Hardware Switch Level Simulator for MOS Circuits

A microfiche appendix is included in the application. One microfiche having forty one frames of computer program listings appear in the microfiche appendix.

The present invention relates to MOS transistor circuit simulators for use in testing integrated circuit designs.

Switch level simulation of digital circuits is an important design verification tool for designers of MOS integrated circuits. However, switch level simulation software running on a general purpose computer can take days to accomplish an exhaustive simulation on a large circuit of about 100,000 transistor devices. As the complexity of VLSI circuits increases, the time and cost to perform a complete software simulation on a general purpose computer is becoming prohibitive. One approach being developed is the construction of hardware specifically designed to handle switch level simulation of MOS transistor circuits. Performance factors which are important include not only speed, but also capacity to load and process large netlists corresponding to realistic LSI/VLSI circuit designs, timing analysis capability in addition to unit delay simulation, the ability to handle commonly used circuit design techniques, such as steering or pass transistor logic, and accuracy. The hardware simulator must be based on an algorithm that is sufficiently accurate to be useful, yet not so complex as to be unable to be put into hardware.

In IEEE Transaction of Computers, vol. C-33, no. 2, February, 1984, pp. 160-177, Bryant describes a switch level model that has been developed to describe the logical behavior of MOS transistor circuits, and then describes an algorithm for a logic simulator based on the model. A hardware architecture for implementing this algorithm is described by Dally and Bryant in IEEE Transactions on Computer-Aided Design, vol. CAD-4, no. 3, July, 1985, pp. 239-250.

In the model a network consists of a set of nodes connected by transistor switches. Each node has a state 0, 1 or X representing low, high and invalid voltages. Likewise, each transistor has a state 0, 1 or X representing open (nonconducting or OFF) closed (conducting or ON) and indeterminate conditions. Input nodes, including power and ground nodes, as well as any clock or data inputs, provide signals to the system and are not affected by the actions of the network. Storage nodes have states determined by the operation of the network, and these states remain on the nodes in the absence of applied inputs. Each storage node is assigned one of a number of discrete size values K.sub.1, . . . , K.sub.max that indicates its approximate capacitances in relation to other nodes. Each transistor has a strength .gamma..sub.1, . . . , .gamma..sub.max, indicating its conductance when closed relative to other transistors which may form part of a ratioed path.

The algorithm simulates the behavior of a circuit by taking a series of unit steps, where within each step the new excitation states of the nodes, i.e. the steady-state response of the nodes for an initial set of nodes states when the transistors are held fixed in states determined by the states of their gate nodes, are computed and the nodes are updated to their new states. This is done repeatedly until either a stable state is reached or a user defined step limit is exceeded. Determining the new excitation states is a four step process comprising a "logic update" step, a "perturbation" step, a "blocking strength" step and a "up/down" step. During the "logic update" step, the conduction states of the transistors whose gate nodes have changed state in the last node update are updated, and the source and drain nodes of these transistors are queued for the next step. During the "perturbation" step, the set of all nodes that could be affected by the changing transistor states are found by starting at the nodes queued in the logic update step and traversing the links representing transistors in the 1 or X state. Each time a new node is encountered, it is added to the queue. The "blocking strength" step determines the strength of the strongest definite path to each node in the queue, a path being definite if none of its edges correspond to transistors in the X state, and the strength of a path being defined as the minimum of the size of the root node and the strengths of the transistors corresponding to the path edges, where strength values are ordered K.sub.1, < . . . <K.sub.max <.gamma..sub.1 < . . . <.gamma..sub.max <.omega.. The "up/down strength" step computes the up (respectively, down) value for each node, i.e. the strength of the strongest unblocked path to the node having a root node with state 1 or X (respectively, 0 or X). If no such path exists, the value is set to 0. Once this computation terminates the steady-state response of the node equals 1 if the down value equals 0, 0 if the up value equals 0, and X otherwise. This algorithm just described will henceforth be called the "Bryant algebra".

The Bryant algebra and the Dally and Bryant hardware architecture which implements the algebra does not deal with MOS circuits employing pass transistors. In that architecture an enhancement device is considered to be ON if the gate has the correct logic level, even if the gate and source are shorted. In this case, the gate to source voltage V.sub.gs would be zero and would be less than a positive threshold voltage V.sub.th. Thus, the transistor would in fact be OFF.

