Async timer with 32kHz crystal on ATtiny861

mega128 · Joined: 08 Feb 2009 Posts: 28

Hi all,

I am new to BacomAVR and AVR so please bear with me.

I am trying to put micro in sleep mode to save coin cell battery and wake up every second to check the status of ADC. If ADC is higher than usual it should carry out some task and go back to sleep.

Now, the problem is I am using ATtiny861 since it has 32x gain capabilities in ADC. But can't find TOSC1/2 pins on this micro. Does it mean 261/461/861A don't have these pins to connect external watch crystal? If there is a way to connect watch crystal to it what pins?

Thanks Very Happy

MWS · Joined: 22 Aug 2009 Posts: 2262

According the data sheet the only way to connect a 32768Hz crystal is as main clock source. This may make execution of your code a tad slow. Very Happy

Better calculate the clock cycles needed for the awake period, before you take that approach. Be also aware that you need then an ISP frequency lower than 8kHz.

If the above doesn't work for you, use an external RTC with 1Hz clock output connected to Int0/1.

Or a different µC.

mega128 · Joined: 08 Feb 2009 Posts: 28

Thanks for quick reply. I intend to use 16Mhz crystal as main clock source. This means I can't have two xtals at same time. Interrupt timing is not critical as I am not going to build a internal clock. As long as I can put micro on sleep and wake it up approx 1 second, do the ADC task and go back to sleep I am fine.

Would you recommend using watchdog or internal oscillator or something else that will consume least power similar to 32khz clock?

Thanks

MWS · Joined: 22 Aug 2009 Posts: 2262

Hmm, everything depends on your requirements of speed / complexity of the circuit / power consumption. I would recommend to check the diagrams at the end of the Tiny data sheet.

The WDT Int combination shouldn't work bad, 4µA at 3V, but again, if you'd use an external RTC which would take about 400nA and the µC in power down at approx. 170nA, this combination would work with less than 600nA.

Of course the consumption for the time the µC is awake and running with 16MHz has to be added in this calculation, for example 1000 cycles would add about 500nA for an one second period, that's 1.1µA for the RTC versus 4.5µA for the watchdog.

Maybe you could also use a lower main clock, this would additionally reduce power consumption. Also be aware that the Tiny needs a certain voltage to run at 16MHz, with one lithium cell of 3V you will not get more than 10MHz.

But again, check the diagrams, everything's in there and then decide what's most important for you.

Visovian · Joined: 31 Oct 2007 Posts: 584 Location: Czech

Hi,

You can divide the system clock with IO register CLKPR.
Eg if you use 16 MHz crystal and CLKPR = 7, you will get clock 125 kHz.
See datasheet how to set CLKPR.

mega128 · Joined: 08 Feb 2009 Posts: 28

MWS - WDT Int 4µA at 3V does not sound too bad to me as it reduces one external component. The reason I can't change 16Mhz is that is required by another chip. I am trying to use 16Mhz for both even though AVR can't run more than 10Mhz on 3V.

Visovian - "16 MHz crystal and CLKPR = 7, you will get clock 125 kHz" Can you explain little bit more. Do you mean that I should use watchdog timer or some internal timer and divide it to get 125khz. Will it consume less power and wake up the micro from sleep on int?

Thanks both of you for your suggestions.

Visovian · Joined: 31 Oct 2007 Posts: 584 Location: Czech

Hi,
With CLKPR register you can change the main system clock during runtime.
When you have external crystal 16 MHz and in the code you set CLKPR to divide by 128, the clock is 128 kHz from this moment (16000000/128=128000).
Same as if you changed crystal for 128 kHz one.

Do you realy need 16 MHz in your application?

MWS · Joined: 22 Aug 2009 Posts: 2262

Visovian,

If the µC has to finish certain code, let's say 1000 clocks, it is more energy efficient to drive the core with 16MHz, than with 128kHz. If it's a wake-up for a fixed time, it's different.
Second, if energy effectiveness is an requirement, what sense should it make to run the µC in idle mode, current is approx. 60µA then, in contrary to 4µA at power down if done with the watchdog ?
The only sense to use CLKPR would be to use a common clock source of 16MHz and scale core frequency down below 10Mhz, so the µC runs stable at 3V.

Visovian · Joined: 31 Oct 2007 Posts: 584 Location: Czech

MWS:

You are right.
Power down and WDT interrupt will consume the least.

mega128 · Joined: 08 Feb 2009 Posts: 28

Thanks all for your input. WDT is working great for me.