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'[PIC]: Oscillator for multiple pic's'
2001\02\17@145407 by Aart

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Hello,

is there a simple way to clock  4 pic's (16f84)  with one Xtal?

Or do i need to build an external oscillator?  The one at page 40 from the datasheet (fig 8-6) does'nt work, well at least not with a 74hct04 or an 7406 Hex inverter. The inverter mentioned in the sheet (74AS04) i do not have at the moment.

I want to make this one or is there a simpler solution?

http://www.radiomods.co.nz/xtalosc.html    
It might be interfering with other oscillaters, i'm using it in a transceiver.

Thanks,
Aart.

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2001\02\17@153940 by David VanHorn

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At 08:52 PM 2/17/01 +0100, Aart wrote:
>Hello,
>
>is there a simple way to clock  4 pic's (16f84)  with one Xtal?
>
>Or do i need to build an external oscillator?  The one at page 40 from the
>datasheet (fig 8-6) does'nt work, well at least not with a 74hct04 or an
>7406 Hex inverter. The inverter mentioned in the sheet (74AS04) i do not
>have at the moment.

There's a three-inverter circuit that's pretty simple, or try 74HCU04s.

Beware! You'll want series resistors in the clock lead to each pic, and a
short, solid ground return from each pic to the clock circuit.
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2001\02\17@164557 by jhancock

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I'm new at PICs, but why not use a crystal clock oscillator (a complete
oscillator in a 4 pin can for 14 pin DIP socket ready to plug in & go)?  RS
shows frequencies including 4, 8, 10 MHz.   You can order them from Radio
Shack, Jameco, Newark or other suppliers (RS 2001 catalog price <$4.00, pg
264).

Jess

{Original Message removed}

2001\02\17@172627 by jhancock

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What value of resistor should be used in the clock leads?  Is this for
isolation of the pic clocks?

Jess

{Original Message removed}

2001\02\17@173036 by Jim Paul

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This is perfectly acceptable.  And it's the way I clock most of the PIC's I
work with.
The main reason for using a crystal is cost.  I have found that most of the
crystals
I get are less expensive than the clock oscillators.  There are a few
exceptions,
but not many.


Regards,


Jim
{Original Message removed}

2001\02\17@173658 by David VanHorn

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At 05:28 PM 2/17/01 -0500, jhancock wrote:
>What value of resistor should be used in the clock leads?  Is this for
>isolation of the pic clocks?

No, there's no need to isolate them. It's for EMI control.

You're talking about feeding a track a signal with edges that have energy
up through 500 MHz.
Adding some fixed resistance on the source end cuts down on the EMI quite a
bit, by slowing down the risetimes.
Check the PIC spec, but I know you don't need that fast a rise time. Stands
to reason, if it can take a crystal input, which is nearly a sine, then it
can probably take a pretty much slowed down square wave.

The resistors should all be on the driving end of the tracks, not the pic end.
As to value, if you have a scope, then as much as you can take.
If no scope, start out at about 100 ohms, and see what that does for you.


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2001\02\17@180231 by Chris Carr

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> At 05:28 PM 2/17/01 -0500, jhancock wrote:
> >What value of resistor should be used in the clock leads?  Is this for
> >isolation of the pic clocks?
>
> No, there's no need to isolate them. It's for EMI control.
>
> You're talking about feeding a track a signal with edges that have energy
> up through 500 MHz.
> Adding some fixed resistance on the source end cuts down on the EMI quite
a
> bit, by slowing down the risetimes.
> Check the PIC spec, but I know you don't need that fast a rise time.
Stands
> to reason, if it can take a crystal input, which is nearly a sine, then it
> can probably take a pretty much slowed down square wave.
>
> The resistors should all be on the driving end of the tracks, not the pic
end.
> As to value, if you have a scope, then as much as you can take.
> If no scope, start out at about 100 ohms, and see what that does for you.
>
I have not tried this with a PIC so please don't flame me if you
try it and it doesn't work or if you are one of the Guru's and I am
missing something really obvious.

Why not configure one PIC as a xtal osc and connect a gate to the oscillator
output to act as a buffer for distribution of the clock signal to the other
PIC's

By the way you may care to look at

http://www.microchip.com/download/appnote/category/articles/fact001.pdf

which is entitled "Basic PIC16/17 Oscillator Design" on the last page (4)
there is a note on External Clocks, and quote

"For really power stingy applications, with high speed external clocks
approaching 20MHz, the device will draw less power if the clock is injected
at the osc-out pin ............."

Regards

Chris

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2001\02\17@182014 by David VanHorn

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>
>Why not configure one PIC as a xtal osc and connect a gate to the oscillator
>output to act as a buffer for distribution of the clock signal to the other
>PIC's


What's it say about output drive specs?
A friend of mine got into deep trouble this way. All 10 of his protos
worked, but half the production line didn't..
Made for a Christmas day flight to Taipei for me..


>"For really power stingy applications, with high speed external clocks
>approaching 20MHz, the device will draw less power if the clock is injected
>at the osc-out pin ............."

Ewwww.
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2001\02\17@193915 by Olin Lathrop

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> Why not configure one PIC as a xtal osc and connect a gate to the
oscillator
> output to act as a buffer for distribution of the clock signal to the
other
> PIC's

With the right gate this should work.  What makes me uneasy is that I don't
remember that the voltage levels at the oscillator output pin are specified.
(Maybe they are, I haven't looked).  Without having a guaranteed min level
for high and max level for low, how do you know the buffer gate will work?
I don't think OSC OUT is driven as a normal digital output in XT or HS mode.

I think doing this would be fine for a one-off personal project after
inspecting the signal with a high impedence scope probe.  However, I
wouldn't want to go into production without solid specs.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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2001\02\17@194931 by David VanHorn

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>
>I think doing this would be fine for a one-off personal project after
>inspecting the signal with a high impedence scope probe.  However, I
>wouldn't want to go into production without solid specs.

That's what I said, but they did it anyway..

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