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Caltech Intermediate Format (CIF) is a recent form for the description of integrated circuits. Created by the university community, CIF has provided a common database structure for the integration of many research tools. CIF provides a limited set of graphics primitives that are useful for describing the two-dimensional shapes on the different layers of a chip. The format allows hierarchical description, which makes the representation concise. In addition, it is a terse but human-readable text format. CIF is therefore a concise and powerful descriptive form for VLSI geometry.
Each statement in CIF consists of a keyword or letter followed by parameters and terminated with a semicolon. Spaces must separate the parameters but there are no restrictions on the number of statements per line or of the particular columns of any field. Comments can be inserted anywhere by enclosing them in parenthesis.
There are only a few CIF statements and they fall into one of
two categories: geometry or control.
The geometry statements are: LAYER to switch mask layers,
BOX to draw
a rectangle, WIRE to draw a path, ROUNDFLASH to draw a circle,
POLYGON to draw an arbitrary figure, and CALL to draw a
subroutine of other geometry statements.
The control statements are DS to start the definition of a subroutine,
DF to finish the definition of a subroutine, DD to delete
the definition of subroutines, 0 through 9 to include additional
user-specified information, and END to terminate a CIF file.
All of these keywords are usually abbreviated to one or two letters that are
unique.
The LAYER statement (or the letter L) sets the mask layer
to be used for all subsequent geometry until the next such statement.
Following the LAYER keyword comes a single layer-name parameter.
For example, the command:
L NC;
sets the layer to be the nMOS contact cut (see Fig. B.1 for some typical
MOS layer names).
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| FIGURE B.1 CIF layer names for MOS processes. | ||||||||||||||||||||||||||||||||||
The BOX statement (or the letter B) is the most commonly used
way of specifying geometry.
It describes a rectangle by giving its length, width, center position,
and an optional rotation.
The format is as follows:
B length width xpos ypos [rotation] ;
Without the rotation field, the four numbers specify a box the
center of which is at (xpos, ypos) and is length across
in x and width tall in y.
All numbers in CIF are integers that refer to centimicrons of distance,
unless subroutine scaling is specified (described later).
The optional rotation field contains two
numbers that define a vector endpoint starting at the origin.
The default value of this field is (1, 0), which is a right-pointing vector.
Thus the rotation clause 10 5 defines a 30-degree counterclockwise
rotation from the normal.
Similarly, 10 -10 will rotate clockwise by 45 degrees.
Note that the magnitude of this rotation vector has no meaning.
The WIRE statement (or the letter W) is used to construct a
path that runs between a set of points.
The path can have a nonzero width and has rounded corners.
After the WIRE keyword comes the width value and then an arbitrary number
of coordinate pairs that describe the endpoints.
Figure B.2 shows a sample wire.
Note that the endpoint and corner rounding are implicitly handled.
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The ROUNDFLASH statement (or the letter R) draws a
filled circle, given the diameter and the center coordinate.
For example, the statement:
R 20 30 40;
will draw a circle that has a radius of 10 (diameter of 20),
centered at (30, 40).
The POLYGON statement (or the letter P) takes a series of
coordinate pairs and draws a filled polygon from them.
Since filled polygons must be closed, the first and last coordinate points
are implicitly connected and need not be the same.
Polygons can be arbitrarily complex, including concavity and self-intersection.
Figure B.3 illustrates a polygon statement.
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The CALL statement (or the letter C) invokes a collection of
other statements that have been packaged with DS and
DF.
All subroutines are given numbers when they are defined and these numbers
are used in the CALL to identify them.
If, for example, a LAYER statement and a BOX statement are
packaged into subroutine 4, then the statement:
C 4;
will cause the box to be drawn on that layer.
In addition to simply invoking the subroutine, a CALL statement can
include transformations to affect the geometry inside the subroutine.
Three transformations can be applied to a subroutine in CIF: translation,
rotation, and mirroring.
Translation is specified as the letter T followed by an x, y
offset.
These offsets will be added to all coordinates in the subroutine,
to translate its graphics across the mask.
Rotation is specified as the letter R followed by an x, y
vector
endpoint that, much like the rotation clause in the BOX statement,
defines a line to the origin.
The unrotated line has the endpoint (1, 0), which points to the right.
Mirroring is available in two forms: MX to mirror in x
and MY to mirror in y.
Mirroring is a bit confusing, because MX causes a negation of
the x coordinate, which effectively mirrors about the y axis!
| Any number of transformations can be applied to an object and their listed order is the sequence that will be used to apply them. Figure B.4 shows some examples, illustrating the importance of ordering the transformations (notice that Figs. B.4c and B.4d produce different results by rearranging the transformations). |
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Defining subroutines for use in a CALL statement is quite simple.
The statements to be packaged are enclosed between
DS (definition start) and DF (definition finish) statements.
Arguments to the DS statement are the subroutine number and a subroutine
scaling factor.
There are no arguments to the DF statement.
The scaling factor for a subroutine consists of a numerator followed by a
denominator that will be applied to all values inside the subroutine.
This scaling allows large numbers to be expressed with fewer digits
and allows ease of rescaling a design.
The scale factor cannot be changed for each invocation of the
subroutine since it is applied to the definition.
As an example, the subroutine of Fig. B.4 can be described formally
as follows:
DS 10 20 2;
B10 20 5 5;
W1 5 5 10 15;
DF;
Note that the scale factor is 20/2, which allows the trailing zero to be
dropped from all values inside the subroutine.
Arbitrary depth of hierarchy is allowed in CIF subroutines. Forward references are allowed provided that a subroutine is defined before it is used. Thus the sequence:
DS 10;
...
C 11;
DF;
DS 11;
...
DF;
C 10;
is legal, but the sequence:
C 11;
DS 11;
...
DF;
is not.
This is because the actual invocation of subroutine 11 does not occur
until after its definition in the first example.
CIF subroutines can be overwritten by deleting them and then redefining
them.
The DD statement (delete definition) takes a single parameter and deletes
every subroutine that has a number greater than or equal to this value.
The statement is useful when merging multiple CIF files because
designs can be defined, invoked, and deleted without causing naming
conflicts.
However, it is not recommended for general use by CAD systems.
Extensions to CIF can be done with the numeric statements 0 through
9.
Although not officially part of CIF, certain conventions have evolved for
the use of these extensions (see Fig. B.5).
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| FIGURE B.5 Typical user extensions to CIF. |
The final statement in a CIF file is the END statement
(or the letter E).
It takes no parameters and typically does not include a semicolon.
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