Introduction to LED Schematic Capture

Led Basics: How to Create an Inverter Schematic

Introduction

Led is the front end schematic entry and layout editing tool we will be using for the VLSI class. In this tutorial you will create the schematic for a CMOS inverter. Later, we will use Led for creating layouts; the editing commands used to create either a schematic or layout are mostly the same.

Setup

Before running Led, you must add the following statement to your .cshrc file:

 swsetup ecad:meta:scs:mgc

 Led -t tech  -l tech_dir  & 

 Led -t scn06hp -l /ecad/local/tech/rel/gdt &

alias Led_tech "Led -t \!\!:1 -l /ecad/local/tech/rel/gdt &"

 Led_tech scn06hp

Before starting Led, make a directory for your work and change to that directory.

Led Exit, Interrupt, Menus

• Once Led is started, you can use CTRL-C to interrupt any current command it is executing.
• To exit Led:
  • Position the cursor over the window background (the grey area)
  • Press the right button on the mouse to display the background Menu
  • Select the Exit Led option
  • If the background Menu does not display, then you might be in the middle of a command in Led. If the menu that appears has a cancel entry, select cancel to discontinue the command. If there is no cancel entry, then press CTRL-C to cancel the command.
• There are two main menus in Led: Background Menu and Led Edit Menu
  • The background menu enables you to:
    • Add and Manipulate windows
    • Change cursor
    • Open icons
    • Print
    • Exit
  • The Led Edit Window Menu enables you to access all of the layout and schematic commands
• To display the background Menu:
  • Move the cursor to the background area
  • Press and hold the right mouse button to display the Background Menu
• To display the Led Edit Menu
  • Move the cursor into an Led window
  • Press and hold the right mouse button to display the Led Edit Menu

Creating the Inverter

1. The first step is create a new schematic cell
   • Select Cells from the Led Edit Window Menu
   • Select NEW SCHEMATIC CELL from the cells menu to display the NEW Cell form
   • In the text field, enter inva. This will be the name of the schematic.
   • Select the ok button

   Once you create the inverter cell, you can add circuit elements like resistors, transistors, etc. and alter their properties.
   

2. To Add an NMOS transistor
   • Left click on a point in the schematic where you want the transistor added.
   • Use the 'v' hotkey to popup the transistor list menu (Add->'Add transistor' menu choice).
   • Select the NMOS transistor choice which is an enhancement N-type transistor. You can also select a PMOS enhancement (P-type) transistor.
   • The transistor will now appear in your schematic and will be 'selected' as indicated by the blinking 'X'.
   • To move the transistor, hit the 'w' hotkey and then move the cursor to the new location. Left-click to place the transistor.
   
   Zooming, Panning, Screen Refresh
   • The '2' hotkey zooms in 2X.
   • The '4' hotkey zooms out 2X.
   • The '3' hotkey zooms to a region. After hitting '3', do left-click-hold and drag out a rectangular selection box to zoom to this area.
   • The '1' hotkey pans to a selected point. Hit '1', then click on the spot you wish to pan to.
   • Led does not automatically refresh the screen after editing operations. Use the 'tab' key to refresh the screen.
   Object Selection
   • Most editing commands work on the currently selected object or group of objects.
   • To select an object or group of objects, left-click-hold and drag to form a rectangular selection box. Release the left mouse button to pop up the 'Find Objects Menu'
   • All objects in the rectangular selection region will be listed in the 'Find Objects Menu'. Clicking on one of the objects will select that object. Clicking on '+' will select all of the objects in the menu; clicking on '-' will deselect all of the objects.
   NMOS Transistor Selection
   • Using the object selection commands above, select everything within the region around the NMOS transistor.
   • The 'Find Objects Menu' should list four items. Three of the items are the drain, source, and gate terminals of the NMOS transistor, the other item ('NMOS nmos0 W=0.9 L=0.6') is the nmos transistor itself. Some editing commands require that you select you entire transistor; other commands such as adding wires (nets) require that you just select a terminal.
   • Note that the transistor has a 'dot' near one of the source/drain terminals. The 'dot' marks the drain terminal of the transistor.
   Other Editing Commands, (delete, duplicate, rotation, label)
   • Most of these commands require that the object be selected.
   • To Duplicate an element, select the element and then use the 'd' hotkey (Add-> 'Duplicate element' menu choice)
   • If you want to move the transistor after it is placed, select the transistor and use the 'w' hotkey to move it (Alter->Change.location)
   • If you wish to delete an element, select it, and delete with the 'q' hotkey (Alter->'Delete element' menu choice)
   • An element can be rotated via the 'Alter->'Ch.rotation' menu choice.
   • An element can be reflected about the X-axis via the 'Alter->'Ch. X reflection' menu choice.
   • An element can be reflected about the Y-axis via the 'Alter->'Ch. Y reflection' menu choice.
   • The default name for the first NMOS transistor is 'n0'. To change the name, use the 'n' hotkey (Alter->Ch. name).
   • The Width/Length of the transistor can altered via the 'i' (Alter->Ch. size) command.
   
