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### Topic: Attiny85 (Read 5721 times)previous topic - next topic

#15
##### Nov 14, 2012, 08:22 am
If I  was using an ATtiny85 using the internal oscillator at 8Mhz divided down to 1 Mhz via the prescaler, which option would I use in the IDE:

ATtiny85 @ 1 MHz  (internal oscillator; BOD disabled)

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Is there any difference in power usage,

1 MHz = less power.

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speed of processing,

1 MHz = 8 times slower.

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running at 1Mhz with no prescaler?

Using what clock?

#### Vincent19

#16
##### Nov 14, 2012, 08:24 am
yea..just stay with those that I want to use.

Thanks

#### Vincent19

#17
##### Nov 14, 2012, 08:29 amLast Edit: Nov 14, 2012, 08:38 am by Vincent19 Reason: 1

If I  was using an ATtiny85 using the internal oscillator at 8Mhz divided down to 1 Mhz via the prescaler, which option would I use in the IDE:

ATtiny85 @ 1 MHz  (internal oscillator; BOD disabled)

So means if 8MHz, delay(1000) will run it as delay 1s? how if 16MHz?

#18
##### Nov 14, 2012, 09:09 am
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So means if 8MHz, delay(1000) will run it as delay 1s? how if 16MHz?

No.  As long as you have the correct board selected, delay will work correctly.

#### lemming

#19
##### Nov 15, 2012, 11:55 am
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Using what clock?

I thought that 1 Mhz internal was an option - I was wrong . But assume a 1 Mhz crystal.

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1 MHz = less power.
1 MHz = 8 times slower.

I realise that, but imagine two Atmega328: system #1 - 8Mhz divided down to 1 Mhz.  System #2 - 1 Mhz crystal, no divisor.

Both systems have a 'final' clock of 1 Mhz. I assume that both would complete a loop of (say) 100 iterations in the same time.

Are there any any notable performance/efficiency differences between system #1  and system #2?

Also, if I programmed an Arduinos fuses to use the internal oscillator, do I need to remove the crystal to ensure stability?

#### fungus

#20
##### Nov 15, 2012, 12:42 pmLast Edit: Nov 15, 2012, 08:01 pm by fungus Reason: 1

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Using what clock?

I thought that 1 Mhz internal was an option - I was wrong . But assume a 1 Mhz crystal.

On an ATtiny85? The hardware can certainly do it, it's how the chips are configured at the factory.

If you're not seeing that option in the IDE then you have an incorrect IDE setup.

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1 MHz = less power.
1 MHz = 8 times slower.

I realise that, but imagine two Atmega328: system #1 - 8Mhz divided down to 1 Mhz.  System #2 - 1 Mhz crystal, no divisor.

Both systems have a 'final' clock of 1 Mhz. I assume that both would complete a loop of (say) 100 iterations in the same time.

Are there any any notable performance/efficiency differences between system #1  and system #2?

1mHz is 1mHz no matter where it comes from. Performance is the same.

But...the internal oscillators aren't as accurate as an external crystal. They vary a little bit with supply voltage and temperature (see the datasheet). If you need very precise timing you have to use external components (this is the reason Atmel gives us options).

Also, if I programmed an Arduinos fuses to use the internal oscillator, do I need to remove the crystal to ensure stability?

It won't make any difference to stability because the crystal is completely ignored.

OTOH removing the crystal gives you two extra I/O pins - worth having on a Tiny85!
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

#21
##### Nov 15, 2012, 07:56 pm
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1 MHz = less power.
1 MHz = 8 times slower.

I realise that, but imagine two Atmega328: system #1 - 8Mhz divided down to 1 Mhz.  System #2 - 1 Mhz crystal, no divisor.

Both systems have a 'final' clock of 1 Mhz. I assume that both would complete a loop of (say) 100 iterations in the same time.

Almost.  As @fungus mentioned there will very likely be a difference in clock speeds.  The crystal will very likely be very close to exactly 1 MHz.  The internal oscillator will very likely not be close to 1 MHz.  (Tuning will get the internal oscillator to ±1%.)

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Are there any any notable performance/efficiency differences between system #1  and system #2?

Yes.  The crystal will very likely be more stable and more accurate over the range of temperatures and voltages.

Yes.  Driving an external crystal takes more energy.  (Also, I think the startup time is longer.)

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Also, if I programmed an Arduinos fuses to use the internal oscillator, do I need to remove the crystal to ensure stability?

No.  When using the internal oscillator, the XTAL1 and XTAL2 pins become normal I/O pins.  You can have whatever you want (within reason) connected to the two pins.  The internal oscillator does not care.

#### lemming

#22
##### Nov 15, 2012, 11:24 pmLast Edit: Nov 15, 2012, 11:27 pm by lemming Reason: 1
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On an ATtiny85? The hardware can certainly do it, it's how the chips are configured at the factory.

I think what you are referring to is a factory configuration of 8Mhz divided down to 1 Mhz. What I was talking about was an internal clock of 1 Mhz with no divisor.

I am trying to understand why the chip has two configurations that are essentially the same: 8 Mhz divided down to 1Mhz and 1 Mhz with no divisor. What is the point of the dividing of the clock rate? Why don't they just get us to set the clock rate that we require rather than selecting one that is too high and then dividing back?

It s bit like buying and eight cylinder car and removing the spark plugs from four of the cylinders because you only want a four cylinder car.  Why not just select a four cylinder engine in the first place?

#23
##### Nov 16, 2012, 12:11 am
What I was talking about was an internal clock of 1 Mhz with no divisor.

Just so we're clear on the matter, that does not exist on an ATtiny85.  It has two internal clock sources: an 8 MHz oscillator and the 128 KHz watchdog oscilator.

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I am trying to understand why the chip has two configurations that are essentially the same: 8 Mhz divided down to 1Mhz and 1 Mhz with no divisor.

It does not.  It has an 8 MHz internal oscillator.

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What is the point of the dividing of the clock rate?

Save energy.

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Why don't they just get us to set the clock rate that we require rather than selecting one that is too high and then dividing back?

Cost and complexity.

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It s bit like buying and eight cylinder car and removing the spark plugs from four of the cylinders because you only want a four cylinder car.

Which several modern vehicles do automatically to save energy.

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Why not just select a four cylinder engine in the first place?

Because some applications require the extra horsepower.

#### fungus

#24
##### Nov 16, 2012, 10:36 am

I am trying to understand why the chip has two configurations that are essentially the same: 8 Mhz divided down to 1Mhz and 1 Mhz with no divisor.

I don't think it has an internal 1MHz clock.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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