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I dug up the forum discussion below on AVR Freaks (THE community
resource for this family of chips). The closest to a money line is
here (with useful response in the very last post of the thread):<br>
<br>
"<span style="color: rgb(0, 0, 0); font-family: Verdana, Arial,
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255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">What start up
time does this result in (would be nice if you gave this in number
of CK-cycles). We actually wake an ATmega168 from power down with
the internal clock oscillator running at 4MHz and fuse settings
like this: CKSE3..0 = 0010 SUT1..0 = 00 (where 0 means programmed
and 1 means unprogrammed). This results in a start up time of
approx. 1ms whereas the datasheet claims 6CK for this kind of
operation which is 1.5us. Do you get a start up time of 6Ck and if
so what FUSE-settings do you use?"<br>
<br>
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<p>I'm personally 99% sure they didn't have a means of more precise
measurement, or else there's a mystery factor that makes my
measurement lucky by about a factor of three and we're left
wondering why the chip would need three times as long for a deep
sleep wakeup as for a cold start.<br>
</p>
<p>But based on my super small sample, if this thread wasn't 11
years old, I could jump in with the suggestion that the carefully
measured latency might turn out to be 300usec for a 328 if the
deep sleep wakeup is like a cold start but without the supply
voltage delay. <br>
</p>
<p>The 328 (and other AVR) data sheets beat around the bushes about
SUT fuse settings with the internal clock, and this made Shane
doubtful about the various constraints. But I think the different
scenarios run like this for deep sleep wakeup:<br>
</p>
<p>1) External, running clock source an zero startup delay with the
fuses: the few clocks everybody is dreaming about<br>
</p>
<p>2) Internal RC, most likely something like the 300usec for a 328p
at room temp *or maybe less*, or, if Atmel land is a complete
mystery, *maybe more*. I didn't do my test from deep sleep wakeup.
(aside: is it conceivable the RC clock design and behavior would
differ between the 328 and the 168 mentioned in the form thread?
Maybe the lack of clear documentation suggests some chips DO
require a nonzero SUT fuse setting? Ugh)<br>
</p>
<p>3) External resonator or crystal (I read someplace a resonator
starts faster). The startup is the same few clocks plus the SUT
fuse choices that are about how long the crystal or resonator
needs to achieve resonance and complete stability: 0ms, 4.1ms, or
65ms. Based on what I've seen so far, the reliability issue comes
into play here: pick a startup delay too short for the crystal or
resonator and the system is flakey.<br>
</p>
<p>Note that if you have a vanilla Arduino with the vanilla optiboot
bootloader there may be an additional, huge delay as the loader
listens to the serial port for programming instructions and that
times out.<br>
</p>
<div class="moz-text-html" lang="x-unicode"><a
moz-do-not-send="true"
href="http://www.avrfreaks.net/forum/query-wake-power-down-mega16l?skey=0ms%20startup%20delay">http://www.avrfreaks.net/forum/query-wake-power-down-mega16l?skey=0ms%20startup%20delay</a></div>
<br>
-Pete<br>
<br>
<div class="moz-cite-prefix">On 01/10/2018 08:55 AM, Pete Soper
wrote:<br>
</div>
<blockquote type="cite" cite="mid:md5:bW8z801CyTOCZjRVkS0kwQ==">
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This was an experiment to see the best case cold start latency. I
got it down to five machine instructions plus the internal startup
of the on-chip oscillator so Shane and others in the discussion
can see the answers to the questions left hanging: how fast the
RC-based clock gets going and whether the latency tends to be
deterministic. For one chip, one VCC and one temperature a half
dozen repetitions suggest it is deterministic.
<div>Pete</div>
<div><br>
</div>
</blockquote>
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