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'[PIC]: Using PIC oscillator output to drive divide'
2000\11\17@125421 by D Lloyd

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Hi,

I would like to use the PIC oscillator pin (OSC1/2) to drive an external
divider as described by the bad ASCII art below

eg

g=gnd, x=Xtal, inv=internal inverter for HS/XT mode. There is a resistor
(or two) missing, potentially.

                              | PIC internal
g----| |---------------|-----   osc1
                |             |      |
               X            |    INV
                |             |      |
g----| |---------------|-----   osc2
                     |        |
                     |        |_______________
                     |
                     |____| 74HC4040N clk-->QE|_________|Seiko 7600 CLK

This is similar to the diagram given in the PIC application notes on the
page for crysal operation/capactor selection etc

Basically, I will be using the crystal to give a nice baud rate multiple
but I need to divide this to drive the clock of a Seiko 7600A.

What I need to know is will I be able to drive this CMOS divider direct
from the PIC oscillator pins or do I need to do some buffering inbetween? I
would prefer to not have the xtal frequency affected by this being
loaded......

I had thought about making an external clock generator (using an unbuffered
inverter and the clock crystal config shown) but why bother when that
effectively is the same as what is on the PIC?

Any help/suggestions, please?

Best regards,

Dan

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2000\11\17@131018 by Andrew Kunz

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Dan,

That's the same question as, "Can I run two PICs (CMOS devices) off the same
xtal" that comes up all the time.

Yes.  If you are using an xtal, take into account that the second ("slave")
device has some capacitive input which must be subtracted from the capacitor
normally on the OSC2 pin.  (Yes, use OSC2 to drive the slave chip).

If your slave device has an OSC2 pin, you can connect it to a third device, etc.

You might want to allow for some resistors to prevent overdriving the next
stage, too.

Andy










D Lloyd <.....dan.lloydKILLspamspam@spam@GB.ABB.COM> on 11/17/2000 08:25:31 AM

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cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: [PIC]: Using PIC oscillator output to drive
         divider?








Hi,

I would like to use the PIC oscillator pin (OSC1/2) to drive an external
divider as described by the bad ASCII art below

eg

g=gnd, x=Xtal, inv=internal inverter for HS/XT mode. There is a resistor
(or two) missing, potentially.

                              | PIC internal
g----| |---------------|-----   osc1
                |             |      |
               X            |    INV
                |             |      |
g----| |---------------|-----   osc2
                     |        |
                     |        |_______________
                     |
                     |____| 74HC4040N clk-->QE|_________|Seiko 7600 CLK

This is similar to the diagram given in the PIC application notes on the
page for crysal operation/capactor selection etc

Basically, I will be using the crystal to give a nice baud rate multiple
but I need to divide this to drive the clock of a Seiko 7600A.

What I need to know is will I be able to drive this CMOS divider direct
from the PIC oscillator pins or do I need to do some buffering inbetween? I
would prefer to not have the xtal frequency affected by this being
loaded......

I had thought about making an external clock generator (using an unbuffered
inverter and the clock crystal config shown) but why bother when that
effectively is the same as what is on the PIC?

Any help/suggestions, please?

Best regards,

Dan

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2000\11\17@133130 by Bob Blick

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You should have no problems. Andy already mentioned subtracting the ~3 to
10 pF load of the HC4040 from your load capacitor.

I'd also like to add the maximum frequency limit of the HC4040 is around
20 MHz, they get real flaky real fast when you hit their limit.

Cheers,

Bob

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2000\11\17@145655 by David VanHorn

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>You might want to allow for some resistors to prevent overdriving the next
>stage, too.


And an application for an international broadcasting licence, unless you
pay careful attention to ground. A short, low Z path between the chips will
be essential.
I'd put series Rs in the clock lines, about 100 ohms, but not becuase of
any "overdrive" mythology.. (how do you propose to "overdrive" a cmos
input, with a signal that runs from ground to VCC?)

The series R changes the waveform to trapezoidal, and keeps EMI down.
Hard edges will get you hard EMI problems.

