Redwood Shores, 26 juni 2008.

1) I'm not sure if your driver chips will be able to handle a 5V input if
their Vdd is 3V. They might have protective diodes that might get too hot
under these circumstances.
2) Your batteries will deplete rather quickly: you seem to be powering both
the driver chips AND the regulator continuously. I assume there is a reason
why you don't want to switch the batteries directly?
3) Yes, near each chip you will need to put a 0.1uF capacitor between Vdd
and Vss. This will ensure that the voltage doesn't drop if there is sudden
current use elsewhere. I use ceramic 0.1uF capacitors, and they have never
failed me. However, tantalum is recommended according to some books. You
might also want to put a large capacitor (10uF or higher) after the switch.
4) If your PIC16F887 indeed allows the reset pin to be internal, you won't
need the pull up network on your Vpp line.

I might have misinterpreted parts of the system, so please correct me if
some of the issues I brought up are wrong.

Greetings,
Maarten Hofman.


2008/6/26, Tom=E1s =D3 h=C9ilidhe <toe@lavabit.com>:
>
>
> I have the schematic for my Connect4 project very nearly finished.
>
> I'm going to use two AA batteries in series as my power supply, thus
> giving me 3 V. I'm going to use a voltage regulator called the LT1173-5
> to give me 5 V to power the PIC16F887.
>
> I'll use the 5 V to power the microcontroller and also as a voltage
> reference for any pull-up resistors, but everywhere else I'm going to
> use the 3 V directly from the batteries for the driver chips which drive
> the LED's and also the piezo speaker.
>
> My board will have a switch for turning the game on and off. This switch
> will make or break the connection to the output of the 5 V voltage
> regulator. This switch will have *no* effect on the 3 V supply that
> comes directly from the batteries to the driver chips. So long as
> there's batteries in the device, there'll always be a 3 V supply going
> to the driver chips regardless of whether the power switch is on or off.
>
> My board will have a 6-pin header for hooking the Pickit2 up to it to
> program and debug it. Before the Pickit2 is hooked up, the power switch
> should be turned off. When the power switch is turned off, there'll be
> no 5 V supply for the microcontroller, nor will there be a 5 V reference
> for the pull-up resistors. There *will* however always be 3 V going to
> the driver chips.
>
> When you hook the Pickit2 up to it, the Pickit2 will detect that there's
> no 5 V supply and so it will supply 5 V itself. That's the idea in anyway.
>
> Here's my current schematic:
>
> http://freepdfhosting.com/uploads/6640a7ea8a.pdf
>
> The PDF file is high resolution so you can zoom in.
>
> Here's my first question:
> 1) Do I need the RC circuit on the Vpp pin of the 887? I heard something
> about being able to use an internal pull-up?
> 2) I remember someone saying something about putting capacitors across
> the power supply as well... so if you could please give me pointers on
> that.
>
> Please feel free to offer whatever *constructive* criticism you can.
> Also, you'll notice that I have 3 pins to spare on the microcontroller.
> If you can think of something frivolous to do with them then please let
> me know!
>
> Also, can anyone suggest a good 3-pin LED for me to use? The forward
> voltage must be less than 3 V and also I'd rather if the colour
> combination wasn't Red and Green because most colour-blind people have
> difficulty distinguishing these colours. Also, I want the LED's to be
> nice and bright to that they can be seen lit outside in daylight. (My
> last project board looked crap because the LED's were very dim in
> daylight).
>
> Comments, questions, suggestions welcomed :-)
>
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