Format of .ext files produced by magic's hierarchical extractor
Magic's extractor produces a .ext file for each cell in a hierarchical design. The .ext file for cell name is name.ext. This file contains three kinds of information: environmental information (scaling, timestamps, etc), the extracted circuit corresponding to the mask geometry of cell name, and the connections between this mask geometry and the subcells of name.
A .ext file consists of a series of lines, each of which begins with a keyword. The keyword beginning a line determines how the remainder of the line is interpreted. The following set of keywords define the environmental information:
Identifies the technology of cell name as techname, e.g, nmos, cmos.
Identifies the time when cell name was last modified. The value time is the time stored by Unix, i.e, seconds since 00:00 GMT January 1, 1970. Note that this is not the time name was extracted, but rather the timestamp value stored in the .mag file. The incremental extractor compares the timestamp in each .ext file with the timestamp in each .mag file in a design; if they differ, that cell is re-extracted.
Identifies the version of .ext format used to write name.ext. The current version is 5.1.
Identifies the style that the cell has been extracted with.
scale rscale cscale lscale
Sets the scale to be used in interpreting resistance, capacitance, and linear dimension values in the remainder of the .ext file. Each resistance value must be multiplied by rscale to give the real resistance in milliohms. Each capacitance value must be multiplied by cscale to give the real capacitance in attofarads. Each linear dimension (e.g, width, height, transform coordinates) must be multiplied by lscale to give the real linear dimension in centimicrons. Also, each area dimension (e.g, transistor channel area) must be multiplied by scale*scale to give the real area in square centimicrons. At most one scale line may appear in a .ext file. If none appears, all of rscale, cscale, and lscale default to 1.
resistclasses r1 r2 ...
Sets the resistance per square for the various resistance classes appearing in the technology file. The values r1, r2, etc. are in milliohms; they are not scaled by the value of rscale specified in the scale line above. Each node in a .ext file has a perimeter and area for each resistance class; the values r1, r2, etc. are used to convert these perimeters and areas into actual node resistances. See ``Magic Tutorial #8: Circuit Extraction'' for a description of how resistances are computed from perimeters and areas by the program ext2sim.
The following keywords define the circuit formed by the mask information in cell name. This circuit is extracted independently of any subcells; its connections to subcells are handled by the keywords in the section after this one.
node name R C x y type a1 p1 a2 p2 ... aN pN
Defines an electrical node in name. This node is referred to by the name name in subsequent equiv lines, connections to the terminals of transistors in fet lines, and hierarchical connections or adjustments using merge or adjust. The node has a total capacitance to ground of C attofarads, and a lumped resistance of R milliohms. For purposes of going back from the node name to the geometry defining the node, (x,\|y) is the coordinate of a point inside the node, and type is the layer on which this point appears. The values a1, p1, ... aN, pN are the area and perimeter for the material in each of the resistance classes described by the resistclasses line at the beginning of the .ext file; these values are used to compute adjusted hierarchical resistances more accurately. NOTE: since many analysis tools compute transistor gate capacitance themselves from the transistor's area and perimeter, the capacitance between a node and substrate (GND!) normally does not include the capacitance from transistor gates connected to that node. If the .sim file was produced by ext2sim\|(1), check the technology file that was used to produce the original .ext files to see whether transistor gate capacitance is included or excluded; see ``Magic Maintainer's Manual #2: The Technology File'' for details.
attr name xl yl xh yh type text
One of these lines appears for each label ending in the character ``@'' that was attached to geometry in the node name. The location of each attribute label (xl yl xh yh) and the type of material to which it was attached (type) are given along with the text of the label minus the trailing ``@'' character (text).
equiv node1 node2
Defines two node names in cell name as being equivalent: node1 and node2. In a collection of node names related by equiv lines, exactly one must be defined by a node line described above.
fet type xl yl xh yh area perim sub GATE T1 T2 ...
Defines a transistor in name. The kind of transistor is type, a string that comes from the technology file and is intended to have meaning to simulation programs. The coordinates of a square entirely contained in the gate region of the transistor are (xl, yl) for its lower-left and (xh, yh) for its upper-right. All four coordinates are in the name's coordinate space, and are subject to scaling as described in scale above. The gate region of the transistor has area area square centimicrons and perimeter perim centimicrons. The substrate of the transistor is connected to node sub.
