PIC18F452 Schematic/Layout Model

[skeatz]

I've tried creating a custom model for the PIC18F452 (for schematic & layout only), since some others are interested in this MCU. The pin arrangement follows that in one of Microchip's application note. I've attached the multisim circuit for those who might want it.

I've 2 questions regarding the symbol library:
- is it possible to have an 'overscore' over the pin name to indicate active low input?
  Multisim does it for the PIC16F84, but I am unable to do it in my custom component
- the PIC18F452 has 2 x Vss and 2 x Vdd pins. Multisim does not allow me to use duplicate names for the pins.
  Is there any work-around, other than using different names, e.g. Vss & Vss1, Vdd & Vdd1 ?

Thanks.

[Biscom]

Hi,

I bought MultiSim 2001 Power Pro a while ago. I don't know how much the system has changed.

Using this older system, I have just made a quick component called, "fred", to show that it is possible to easily insert an overscore.

When you construct or copy the circuit layout in the Symbol Editor, one can draw a line above the text as shown.

When one saves the contents of the Symbol Editor the overscore line is also saved, and become a part of the component as shown.

There are other ways in which you could indicate active low inputs.

Also in the Symbol Editor one can select different pin shapes. One needs to click on the text next to "Shape" to reveal the drop down selection, as shown. (A bit of advice: Don't use Zero Length, it becomes very hard to find the pin, especially if one moves the pins around.)

One can also hide the text of the "Logical Pin" and "Physical Pin" (by clicking on Visible/Hidden which reveals a box to click in) and place ones own text in its position. This can take a long time. I find it easier to define/build the component first, then edit the component, hiding the "Logical Pin" and "Physical Pin" and replacing with my own text.

I hope this is helpful.

[skeatz]

Hi Biscom,

Thanks for your reply. I've actually considered & tried those methods before, but found them unsuitable (for me). My reasons are as follows:
- using active low pin symbol: some MCU pins are multi-functional, as both active high & active low (e.g. ~MCLR/Vpp/RE3)
- drawing 'overscore' as line: may have many such signals & is tedious to align. I was hoping that there would be an easier approach
- replacing with own text: symbols will be used by many others. Seeing a different pin name (from the one shown in the graphic) may confuse some of these users.

Partial pinout for the PIC18F4520 attached to show what I mean.

rgds,

[Olaf van den Berg]

Hye skaetz,

An overscore is placed in the symbol editor by putting ^ signs around the part you want to overscore. So if you want the pinname CLK_EN to be completely overscored, name it: ^CLK_EN^.

Multifunction pin example:  ^CLK_EN^/IOA/^IOB^

Here the CLK_EN and the IOB are overscored.

As for the duplicate pinnames, there is no solution to my knowledge.

I usually do the following:

In your symbol editor in multisim create just one pin for the VDD (so your schematic symbol has only one VDD pin), and assign multiple footprint pins to it. (so the VDD pin in your schematic is linked to pin 1 and 5 and 12 etc. etc.)

Hope this helps,

Kind regards,

Olaf

[nestor-sg]

That is correct Olaf, if you look in the Help File while in the Symbol Editor, search for "negation bar"... it will explain it there. Basically if you want to negate the whole name, just put one ^ at the beginning or at the end of the name. If you want to negate just one section of the name enclose it between ^section^ and it will negate only that part.

[skeatz]

Thanks, Olaf & Nestor. I've tried it out & it works perfectly. I'm still learning to use the software & there are not many experts that I can consult where I'm at, so I find the forum very helpful.

best rgds,
Steven

[nestor-sg]

I thought it was a good idea to create a support document about it, so now you can also look for KnowledgeBase 4JRGFBXL: Placing a Negation Bar in a Symbol Pin Name. In this way it will be available for other users with the same question.

[$shanthi]

Hi

   I need a help about How to place overline for net names to indicate active low signal?