PSpice Capture v9.2 74121 simulation problem

Graham Macpherson

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Nov 22, 2001, 5:23:04 PM11/22/01

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Hello,

I'm pretty new to PSpice and I am trying to simulate a 74121 non
retriggerable monostable.

I have the datasheet form the real device...

http://focus.ti.com/docs/logic/catalog/products/productlist.jhtml?familyId=3
02&techFamId=22&tfsection=product_list

...and I am trying to apply an external timing capacitor and resistor to
control the pulse width. The problem is no matter what I apply as C and R
then I get the minimum pulse width of app 30ns after the clock edge.

The first problem I had was that I got the green DRC washers on the
Rext/Cext, Cext, and Rint pins saying that they were unmodelled so I changed
the pin FLOAT properties for them to Uniquenet. This did not work.

I have connected R between Rext/Cext and Vcc and C between Cext and
Rext/Cext

Can anyone suggest what the problem is?

Thanks in advance

Graham

--
**********************************************
Graham Macpherson
University of Strathclyde
2nd Year Electrical and Mechanical Engineering
graham.m...@strath.ac.uk
**********************************************

Graham Macpherson

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Nov 22, 2001, 6:00:38 PM11/22/01

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"Graham Macpherson" <gbmacp...@btclick.com> wrote in message
news:cYeL7.10072$JZ3.109222@NewsReader...

> Hello,
>
> I'm pretty new to PSpice and I am trying to simulate a 74121 non
> retriggerable monostable.
>
> I have the datasheet form the real device...
>
>
http://focus.ti.com/docs/logic/catalog/products/productlist.jhtml?familyId=3
> 02&techFamId=22&tfsection=product_list
>
> ...and I am trying to apply an external timing capacitor and resistor to
> control the pulse width. The problem is no matter what I apply as C and R
> then I get the minimum pulse width of app 30ns after the clock edge.
>
> The first problem I had was that I got the green DRC washers on the
> Rext/Cext, Cext, and Rint pins saying that they were unmodelled so I
changed
> the pin FLOAT properties for them to Uniquenet. This did not work.
>
> I have connected R between Rext/Cext and Vcc and C between Cext and
> Rext/Cext
>
> Can anyone suggest what the problem is?
>
> Thanks in advance
>
> Graham

CUH! Just when I finally decide to post a question (after *hours* of
struggling) I spot the small "Pulse" property in Capture for the 74121 - I
can get the width of pulse I want from this but does that mean that I can't
use the external components to control it? - Seems to defeat the purpose of
simulating it as a distinct chip if so.

I think I still as confused.

Graham

Jim Thompson

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Nov 23, 2001, 10:48:17 AM11/23/01

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On Thu, 22 Nov 2001 22:23:04 -0000, "Graham Macpherson"
<gbmacp...@btclick.com> wrote:

|Hello,
|
|I'm pretty new to PSpice and I am trying to simulate a 74121 non
|retriggerable monostable.
|
|I have the datasheet form the real device...
|
|http://focus.ti.com/docs/logic/catalog/products/productlist.jhtml?familyId=3
|02&techFamId=22&tfsection=product_list
|
|...and I am trying to apply an external timing capacitor and resistor to
|control the pulse width. The problem is no matter what I apply as C and R
|then I get the minimum pulse width of app 30ns after the clock edge.
|
|The first problem I had was that I got the green DRC washers on the
|Rext/Cext, Cext, and Rint pins saying that they were unmodelled so I changed
|the pin FLOAT properties for them to Uniquenet. This did not work.
|
|I have connected R between Rext/Cext and Vcc and C between Cext and
|Rext/Cext
|
|Can anyone suggest what the problem is?
|
|Thanks in advance
|
|Graham

Post the error message you are getting, as well as the model
(subcircuit) of the 74121.

(If replying by E-mail please observe obscure method of anti-spam.)

For proper E-mail replies SWAP "-" and "_".

"Things turn out best for those who make the best of how things turn out."

