Caltech Intermediate Form

Caltech Intermediate Form (CIF) is a file format for describing integrated circuits. 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.

Overview
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 parentheses.

There are only a few CIF statements and they fall into one of two categories: geometry or control. The geometry statements are:  to switch mask layers,   to draw a rectangle,   to draw a path,   to draw a circle,   to draw an arbitrary figure, and  to draw a subroutine of other geometry statements. The control statements are  to start the definition of a subroutine,   to finish the definition of a subroutine,  to delete the definition of subroutines,   through   to include additional user-specified information, and   to terminate a CIF file. All of these keywords are usually abbreviated to one or two letters that are unique.

Geometry
The  statement (or the letter  ) sets the mask layer to be used for all subsequent geometry until the next such statement. Following the  keyword comes a single layer-name parameter. For example, the command:  L CC; sets the layer to be the CMOS contact cut (see Fig. B.1 for some typical MOS layer names).

The  statement (or the letter  ) 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  defines a 26.6-degree counterclockwise rotation from the normal. Similarly,  will rotate clockwise by 45 degrees. Note that the magnitude of this rotation vector has no meaning.



The  statement (or the letter  ) 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  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.

The  statement (or the letter  ) 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  statement (or the letter  ) 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.

Hierarchy
The  statement (or the letter  ) invokes a collection of other statements that have been packaged with  and. All subroutines are given numbers when they are defined and these numbers are used in the  to identify them. If, for example, a  statement and a   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  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  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  followed by an x, y vector endpoint that, much like the rotation clause in the  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:  to mirror in x and   to mirror in y. Mirroring is a bit confusing, because   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).

Defining subroutines for use in a  statement is quite simple. The statements to be packaged are enclosed between  (definition start) and   (definition finish) statements. Arguments to the  statement are the subroutine number and a subroutine scaling factor. There are no arguments to the  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.

Control
CIF subroutines can be overwritten by deleting them and then redefining them. The  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  through. Although not officially part of CIF, certain conventions have evolved for the use of these extensions (see Fig. B.5).

The final statement in a CIF file is the  statement (or the letter  ). It takes no parameters and typically does not include a semicolon.

BNF grammar
The following is the grammar for the CIF format with cifFile being the top level grammar node.

cifFile ::= (blank* command? semi)* endCommand blank* command ::= primCommand | defDeleteCommand | defStartCommand semi (blank* primCommand? semi)* defFinishCommand primCommand ::= polygonCommand | boxCommand | roundFlashCommand | wireCommand | layerCommand | callCommand | userExtensionCommand | commentCommand polygonCommand ::= "P" path boxCommand ::= "B" integer sep integer sep point (sep point)? roundFlashCommand ::= "R" integer sep point wireCommand ::= "W" integer sep path layerCommand ::= "L" blank* shortname defStartCommand ::= "D" blank* "S" integer (sep integer sep integer)? defFinishCommand ::= "D" blank* "F" defDeleteCommand ::= "D" blank* "D" integer callCommand ::= "C" integer transformation userExtensionCommand ::= digit userText commentCommand ::= "(" commentText ")" endCommand ::= "E" transformation ::= (blank* ("T" point |"M" blank* "X" |"M" blank* "Y" |"R" point)*)* path ::= point (sep point)* point ::= sInteger sep sInteger sInteger ::= sep* "-"? integerD integer ::= sep* integerD integerD ::= digit+ shortname ::= c c? c? c? c ::= digit | upperChar userText ::= userChar* commentText ::= commentChar* | commentText "(" commentText ")" commentText semi ::= blank* ";" blank* sep ::= upperChar | blank digit ::= "0" | "1" | ... | "9" upperChar ::= "A" | "B" | ... | "Z" blank ::= any ASCII character except digit, upperChar, "-", "(", ")", or ";" userChar ::= any ASCII character except ";" commentChar ::= any ASCII character except "(" or ")"