   Writing our your design, Reading a designs
   • It is a good idea to save your work periodically.
   • To save your design, use the 'W' hotkey (Files->Write cell(s)) command. Note that this is a CAPITAL 'W', hotkey commands in Led are case sensitive.
   • To read in a previous design, use the 'R' hotkey (Files->Read an L file).
   
   Naming Elements, Making Names visible
   • The default name for the first NMOS transistor is 'n0'. To change the name, use the 'n' hotkey (Alter->Ch. name).
   • By default, the name of an element is not plotted. To make the name of element visible, the element must be selected and the 'VIEWNAME' property added.
   • Select the NMOS transistor, and use the 't' hotkey (Add-> Property) command to bring up the 'Properties' menu.
   • Select the 'VIEWNAME' property to make the element name visible. Hit the 'tab' key to refresh the display, you should now see the element name.
   • To change the font size for the element name, you need to select the 'VIEWNAME' property for the element. It will look something like 'VIEWNAME for nmos0 ...' in the 'Find Objects' selection menu. After the VIEWNAME property is selected, the 'i' hotkey (Alter-> Ch. size) can be used to change the font size.
   
   
3. Add a PMOS transistor
   

   Use the 'v' hotkey to add a PMOS transistor. Reflect the PMOS transistor about the X axis so that the drain of the PMOS transistor is pointing down; move the two transistors so that the drain terminals are aligned but leave a gap so that a wire can be added to connect the two terminals.

   
   

4. Add a wire to connect the drain terminal of the PMOS to the drain terminal of NMOS
   • Click left and drag a small box around the drain of the NMOS transistor. If your box encloses other objects, then the Find Objects Form pops up.
   • Select nmos0.d from the Find Objects Form. The blinking cursor (x) appears attached to the selected object.
   • Next, add a wire via the 'a' hotkey (Add->Add Wire menu choice). A 'wire' should appear whose end is connected to the nmos.d terminal; the wire will stretch as the cursor is moved.
   • To connect the wire to the PMOS drain terminal, move the cursor to the PMOS drain terminal, then click left and drag a small box around the PMOS drain terminal to select it. Make sure the box is small so that only the PMOS drain terminal is selected.
   • The wire should now be connected between the NMOS drain and the PMOS drain terminals.
   • You can verify this by selecting the region around the wire including the terminals; the 'Find Objects' box should include a 'wire' element that looks like: 'Wire nmos0.d to pmos0.d'.
   
   
5. Add a wire which connects the gate terminals of the two transistors.
   • Use the procedure above to connect the nmos0.g terminal to the pmos0.g terminal.
   
   
6. Add power terminals
   • To Add a Vdd terminal
     • Left click at a point above the PMOS source terminal to place the Vdd terminal
     • Use the 'x' hotkey to bring up the Terminal menu list (Add->Add Terminals menu choice)
     • Select VDD from the terminals list
     • Add wire between the Vdd and the resistor
   • To Add a GND terminal, use the same procedure as above except select a 'GND' terminal from the Terminals menu choice. The 'GND' terminal should be connected to the source terminal of the NMOS transistor.
   
   
7. Add IN/OUT Terminals
   • Use the procedure above to add a terminal of type 'IN' to the schematic. This terminal should be connected the wire which connects the two gate terminals. The gate terminals will already have a wire connecting them; when drawing the wire between the IN terminal and the gate wire you can select a small portion of the gate wire and this will create a 'node' connection point in the wire.
   • Use the procedure above to add a terminal of type 'OUT' to the schematic. This terminal should be connected the wire which connects the two drain terminals.
   
   
8. Change the name of the IN terminal to 'A', OUT terminal to 'Y'
   • Select the terminal, and use the 'n' hotkey (Alter->Ch.name) to change the name to 'A'.
   • Make the name visible by adding a VIEWNAME property to the 'A' terminal. Change the size of the VIEWNAME property so that the terminal name is easily visible.
   • Follow the same procedure to change the OUT terminal name to 'Y'.
   
   
9. Document your Schematic
   • Place the cursor where the text needs to be added
   • Select text from the 'Add->Properties' ('t' hotkey) menu
   • A window will pop up. Enter the text in the window.
   • Select ok
   
   
10. To plot your schematic
    • Use the 'Print->Postscript object' menu choice from the Led Edit Window to print out your file to either a printer or file. The default filename choice is '/tmp/printfile'.