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Thread: Yet another squeezed-app challenge...
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Well, last night I re-designed the board, and managed to (barely) fit the 16F628 and 12F629 on the board.  I had to move some resistors under the 7-seg display (soldered before the display and on the back of the board).  Would've been so much nicer if I didn't have to do that, but it should work.

That said, the DS1302 would fit instead of the 12F629, since it's the same 8-dip as the 12F629.  But it's a bunch more expensive (3 times the 12F629 price), and still requires a crystal.  Also, I now need 3 free lines on the 16F628 to communicate with the DS1302, which I don't have.  Instead of this, yesterday I tried an arrangement which uses a 12F629 in the same fashion as the DS1302 (keeps time info and sends to 16F628 to display it), but required 2 lines to communicate with the 16F628 (my own protocol).  The crystal would've been on the 12F629, and the 16F628 would operate on it's internal oscillator since timing is not important on that.  But alas, I still would *not* have 2 free pins on the 16F628.

The arrangement with the 12F629 being a 3-line to 5-line decoder/demux would work, so that's still in the possible-answers bucket.  In case anyone's wondering, it's actually a 2-line to 4-line decoder, and the 3rd line from the 16F628 would go straight to the display.

So another thought occurred to me yesterday, and that is to multiplex the switches with the LED-column-scan lines.  I have to do some thinking/designing, but it seems like it would work at a high-level -- of the 18 lines on the 16F628, 2 are for power, 2 for 32.768khz xtal (on timer 1), (int osc still used for the higher-speed stuff), 7 for LED segments, 5 for LED columns.  That leaves 2 lines.... one for the buzzer and one for the input from the multiplexed switches.  Since I already have 2 transistors on the multiplexed switches, I may not have add much more.  I'll hash it out over the next few hours and then post.

Finally, I worked out a way to move all the (3) switches onto one comparator input, but have fears about unreliability due to the multiple levels.  I'll post that separately since this email is stretching out a bit too much.

Cheers,
-Neil.



On Sunday 08 June 2003 04:42, Hugo scribbled:
> I know you are short of PCB space, but wouldn't it be possible to fit a
> DS1302 somewhere? 8 pin DIP or smaller SMD versions.
> Great clock stability, low power and some extra features.
> You might even go with a slower PIC crystal to save more power.
>
> Hugo

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