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<div class="moz-cite-prefix">On 12/15/2013 11:27 PM, Charles
McClelland wrote:<br>
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Pete,
<div><br>
</div>
<div>I think that your idea of a “start to finish” process for
making a printed circuit board would be fantastic. My hope is
to be ready to start that process in the coming month or two. I
hope that in addition to making the circuit smaller and more
reliable, i can take out some of the high costs of these
“breakout” boards I have been buying from Sparkfun.</div>
<div><br>
</div>
</blockquote>
<br>
OK. <br>
<br>
<blockquote
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<div>For all, </div>
<div><br>
</div>
<div>I have taken the advice of the group to heart and moved my
trail traffic counter from a Piezo electric sensor to an
accelerometer - v4. </div>
<div><br>
</div>
<div>Here are some lessons learned in this process and a couple
areas where I am very open to suggestions:</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
Accelerometers are very cool. I have created sketches to
characterize the orientation of my board and I have a number of
ideas for other uses - thank you all for the suggestion.</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
The inexpensive accelerometers like the <a
moz-do-not-send="true"
href="https://www.sparkfun.com/products/9652">MMA7361</a> suggested
in a previous post are all analog. I got it working but saw a
few disadvantages primarily that the accelerometer and Arduino
need to be close to one another and it tied up three analog
lines which needed to be read and summed to create a “trigger”
for a knock.</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
The accelerometers which have I2C communications are more
expensive but they added the interrupt functions I was looking
for. I went with the <a moz-do-not-send="true"
href="https://www.sparkfun.com/products/10955">MMA8452</a> which
was the lowest priced but also had a “tap” sensor <br>
</div>
</blockquote>
Until you need to worry about optimizing cost the difference between
these two chips is surely noise but the convenience difference
between the analog vs serial digital interface is immense. Although
I suspect it isn't relevant for your app, the 8452 offers 12 bit
resolution vs the 10 bits of the 7361 via your Atmega chip's ADCs,
and it may be that without some signal conditioning of some sort you
can't even get the full 10 bits.<br>
<br>
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<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
I could not find an inexpensive i2C real time clock which could
work on 3.3V. So, I am stuck with the expensive <a
moz-do-not-send="true"
href="https://www.sparkfun.com/products/10160">DS3234</a> on
the SPI bus with the SD Card. Any suggestions here? A 3.3V
1307 perhaps?</div>
</blockquote>
I've been exploring the <a
href="http://www.digikey.com/product-detail/en/M41T62LC6F/497-11275-1-ND/2673274">ST
M41T62</a> which, together with a temp sensor common in
microcontroller chips, can be calibrated to 2ppm (5 seconds a month.
It can generate a 32khz square wave to run my processor at very low
speed when that's desirable, and comes in two packages: a QFN-16 for
use with an outboard crystal, and an LCC8 that has a crystal inside
it. <br>
<br>
When I get caught up a bit I'm going to make small breakouts for
both flavors of chip. Here's a <a
href="http://triembed.org/images/m41t62-lcc8-DRAFT.png">rough
draft</a> of one layout. I don't know yet if this clock is any
good, but I'm going to find out. :-) By the way, the bypass cap in
this draft is approximately the same size as the outboard crystal
that plays with the QFN-16 version of the clock. (Both boards will
be like this one: .4x.5 inches)<br>
<br>
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<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
Getting sleep to work is still an issue, turns out the at the
Arduino will only come out of sleep when the interrupt goes LOW
and the accelerometer’s interrupt goes to HIGH when it is
triggered. Ugh.</div>
<div><span class="Apple-tab-span" style="white-space:pre"> </span>- Unless
I want to put an inverter chip on the board, I may be stuck
here. Idle mode allows </div>
</blockquote>
An inverter would not be hard. <br>
<br>
But it looks like the Sparkfun breakout for your clock brings the
"INT/SQW" signal out. If you don't need the interrupt functionality
you could program the SQW output to provide an alternate clock
source for your CPU. If software can reliably switch the CPU between
it's regular full speed clock and this much slower clock (1hz, 1, 4,
or 8khz via software config) perhaps instead of "sleep" you could
set your system to "comatose", where the interrupt handlers start
off very slow but goose themselves with a clock source change as
needed. The Atmega chip's current draw will be a direct function of
the clock rate, so running at 8khz vs some number of mhz would
translate to a large savings.<br>
<br>
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<div><span class="Apple-tab-span" style="white-space:pre"> </span>-
My next “to do” is to enable batch writes to the SD Card reader
and see if I can put the card </div>
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Working out power saving strategies is surely a fun of this project.
Securely (eeprom?) buffering data until you can pump out an entire
SD card "sector" would seem to be one way of saving a lot of power,
but I don't know.<br>
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cite="mid:E312C4F2-242D-4683-99B5-B52825B9CCEE@mcclellands.org"
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<div>reader and clock to sleep between events to save power since
my Arduino has insomnia</div>
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I'm scheming to use the MSP430G2955 which has 56kb of flash memory
that the chip can write to with itself. And the MSP430 was designed
from day one to run at low power. Insanely low power. My goal is to
get one app that currently uses almost eight square inches of PCB
space and six AA batteries down to something the size of a thumb
drive and running on a single small lithium coin cell. <br>
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<div><br>
</div>
<div>Thanks again for all your help. I was able to finish my v3
board and send off to a friend in Connecticut for field testing
so I am making progress.</div>
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It's very exciting to follow your project with the msgs to the list
and sharing at meetings. Keep us posted!<br>
<br>
Coincidentally I'm assembling breakout boards for the FXOS8700CQ
accelerometer/magnetometer chip (also Freescale). With luck and a
tail wind I may have one assembled to play with at tomorrow's
meeting at Splatspace. (Meetup page for this meeting <a
href="http://www.meetup.com/splatspace/events/155648802/">here</a>.)
But I'm just learning what it's like to apply solder paste for a
QFN-16 package. The first attempt was simply comical, as I couldn't
keep the stencil stuck to the PCB and paste went through the holes
and underneath the stencil, creating a very neat, roughly square
blob over the top of the chip's pads.<br>
<br>
-Pete<br>
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<div><br>
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<div>Chip</div>
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