A limitation of other prior art simulators is that they are adapted to perform only in a unit delay mode and thus cannot do timing analysis. In some circuits, a proper prediction of behavior requires delay times to be taken into account.

In IEEE Transactions on Computer-Aided Design, vol. CAD-3, no. 4, October, 1984, pp. 331-349, Lin and Mead disclose a method for determining a single value of Elmore's delay for any node changing state in a general RC network. The effects of parallel connections and initial stored charges on nodes are taken into consideration. A simulation algorithm based on Bryant's algebra is described.

In Proceedings of the IEEE International Conference on Computer Design 1985, pp. 586-589, Smith discloses a simulation algorithm which accurately predicts the behavior of MOS transistor circuits including pass transistors. Smith modifies the "up/down strength" step of the Bryant algebra for MOS transistor circuits including pass transistors to determine if there is an unblocked path between gate and source node of a transistor.

It is an object of the present invention to produce a hardware switch level simulator for MOS transistor circuits which is able to accurately simulate the behavior of circuits using pass transistors.

It is another object of the invention to produce such a hardware simulator which is also fast, has the capacity to handle realistic VLSI circuit designs, and offers the option of performing timing analysis in addition to unit delay simulation.

The above objects have been met with a hardware switch level simulator using a modified Bryant algebra for accurately performing unit delay and timing simulation of VLSI MOS transistor circuits including pass transistors. The simulator includes a traversal unit having a link memory storing a netlist of transistor switches with parameters such as identification of the source and drain nodes with which a transistor switch connects, the transistor's current switch state, and the transistor switch's predetermined strength. The traversal unit also includes a gate memory storing a gate list of nodes identifying the transistor switches whose gates are connected to said nodes. The simulator also includes a solve unit having a node memory storing a node list of all nodes in a circuit, together with parameters such as whether a node is a member of a specific set in a stack memory, identification of netlist and gate list entries containing that node, the node's current logic state, and strengths of the node and of the dominant charging path to the node. The solve unit also includes programmed logic arrays which communicate with the various link, gate, node and stack memories and perform the Bryant algebra simulation steps.

In order to accurately simulate the behavior of circuits with pass transistors, a SOLVE.sub.-- O programmed logic array detects the condition where the gate to source voltage V.sub.gs is less than the threshold voltage V.sub.th, by determining whether an unblocked path exists from the source node to a controlling gate node of a pass transistor However, since SOLVE.sub.-- O is called from within the "up/down strength" step SOLVE.sub.-- U/D in the modified Bryant algebra, the gate memory addressing circuitry in the traversal unit is capable of being multiplexed into link memory to retrieve netlist information corresponding to transistor switches and nodes for use by the SOLVE.sub.-- O programmed logic array.

The simulator may also include a timing unit with programmed logic arrays for performing capacitance and delay calculations for nodes changing state in a circuit. The node memory of the solve unit includes parameters for use in the timing calculations, such as cumulative capacitance and delay at each node and identification of the adjacent node upstream in the charging path, the stack unit can store multiple lists of nodes being updated for each time delay calculated in time analysis.

FIG. 1 is a block diagram of the hardware switch level simulator of the present invention together with a host computer and host controller communicating with the simulator.

FIG. 2 is a block diagram of a functional section of the solve unit in the simulator of FIG. 1 dedicated to a single step in the Bryant algebra.

FIG. 3 is a block diagram of the traversal unit in the simulator of FIG. 1.

FIG. 4 is a state table indicating the resulting switch states for various transistor types, gate logic states and source-ground voltage determinations.

FIG. 4A is a simplified MOS transistor model.

FIGS. 5-9 are schematic diagrams of MOS circuit portions illustrating the various steps in Bryant algebra.

With reference to FIG. 1, a hardware switch level simulator includes a stack unit 10, a solve unit 12, a traversal unit 14 and a timing unit 16. Stack unit 10, solve unit 12 and traversal unit 14 are all used for unit delay simulation. Timing simulation involves the above named units 10, 12 and 14 with the addition of timing unit 16. Typically, the simulator is built using TTL logic and all memory units are standard dynamic random access memory devices, each typically having 256 K of storage locations.

