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'[PIC]: Two PIC16F877 Questions'
2002\01\14@214652 by Donovan Parks

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Hello,

Two questions:

1. The PIC16F877 has 2 Vss and 2 Vdd pins.  Do both Vss/Vdd pins need to be connected to +5V/GND?  Can I supply 5V to one Vdd pin and then get 5V from the other Vdd pin?

2. I am running 3 PIC16F877 in my project.  Can I use the same clock for all of them?  If so, what are the pro's and con's of doing this?  
Regards,
Donovan

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2002\01\15@175840 by David Duffy

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At 06:52 PM 14/01/2002 -0800, you wrote:
>Hello,
>
>Two questions:
>
>1. The PIC16F877 has 2 Vss and 2 Vdd pins.  Do both Vss/Vdd pins need to
>be connected to +5V/GND?  Can I supply 5V to one Vdd pin and then get 5V
>from the other Vdd pin?

No, you must connect BOTH Vss pins together and BOTH Vdd pins together.

>2. I am running 3 PIC16F877 in my project.  Can I use the same clock for
>all of them?  If so, what are the pro's and con's of doing this?

Check the archives at http://www.piclist.com for that one. I seem to recall that
you *can* but
it's messy unless you use a master clock (crystal & 74xx buffers) to drive
them all.
Using 3 resonators/crystals may be easier/cheaper.
Regards...

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2002\01\15@184550 by Dwayne Reid

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At 06:52 PM 1/14/02 -0800, Donovan Parks wrote:
>Hello,
>
>Two questions:
>
>1. The PIC16F877 has 2 Vss and 2 Vdd pins.
>Do both Vss/Vdd pins need to be connected to +5V/GND?   ** YES **
>Can I supply 5V to one Vdd pin and then get 5V from the other Vdd
>pin?    ** NO **
>
>2. I am running 3 PIC16F877 in my project.  Can I use the same clock for
>all of them?   ** YES **
>If so, what are the pro's and con's of doing this?

A little more work up front.  Essentially, you build the oscillator section
as normal for the 1st PIC, then take the OSC out of the 1st PIC and run it
into the OSC input of the 2nd.  It has worked at low frequencies for many
people (4 MHz) but I would be leery about trying it at 20 MHz.  You
probably need to decrease the crystal loading capacitor on the OSC out pin
by the amount of the input capacitance of the 2nd pin (thats the extra
work).  You may also have EMI and EMC considerations to deal with.

Of course, if you use an external clock to supply ALL the PICs, no problem
at all.

dwayne



Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2002\01\15@184555 by Byron A Jeff

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On Mon, Jan 14, 2002 at 06:52:42PM -0800, Donovan Parks wrote:
> Hello,

A suggestion. It's helpful if you actually wrap your lines...

>
> Two questions:
>
> 1. The PIC16F877 has 2 Vss and 2 Vdd pins.  Do both Vss/Vdd pins need to be
> connected to +5V/GND?

Yes. On larger chips there can be multiple Vcc/Vss pins to route power to
different areas of the chip. Unless the data sheet gives specific
instructions about their usage, you must supply +5/GND to each power
in.

>  Can I supply 5V to one Vdd pin and then get 5V from the other Vdd pin?

NO! (emphatic double hands/STOP gesture!) Each and every Vdd needs to be
connected directly to the power supply.

>
> 2. I am running 3 PIC16F877 in my project.  Can I use the same clock for all
> of them?

As long as the runs are not too long and you use an oscialltor

>  If so, what are the pro's and con's of doing this?

Neither if you do what I outlined above.

BAJ

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2002\01\16@021958 by Josh Koffman

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Isn't the OSC out actually OSC/4? That's just off the top of my head
though...

Josh

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2002\01\17@112504 by Roman Black

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Josh Koffman wrote:

> > >2. I am running 3 PIC16F877 in my project.  Can I use the same clock for
> > >all of them?   ** YES **
> > >If so, what are the pro's and con's of doing this?
> >
> > Of course, if you use an external clock to supply ALL the PICs, no problem
> > at all.

Search the net for a "one transistor crystal
oscillator", they are very simple and have plenty
of output power to drive a few PICs from the one
crystal.
-Roman

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2002\01\17@113334 by Sean Alcorn - Avion Sydney

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Roman,

It's 3:30am in our part of the world! Go to bed! :-)

I can now, 'caus I just found my bug! Oh what a feeling!

Cheers,

Sean

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