[ARDF] Digispark USB Development Board on sale at Microcenter: $5.99

[Matthew Robbins]

    This is the crystal I mentioned: 32.768 kHz:
http://www.atmel.com/Images/doc8333.pdf

    At this URL there a several tech notes about using crystals for
calibration of the internal oscillator:
http://www.atmel.com/devices/attiny85.aspx?tab=documents

    What is cool about the 32,768 Hz crystal is that (1) the
microcontrollers are made to use it, usually without other external
components (although *very* short leads are recommended, and (2) You can
write code that plays nicely with the slower timer without having to worry
about accounting for each op cycle of the processor---as long as you can
"clear the negative" of the signed integer, you've got a wide margin for
keeping track of the time. The way I read it---and I'm *not* good at
this---is that from 0x00 (hex 00) to 0x7F is one second, and then the next
cycle gives 0x80 (which is 32768 cycles), which is negative (signed int).
As long as the code can catch the timer before it hits 0xFF and can clear
the most significant bit and increment the second count, it's all good. The
count of the crystal pulses is maintained in the 15 least significant
bits---say the count is 0xA8 when the code checks the msb and clears it,
then the count becomes 0x28. As long as the code checks that timer about
every second (and never >= 2 seconds), it's basically impossible to lose
the count. Most code is going to check it *a lot* more than that. (I'm
assuming Jennifer knows this, but I am trying to explain it partly for
other people, and partly so people can tell me if I've got it wrong.)

Whether the Arduino software tools can do this---I have no idea. Just to
mount the crystal might take soldering it piggyback to the chip and cutting
the traces to the header.

Matthew
Cincinnati

On Sat, May 18, 2013 at 5:54 PM, Jennifer Harker <harkerjen at yahoo.com>wrote:

> I've spent a few days playing with a couple of the digisparks, and they
> definitely drift without a crystal. I think maybe a slightly more expensive
> Arduino (mini pro?) with a crystal and a couple more I/O is the way to go.
>
> $6 for a digispark was a steal, though, and they will come in handy for
> other projects.
>
> On May 8, 2013, at 4:42 PM, Matthew Robbins <cedarcreek at gmail.com> wrote:
>
> >    This is kinda neat. It's a tiny little arduino-compatible board with 6
> > I/O pins. The normal price is $9 or $10, but they're on sale for a few
> more
> > days for $5.99. It has a built-in USB for programming (and maybe use by
> the
> > arduino program during operation---not sure).
> >
> >    Might be a cheap solution ($6 each) for a PicCon replacement or a
> > foxhunt controller. I'm not positive, but I think since the USB connector
> > is part of the PCB, it only needs a USB extension cable for programming
> (or
> > no cable at all if you can plug it in without the extension cable). I'm
> > thinking it might need a 32K Hz watch crystal to keep good time.
> >
> >    I have four already, but I have six more on order.
> >
> > Matthew
> > AA9YH
> > Cincinnati
> >
> > ---------- Forwarded message ----------
> > From: Matthew Robbins <cedarcreek at gmail.com>
> > Date: Wed, May 8, 2013 at 5:27 PM
> > Subject: Digispark USB Development Board on sale at Microcenter: $5.99
> >
> >
> > I bought the last four they had yesterday, but today I ordered a few more
> > from their website with in-store pickup (free shipping, but tax is
> charged).
> >
> > They have a small variety of prototype shields and i2c lcd display kits
> as
> > well. They are located in the walled-off gaming section about 8 or ten
> feet
> > right of the SparkFun section (in the Cincinnati store).
> >
> > They had a starter kit for $14.95 with the dev board plus a prototype
> > shield (I think). I didn't want to buy that because I think it's cheaper
> > separately with this deal.
> >
> > Matthew
> >
> > On Mon, Apr 29, 2013 at 9:12 AM, Ed Estes wrote:
> >
> >> Microcenter is running a sale on the *Digispark USB Development Board*<
> http://www.microcenter.com/product/409634/USB_Development_Board>.
> >> They are usually $9.95 but are on sale for $5.99 for the next two
> weeks. I
> >> backed them on *Kickstarter<
> http://www.kickstarter.com/projects/digistump/digispark-the-tiny-arduino-enabled-usb-dev-board
> >
> >> * and this is even a cheaper price than I was able to get them for...
> >>
> >> You can't beat this board if you are looking for a small, low powered
> >> Arduino for those disposable projects.
> >>
> >> --Ed
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