A host controller 18 provides an interface between the simulator and some host computer 20, such as a Hewlett Packard 9000-320 computer. This interface allows the host computer 20 to load various lists, derived from a MOS transistor circuit and necessary for the simulator to operate, into the various memory units of the simulator. The interface also allows the host computer 20 to load test vectors and read returned data. A second function of host controller 18 is to control the actions of the simulator by causing it to run as directed by the host computer 20, and to interpret the operating status of the simulator and return this status information to the host computer 20. Host controller 18 comprises a microprocessor 22, such as the commercially available Zilog Z-80 microprocessor, and a buffer 24. Buffer 24 allows test vector and returned data to be temporarily stored, thus reducing the amount of time the simulator waits for host computer interaction. Host controller 18 can also perform some memory list building and other managerial tasks related to simulation that would ordinarily burden the host computer 20. Host controller 18 handles data exchange between computer and simulator. The controller is not essential and computer 20 may interact directly with the simulator if controller is eliminated.

In the unit delay mode, the simulator performs the following algorithm. Details can be found in the Bryant reference noted above as modified by Smith, also referenced above.

Briefly, the algorithm operates on sets of nodes L.sub.-- SET(0), P.sub.-- SET, R.sub.-- SET(x) and U.sub.-- SET(x) stored in stack unit 10. The procedures LOGIC.sub.-- UPDATE, PERTURB, SOLVE.sub.-- Q and SOLVE.sub.-- U/D are performed by hardware functional units in solve unit 12 using various data from stack unit 10 and traversal unit 14. The algorithm is executed repeatedly until a stable state is reached or until the number of repeat steps exceeds a user defined limit.

With reference to FIGS. 4 and 4A, a transistor 62 is represented as a switch consisting of a drain and a source serving as connection ports, and a gate which serves as a control port. When the switch is OFF, it is assumed to have an infinite resistance from source to drain, and when it is ON, it is assumed to have some fixed resistance R that is normally a function of the transistor type and its internal geometry. The simulation algorithm uses this resistance R in the determination of the strength .gamma..sub.1, . . . , .gamma..sub.7 of a charging path to a node, where a device's resistance is inversely proportional to its ON resistance. The manner in which the gate controls the transistor's switch state is shown in FIG. 4 for NMOS depletion, NMOS enhancement, and PMOS enhancement devices.

In FIG. 5, a transistor 64 has a gate 66, a source 68 and a drain 70. Gate 66 connects to a node 72, source 68 connects to a node 74 and drain 70 connects to a node 76. The function of the LOGIC.sub.-- UPDATE procedure is to identify nodes which may be affected by a change in the state of a gate node 72 driven during the previous step, and then to queue these nodes into a node list for further investigation. In order to identify these nodes, information is needed on the source and drain nodes, 74 and 76 that belong to the transistor 64 whose gate 66 is being driven. As is discussed below, this gate traversal information is provided by a gate list stored by traversal unit 14. L-SET [0] is the set of nodes driven in the previous step and the identified source and drain nodes are stored in the set P.sub.-- SET.

In FIG. 6, transistors 64, 78, 80 and 82 are connected together at nodes 74, 84 and 86. In order to determine the complete effect of a state change at gate node 72 of transistor 64, it is next necessary to perform a link traversal, i.e. to find the set of all nodes that could be affected, by starting at the nodes 76 and 74 in P.sub.-- SET queued in the LOGIC.sub.-- UPDATE procedure and adding into the queue those nodes 84 and 86 traversed from node through transistors 78 and 80 in the ON state. Node is not added since transistor 82 is OFF. This procedure, called PERTURB, requires information on the source and drain nodes to which a transistor connects as well as the transistor's current switch state. As is discussed below, this link traversal information is provided by a netlist stored by traversal unit 14. The resulting list of nodes is then sorted by node strength into separate lists R.sub.-- SET(x).

In FIG. 7, the blocking step performed by procedure SOLVE.sub.-- Q, determines the effect on a node 90 from two unblocked paths 92 and 94 of unequal charging strength. Path 92 begins at node 104 of strength K.sub.1, traverses transistor 96 of strength .gamma..sub.1, to node 106 of strength K.sub.2, and traverses transistor 98 of strength .gamma..sub.2 to node 90. Path 94 begins at node 108 of strength K.sub.4, traverses transistor 100 of strength .gamma..sub.4 to node 110 of strength K.sub.3, and traverses transistor 102 of strength .gamma..sub.3 to node 90. SOLVE.sub.-- Q acts on nodes in order of strength, strongest nodes first in order to identify the path with the greatest minimum of the size of the root node 104 or 108 and transistor sizes in the path. Thus, path 94 is identified as the dominant path. Information on transistor strengths is provided by the netlist in traversal unit 14. Node strength information is stored and retrieved from a node list in solve unit 12.

