripheral at a frequency which is not the
 core's default.

2. You want to use a peripheral directly rather than through the Arduino
 core. Either you are going to “take it over” from the core, or it's a
 peripheral the core doesn't use in the first place.

In the first case, you should only touch those bits of the control
registers that you must change. The other bits should be left alone, at
whatever values the core has set them to. This is the approach of
dannyf's answer.

In the second case, it is wiser to _not_ rely on the core's default
setting, and instead write _every single bit_ of the control registers
to the values appropriate for your application. This is the scenario of
the original question, and the solution proposed in my answer.

For the particular case of this question, the `TCCR1B` register would
be set to: ICNC1 = 0†, ICES1 = 0, reserved bit = 0, WGM13
= 0, WGM12 = 0, CS12 = 1, CS11 = 0 and CS10 = 1. Combining those bits
together gives the setting `TCCR1B = 0x05;`. However, for readability
reasons, I prefer to write that value `(1<<CS12)|(1<<CS10)`, as in the
question and in my original answer. Some people may prefer to explicitly
write every bit, like

 TCCR1B = (0<<ICNC1) | (0<<ICES1) | (0<<5) | (0<<WGM13)
 | (0<<WGM12) | (1<<CS12) | (0<<CS11) | (1<<CS10);

Whichever version you prefer is up to you. I personally don't find this
latter version any easier to read.

†: The values of `ICNC1` and `ICES1` are actually
irrelevant in this case.

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Thanks it solved my problem. i have just checked the default value of TCCR1B . it is 0x33 . But in the datasheet default value of TCCR1B = 0x00. i was expecting the value of CS11 bit to be 0.

paras bhanot
– paras bhanot

2017年08月18日 15:33:35 +00:00
Commented Aug 18, 2017 at 15:33
" i have just checked the default value of TCCR1B . it is 0x33 ." that means the code is setting WGM13. With the proposed "beginner" approach, WGM13 is cleared. To the extent that your code uses anything that requires WGM13 being set, like PWM, it wouldn't work.

dannyf
– dannyf

2017年08月18日 16:54:49 +00:00
Commented Aug 18, 2017 at 16:54
"But in the datasheet default value of TCCR1B = 0x00. i was expecting the value of CS11 bit to be 0." you should never rely on the default values. and your code should only change the registers / bits to the extent that you need. For example, if you want to change prescalers, change only those registers / bits that are related to the registers / bits that need to be changed. Otherwise, your code may have unexpected impact on other parts of your code.

dannyf
– dannyf

2017年08月18日 16:56:51 +00:00
Commented Aug 18, 2017 at 16:56
It is because of that, the code proposed to you is very dangerous: it is a bomb that will explode in your face when the conditions are "unexpectedly" right.

dannyf
– dannyf

2017年08月18日 16:57:30 +00:00
Commented Aug 18, 2017 at 16:57
@parasbhanot: "But in the datasheet default value of TCCR1B = 0x00". This only applies after a reset. You can rely on this default if you do not use a bootloader nor the Arduino core. The Uno's bootloader does its best to leave every register at its reset state, but it could be unwise to rely on this. And if you use the main() from Arduino core (the "normal" Arduino practice, where you write setup() and loop() but not your own main()), then many registers will end up in the Arduino's default setting, which is not the same as the hardware-defined default.

Edgar Bonet
– Edgar Bonet

2017年08月18日 19:32:15 +00:00
Commented Aug 18, 2017 at 19:32

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