The remainder of a fet line consists of a series of triples: GATE, T1, .... Each describes one of the terminals of the transistor; the first describes the gate, and the remainder describe the transistor's non-gate terminals (e.g, source and drain). Each triple consists of the name of a node connecting to that terminal, a terminal length, and an attribute list. The terminal length is in centimicrons; it is the length of that segment of the channel perimeter connecting to adjacent material, such as polysilicon for the gate or diffusion for a source or drain.
The attribute list is either the single token ``0'', meaning no attributes, or a comma-separated list of strings. The strings in the attribute list come from labels attached to the transistor. Any label ending in the character ``^'' is considered a gate attribute and appears on the gate's attribute list, minus the trailing ``^''. Gate attributes may lie either along the border of a channel or in its interior. Any label ending in the character ``$'' is considered a non-gate attribute. It appears on the list of the terminal along which it lies, also minus the trailing ``$''. Non-gate attributes may only lie on the border of the channel.
The keywords in this section describe information that is not processed hierarchically: path lengths and accurate resistances that are computed by flattening an entire node and then producing a value for the flattened node.
During resistance extraction, it is sometimes necessary to break a node up into several smaller nodes. The appearance of a killnode line during the processing of a .ext file means that all information currently accumulated about node, along with all fets that have a terminal connected to node, should be thrown out; it will be replaced by information later in the .ext file. The order of processing .ext files is important in order for this to work properly: children are processed before their parents, so a killnode in a parent overrides one in a child.
resist node1 node2 R
Defines a resistor of R milliohms between the two nodes node1 and node2. Both names are hierarchical.
distance name1 name2 dmin dmax
Gives the distance between two electrical terminals name1 (a driver) and name2 (a receiver). Note that these are terminals and not nodes: the names (which are hierarchical label names) are used to specify two different locations on the same electrical node. The two distances, dmin and dmax, are the lengths (in lambda) of the shortest and longest acyclic paths between the driver and receiver.
The keywords in this last section describe the subcells used by name, and how connections are made to and between them.
use def use-id TRANSFORM
Specifies that cell def with instance identifier use-id is a subcell of cell name. If cell def is arrayed, then use-id will be followed by two bracketed subscript ranges of the form: [lo,hi,sep]. The first range is for x, and the second for y. The subscripts for a given dimension are lo through hi inclusive, and the separation between adjacent array elements is sep centimicrons.
TRANSFORM is a set of six integers that describe how coordinates in def are to be transformed to coordinates in the parent name. It is used by ext2sim\|(1) in transforming transistor locations to coordinates in the root of a design. The six integers of TRANSFORM (ta, tb, tc, td, te, tf) are interpreted as components in the following transformation matrix, by which all coordinates in def are post-multiplied to get coordinates in name:
ta td 0 tb te 0 tc tf 1
merge path1 path2 C a1 p1 a2 p2 ... aN pN
Used to specify a connection between two subcells, or between a subcell and mask information of name. Both path1 and path2 are hierarchical node names. To refer to a node in cell name itself, its pathname is just its node name. To refer to a node in a subcell of name, its pathname consists of the use-id of the subcell (as it appeared in a use line above), followed by a slash (/), followed by the node name in the subcell. For example, if name contains subcell sub with use identifier sub-id, and sub contains node n, the full pathname of node n relative to name will be sub-id/n.
Connections between adjacent elements of an array are represented using a special syntax that takes advantage of the regularity of arrays. A use-id in a path may optionally be followed by a range of the form [lo:hi] (before the following slash). Such a use-id is interpreted as the elements lo through hi inclusive of a one-dimensional array. An element of a two-dimensional array may be subscripted with two such ranges: first the y range, then the x range.
Whenever one path in a merge line contains such a subscript range, the other must contain one of comparable size. For example,
merge sub-id[1:4,2:8]/a sub-id[2:5,1:7]/b
is acceptable because the range 1:4 is the same size as 2:5, and the range 2:8 is the same size as 1:7.
When a connection occurs between nodes in different cells, it may be that some resistance and capacitance has been recorded redundantly. For example, polysilicon in one cell may overlap polysilicon in another, so the capacitance to substrate will have been recorded twice. The values C, a1, p1, etc. in a merge line provide a way of compensating for such overlap. Each of a1, p1, etc. (usually negative) are added to the area and perimeter for material of each resistance class to give an adjusted area and perimeter for the aggregate node. The value C attofarads (also usually negative) is added to the sum of the capacitances (to substrate) of nodes path1 and path2 to give the capacitance of the aggregate node.
cap node1 node2 C
Defines a capacitor between the nodes node1 and node2, with capacitance C. This construct is used to specify both internodal capacitance within a single cell and between cells.