With reference to FIG. 8, the procedure SOLVE.sub.-- U/D computes the up and down strengths for each node. The up strength of a node is the strength of the strongest unblocked path to the node having a root node with state 1 or X. If no such path exists, the strength is set to 0. Likewise, the down strength of a node is the strength of the strongest unblocked path to the node having a root node with state 0 or X. The up and down strengths are compared to determine the resulting node value. If the up and down strengths are equal, the resulting node value is indeterminate (X). In FIG. 8, the two paths from FIG. 8 leading to node 90 are compared. The up strength is the strength K of root node 108 with state 1. The down strength is the strength K of root node 104 with state 0. The resulting state of node 90 is therefore 1.

The procedure SOLVE.sub.-- O that is used in the simulation of circuits with pass transistors is called from procedure SOLVE.sub.-- U/D as shown in the algorithm below. SOLVE.sub.-- O is discussed in detail in the Smith reference noted above.

In FIG. 9, an inverter circuit 112 with input state 0 and output state at node 114 of 1 traverses a pass transistor 116 to a source node 118. When pass transistor 116 is ON it will pass the value at node 114 to source node 118 for an output 120 of state 1. Normally, this will occur when gate node 122 of pass transistor 122 is at logic state 1, i.e. when the clock value is at state 1. However, as seen in the table in FIG. 4, when source node 118 is connected via a dominant unblocked path 124 back to gate node 122, the gate-to-source voltage V.sub.gs will equal zero, and may be less than a threshold voltage V.sub.th. In this case, an enhancement transistor will be OFF even though the gate logic state would indicate otherwise. Accordingly, procedure SOLVE.sub.-- O attempts to locate any unblocked path from a gate node of a pass transistor to the source node.

Referring again to FIG. 1, stack unit 10 comprises a number of registers 26 serving as stack pointers, a stack memory 28 and a control subunit 30 for managing the various node sets stored in stack memory 28. A large common pool of memory 28 stores numerous sets of nodes L.sub.-- SET (.tau.), P.sub.-- SET, R.sub.-- SET(x) and U.sub.-- SET(x) to be operated on during the various steps taken during simulation. Typically, about 512K storage locations are provided. Stack pointer registers 26 indicate which node set is to be used with each procedure. Stack control 30 dynamically allocates as much memory as is needed. Stack control 30 includes a finite state machine, which typically consists of three programmed logic arrays, as well as some input and output registers. Computer program listings for a stack unit state machine is provided in the microfiche appendix included herewith.

The node set L.sub.-- SET(.tau.) holds nodes for use in the LOGIC.sub.-- UPDATE procedure. In the case of unit delay, one stack L.sub.-- SET (0) is used. In the timing mode, 1024 stacks can contain nodes, each stack corresponding to a position of a 10 bit time pointer in timing unit 16. During the PERTURB procedure, nodes are pulled from an input set P.sub.-- SET and pushed to any of 15 separate sets R.sub.-- SET(x) sorted by node strength which are to be used as input sets for the SOLVE.sub.-- Q procedure. There are 15 sets because there are 15 total strength levels K.sub.1, . . . , K.sub.7, .gamma..sub.1, . . . , .gamma..sub.7,.omega. corresponding to all possible transistor and node strengths. If a different number of discrete strengths is used, the number of node sets R.sub.-- SET(x) will correspond to that different number. During execution of the SOLVE.sub.-- Q procedure, nodes are acted upon only in order of strength, the strongest set R.sub.-- SET(.omega.) being acted on first and the weakest set R.sub.-- SET(K.sub.1) being operated on last. Likewise, the procedure SOLVE.sub.-- U/D acts on the sets U.sub.-- SET(x) in order of node strength from strongest to weakest. Within a certain strength level, no particular order for node consideration is necessary, so the sets can be arranged in simple stacks.

Traversal unit 14 comprises a control subunit 32, a link memory 34 and a gate memory 36. Traversal unit 14 stores two general lists of data, a netlist stored in link memory 34 and a gate list stored in gate memory 36. Control subunit 32 typically comprising one or more programmed logic arrays together with input and output registers, communicates with solve unit 12 and loads and retrieves data from link memory 34 and gate memory 36 as they are requested. A computer program listing for the traversal unit state machine, i.e. programmed logic arrays, is included in the microfiche appendix herewith.