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2000\11\17@150318 by David VanHorn

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At 06:30 PM 11/17/00 +0000, Bob Blick wrote:
>You should have no problems. Andy already mentioned subtracting the ~3 to
>10 pF load of the HC4040 from your load capacitor.
>
>I'd also like to add the maximum frequency limit of the HC4040 is around
>20 MHz, they get real flaky real fast when you hit their limit.

Can you spell "non-deterministic"? :)

We hit this using a 4040 in the bit clock generator for a thermal dot printer.
Had to back it down a bit.

The CD4040 was hilarous.
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2000\11\17@152012 by Andrew Kunz

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I stand corrected.

I also noticed that the OSC2 pin outputs a sine wave.  Aren't those better than
trapezoids for preventing harmonics?

Andy









David VanHorn <spamBeGonedvanhornspamBeGonespamCEDAR.NET> on 11/17/2000 02:56:23 PM

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Subject: Re: [PIC]: Using PIC oscillator output to drive
         divider?








>You might want to allow for some resistors to prevent overdriving the next
>stage, too.


And an application for an international broadcasting licence, unless you
pay careful attention to ground. A short, low Z path between the chips will
be essential.
I'd put series Rs in the clock lines, about 100 ohms, but not becuase of
any "overdrive" mythology.. (how do you propose to "overdrive" a cmos
input, with a signal that runs from ground to VCC?)

The series R changes the waveform to trapezoidal, and keeps EMI down.
Hard edges will get you hard EMI problems.

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2000\11\17@153858 by David VanHorn

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At 03:22 PM 11/17/00 -0500, Andrew Kunz wrote:
>I stand corrected.
>
>I also noticed that the OSC2 pin outputs a sine wave.  Aren't those better
than
>trapezoids for preventing harmonics?

It's "choose your poison".  Sines have lots of even harmonics, squares have
lots of odds. People often think that a sine dosen't cause EMI because it
dosen't have hard edges. You can't get away from it completely, but you can
control it.

The R serves two functions, toning down the edges, and also providing a
terminating impedance for the track.  The track will be somewhere in around
100 ohms or so. (very dependent on layout)  But, a 100 ohm resistor will be
a lot closer, and support less ringing, than an open circuit (idealized
gate) or short circuit (idealized driver)

You can trim up the resistor values on the production boards, to get the
best match.

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2000\11\17@160924 by Sean Breheny

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Hi Dave,

I'm sure you know that a pure sine wave would have no harmonics, so it
seems to me that you would have to know how it was being distorted in
order to know what harmonics are present. I suppose you could make the
argument that many types of nonlinearities tend to have the effect of
applying an x^2 function to the waveform, thereby introducing mainly 2nd
harmonics, but I wonder how generally that applies.

Sean


On Fri, 17 Nov 2000, David VanHorn wrote:
> It's "choose your poison".  Sines have lots of even harmonics, squares have
> lots of odds. People often think that a sine dosen't cause EMI because it
> dosen't have hard edges. You can't get away from it completely, but you can
> control it.
>

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2000\11\17@170955 by Olin Lathrop

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> ... Sines have lots of even harmonics,

A sine (or cosine) is a "pure" frequency which contains no harmonics by
definition.


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2000\11\17@175115 by David VanHorn

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At 04:58 PM 11/17/00 -0500, Olin Lathrop wrote:
>> ... Sines have lots of even harmonics,
>
>A sine (or cosine) is a "pure" frequency which contains no harmonics by
>definition.


True, but if you've got a pic that outputs a pure sine, let me know, I want
to buy stock.

The osc drive circuits will give a distorted sine, that contains even
harmonics.

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2000\11\20@035008 by D Lloyd

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part 1 1062 bytes content-type:text/plain; charset=us-ascii
Hi,

Thanks to everyone who responded to this.

Regards,

Dan




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At 04:58 PM 11/17/00 -0500, Olin Lathrop wrote:
>> ... Sines have lots of even harmonics,
>
>A sine (or cosine) is a "pure" frequency which contains no harmonics by
>definition.


True, but if you've got a pic that outputs a pure sine, let me know, I want
to buy stock.

The osc drive circuits will give a distorted sine, that contains even
harmonics.

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