piclist 2004\07\02\033310a >
Thread: Driving ATtiny11 with 9v battery
face BY : Russell McMahon email (remove spam text)

> > The supply is a NOMINAL 9 volt battery. These can start out with as
> > much as 9.6v (maybe even more at the very start) and down to say 6v as
> > a reasonable end point. Even at 9v to 6v that's a 3 volt swing which
> > is quite demanding for a microprocessor to handle. Say 2v to 5v or 2.5
> > to 5.5.

> Since the tiny11 is spec'ed at 2.7 to 6V, that sounds like a pretty
> good match...

No - the series zener concept will not work across a full working battery
range with a tiny11.
Actually (or at least according to the Atmel tiny 11 & tiny12 data sheet Rev
1006D--AVR-07/03) the voltages are

2.7 - 5.5    ATtiny11L-2 & L-4        2.8v range
4.0 - 5.5    ATTiny11-6                    1.5v range

Note that the bulk buy part is liable to not be the L version. It would work
only until the battery went from new to 8.1V !!!

To guarantee that datasheet spec is met for the L part (and guaranteeing
that datasheet spec is met is an essential part of acceptable design), you
could allow the battery to go from 9v6 down to 6v8 IF the zener knee was
100% square. In practice the zener is operating from a varying current load
and you probably should allow another 0.2v on knee variability. This gives
battery range of from new down to 7v. You can't use the processor for a UART
with RC control with such a supply. Other timing applications using RC clock
are also essentially impractical.

In fact, while in the above I said "even at 9v to 6v" (= 3v range), I also
made it clear that the real range is liable to be 9.6 - 6 = 3.6v range.
Both of these are outside the maximum Vcc swing of 2.8v for the L part and
1.5v swing for the normal temperature part.

Even if the processors operating range could JUST be made to fit the 3.6v
battery swing (eg 5.6 - 2v operation) I would tend to recommend against it
for all but very very very dedicated jobs that could tolerate it, as such a
wide Vcc swing in operation is liable to cause "issues" in general
applications. While it MAY be OK to run a blink-a-LED program with such a
widely varying supply, the savings of a few cents in the power supply is
exposing beginners to a range of other factors which may confuse and
mislead. A keen beginner will consider playing with the program and giving
them an approximately stable clock is highly desirable. Even having the tone
of a doorbell or code practice oscillator vary with battery voltage is
liable to be unacceptable.

Using Digikey 100 pricing,

BC337    $0.14
Zener       $0.04
Resistor   $0.02 say

$US0.20 buys you the 3 parts needed to build quite a reasonable regulator.

Note that Digikey prices for 78L05 in 100 quantity are as low as $US0.30
OR you can get the superior 7805 for the same money!
But, then you'd have to use them :-)

       Russell McMahon

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See also: www.piclist.com/techref/power/batterys.htm?key=battery
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