The netlist (node.storageconset in algorithm (2) above) stored in link memory 34 is a list of transistor switches containing information in a record with a plurality of fields indicating each transistor's connections to various nodes by its gate, source and drain. Among the information included in each entry are NODE POINTERS 126 (x.node) which point to the nodes connected to by the transistor source and drain. SWITCH STRENGTH 128 (trans.strength) indicates the transistor's predetermined strength. SWITCH STATE 130 (trans.state) indicates the transistor's current switch state, i.e. whether the transistor is ON, OFF or whether the state is INDETERMINATE. RESISTANCE 132 indicates the transistor's predetermined ON resistance used in determining delay in the timing analysis mode. GATE POINTERS 134 (gate.node) indicate the node to which the transistor's gate is connected. Each netlist entry is preferably stored in sequential traversal unit memory addresses, making pointers to the next list entry unnecessarily. A TAG 136 in the list identifies when an element is the last transistor switch listed.

The GATE LIST 138 stored in gate memory 36 identifies those transistor switches that could be affected by changing the gate logic level of interest, i.e. those transistors whose gate connects to the node being driven. The gate list comprises pointers to transistors in the netlist and therefore to appropriate drain and source node identification. Each gate node entry is arranged in a sequential manner, and a TAG 140 in the GATE LIST 138 identifies the last element in the list. The gate list is only used during the LOGIC UPDATE procedure and ensures that only active portions of a circuit are operated on thereby providing more efficient simulation. As is discussed below, circuits ancillary to the gate memory 36 can be converted into a second group of circuits 52 ancillary to the link memory 34 during operation of the SOLVE .sub.-- O procedure.

Solve unit 12 communicates directly with all other units 10, 14, 16 and 18, and is involved in executing all of the simulation procedures LOGIC.sub.-- UPDATE, PERTURB, SOLVE.sub.-- Q, SOLVE.sub.-- U/D and SOLVE.sub.-- O. Solve unit 12 comprises a simulation step subunit 38 an a node memory 40.

Node memory 40 stores a node list containing entries for every node in a circuit. Typically, up to 256K entries may be stored. Each node list entry contains a plurality of parameters arranged in fields. STATUS FLAGS 142 indicate conditions, such as whether a particular node is a member of a particular set stored in stack memory 28. Typically, there may be as many as 8 status flags. NETLIST POINTERS 144 indicate transistor switches in the netlist in link memory 34, in which the source or drain thereof connects to a particular node. Likewise, GATE LIST POINTERS 146 indicate transistor switches in the gate list in gate memory 36, in which the gate thereof connects to the node. NODE LOGIC STATE 148 gives the node's current logic value. STRENGTHS 150 indicate strengths currently associated with the node, such as the intrinsic node strength, as well as strength currently associated with the dominant charging path to the node, such as blocking strength, up strength and down strength. CUMULATIVE CAPACITANCE 152, CUMULATIVE DELAY 154 and PARENT NODE POINTERS 156 are associated with timing analysis and are discussed below.

Referring to FIG. 2, the simulation step subunit 38 is made up of a number of functional units 42, each of which is dedicated to a specific one of the procedures. There are many steps involved in performing a complete circuit simulation, but most of the functionality encountered during execution involves reading strengths and other data from the node list in node memory 40 for nodes indicated by stack and traversal units 10 and 14 respectively. Other data may be retrieved from traversal unit 14, if necessary.

Node memory 40, stack unit 10 and traversal unit 14 can be read and the information transferred to a functional unit 42 via input registers 44 connected to functional unit 42 and units 10, 14 and 40. The data is then processed by a functional unit 42, typically comprising one or more programmed logic arrays, designed to perform the desired calculation, such as PERTURB or SOLVE.sub.-- Q. New strengths and other data are buffered out of the functional unit 42 via output registers 46 connected to functional unit 42 and memory units 10, 14 and 40 and written back into the desired memory structure, usually node memory 40. A finite state machine 48 controls the interaction of functional units 42 and registers 44 and 46 with the various memory structures 10, 14 and 40. A computer program listing for the programmed logic arrays in solve unit 12 is included in microfiche appendix herewith.