Malcolm Reeves

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Nov 23, 2001, 12:02:59 PM11/23/01

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On Thu, 22 Nov 2001 23:00:38 -0000, "Graham Macpherson"
<gbmacp...@btclick.com> wrote:

>CUH! Just when I finally decide to post a question (after *hours* of
>struggling) I spot the small "Pulse" property in Capture for the 74121 - I
>can get the width of pulse I want from this but does that mean that I can't
>use the external components to control it? - Seems to defeat the purpose of
>simulating it as a distinct chip if so.

The reason for this is that if you are simulating digital (on the
whole), analogue will just slow you down. So simulating a monostable
as a digital delay is faster. Also if you want to know the limits of
the pulse period then you can't beat getting out your calculator and
using the data book equations - far faster, and probably more accurate
to boot :-).

...malcolm

--
Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
(mre...@fullcircuit.com, mre...@fullcircuit.co.uk or mre...@iee.org).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - VHDL test bench tool

Graham Macpherson

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Nov 23, 2001, 6:30:08 PM11/23/01

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"Jim Thompson" <Jim-T@analog_innovations.com> wrote in message
news:bfqsvtk5ac868pj1q...@4ax.com...

Model? The "PSpice Template" is:

X^@REFDES %A1 %A2 %B %RINT %CEXT %REXT/CEXT %Q %Q\ %VCC %GND @MODEL
PARAMS:\n+ PULSE=@PULSE IO_LEVEL=@IO_LEVEL MNTYMXDLY=@MNTYMXDLY

and the error message I was getting before I changed the pin status was:

"WARNING: [NET0079]
Unmodeled pin shouldn't be in template
U3 pin 'CEXT' "

Thanks for your help.

Graham

Jim Thompson

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Nov 26, 2001, 2:00:11 PM11/26/01

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On Fri, 23 Nov 2001 23:30:08 -0000, "Graham Macpherson"
<gbmacp...@btclick.com> wrote:

[snip]

Couple of questions:

1) If a pin name is "Qbar" the correct notation is "\Q\"
2) "%REXT/CEXT" puzzles me, the marker for continuing text is "\", so
it appears that PSpice may be balking at an improper pin name.

Sheeeesh! I just pulled up the part (1_shot.lib) library and found
the problem, it is some *SCREWY* digital model (Charlie, Mohi, Brian,
please comment):

* Notes:
* 1. The RINT, CEXT, and REXT/CEXT pins are not functional.
^^^^^^^^^^^^^^^^^^^^^^^
* The output pulse width, tw(out), is controlled with the PULSE
* subcircuit parameter. Note that this means that the pulse width is
* FIXED for the duration of the simulation. You can specify this
* value in the subcircuit call, e.g. X1 ... 74121 PARAMS: PULSE=1us
* 2. Instead of a fixed minimum input pulse width (50ns), this model
* requires the input pulse to be at least as long as the propagation
* delay through the device. Input pulses which are shorter than this
* value produce an X which is tw(out) in duration.

My advice would be to study up on behavioral modeling and spin your
own model :-(

(If replying by E-mail please observe obscure method of anti-spam.)

...Jim Thompson

Charles Edmondson

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Nov 26, 2001, 2:34:48 PM11/26/01

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Hi Jim,
Malcolm already answered, but here it is again.

Most of the digital one-shots were designed to be completely DIGITAL parts.
As such, they don't have an analog component to model the external R and C
components.

If someone want to take on the task of CORRECTLY modeling that oscillator
using the external R and C, more power to them! It is definitely one of the
non-trivial tasks!

For know, you connect the R's and C's appropriately, and then set the pulse
width you want.

Charlie
Edmondson Engineering
Unique Solutions to Unusual Problems

Graham Macpherson

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Nov 27, 2001, 6:29:17 PM11/27/01

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My thanks to all three of you.

I will leave building my own model for the moment - it sounds a bit beyond
the time and skill of a harassed undergraduate :o)

Graham

Jim Thompson

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Nov 27, 2001, 7:39:40 PM11/27/01

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On Tue, 27 Nov 2001 23:29:17 -0000, "Graham Macpherson"
<gbmacp...@btclick.com> wrote:

|My thanks to all three of you.

You are quite welcome!

|
|I will leave building my own model for the moment - it sounds a bit beyond
|the time and skill of a harassed undergraduate :o)
|
|Graham
|

Modeling is nasty even for us old farts ;-)