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

[Matthew Robbins]

I just realized my errors. 

Of course it takes 16 bits for 32768 signed, but it takes 4 hex digits, not two. So one second is 0x0000 to 0x7FFF, and so on. (Duh.)

Matthew

On May 20, 2013, at 6:20 PM, Matthew Robbins <cedarcreek at gmail.com> wrote:

>     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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