With reference to FIG. 3, the procedure SOLVE.sub.-- O, which is used to determine the presence of an unblocked path back to a controlling gate in a pass transistor circuit, presents a special problem for the traversal unit. As noted in procedure (2) above, SOLVE.sub.-- O is called from SOLVE.sub.-- U/D. Thus, there can be two netlist link traversals occuring at the same time, one for each SOLVE procedure. It is necessary that traversal unit 14 have a way of restarting the link traversal for the SOLVE.sub.-- U/D procedure each time SOLVE.sub.-- O finishes. In other words, the traversal unit 14 must be capable of performing two link path traversals without getting the two mixed up. To do this the traversal unit 14 has a state machine 49 common to both link and gate memories 34 and 36 and two sets of memory access counters and gates 50 and 52, one of which can be switched to access link memory 34. An output buffer 60 returns requested data to solve unit 12.

Normally, gate traversal information is needed only during the LOGIC.sub.-- UPDATE procedure, while link traversal information is needed the remainder of the time. State machine 49 switches between access to gate memory 36 via memory access counters and gates 50 and access to link memory 34 via the second set of memory access counters and gates 52. Counters and gates 50 are normally set by a gate list pointer received from solve unit 12 by state machine 49 so as to access the appropriate gate list data location in gate memory 36 via a first bus 54. Likewise, counters and gates 52 are normally set by a netlist pointer received from solve unit 12 by state machine 49 so as to access the appropriate netlist data location in link memory 34 via a second bus 56. During a SOLVE.sub.-- O procedure, state machine 48 can trace a second link pathway be setting counters and gates 50, normally dedicated to access to gate memory 36, with a netlist pointer corresponding to the second link pathway, and by accessing the appropriate data location in link memory 34 via a third bus 58. Thus, traversal unit 14 allows counters and gates 50 to be multiplexed for access to link memory 34, so that pointers in second counters and gates 52 can be retained and restarted after completion of the SOLVE.sub.-- O procedure. In effect, counters and gates 50 replicates the function of second counters and gates 52 to create two simultaneous accesses to link memory 34.

We have so far discussed the simulator in the unit delay mode. The general algorithmic flow for timing simulation is the following:

Referring again to FIG. 1, a timing unit 16 keeps a pointer to time. Stack unit 10 can store a circular queue of 1024 stacks referred to as L.sub.-- SET(.tau.), where .tau. can take on values between 0 and 1023. The simulator finds the dominant paths to nodes in the same manner as in unit delay, except that in procedure SOLVE.sub.-- U/D, rather than placing nodes that have changed state back into a L.sub.-- SET, it assigns pointers to all nodes along dominant paths connecting nodes that may change. These PARENT NODE POINTERS are stored in node list 40 of solve unit 12, and point back through a switch to the adjacent node upstream in the charging path. The last node in the path, one having no pointer referring to it, is called the leaf node, while all others are called parent nodes. SOLVE.sub.-- U/D places all leaf nodes into a C.sub.-- SET(1) in stack unit 12. Control is then passed to timing unit 16 where the procedures CAP.sub.-- CAL and DEL.sub.-- CAL are performed. The procedures CAP.sub.-- CAL and DEL.sub.-- CAL are described in detail in the Lin and Mead reference noted above.

Briefly, CAP.sub.-- CAL traces along a path starting from the leaf node, determining the effective capacitance at each node, and solving for a resultant cumulative capacitance which is the sum of the node's effective capacitance and the cumulative capacitance of the previous node. The effective capacitance is defined as:

if node.state has changed

then effective.capacitance:=node.size

else effective.capacitance:=0;

CUMULATIVE CAPACITANCE 152 for each node is stored in node memory 40 of solve unit 12. CAP.sub.-- CAL continues until the final parent node of each path is encountered.

A procedure DEL.sub.-- CAL is then performed by timing unit 16, operating in the reverse direction on each path from CAP.sub.-- CAL. DEL.sub.-- CAL calculates the cumulative delay for each node, which is defined as the cumulative delay of the parent node added to the product RC. R is the predetermined resistance of the transistor switch connecting the node to the parent node, and is supplied by RESISTANCE 132 in the link memory 34 of traversal unit 14. C is the previously calculated CUMULATIVE CAPACITANCE 152 for the node stored in node memory 40. CUMULATIVE DELAY 154 for each node is stored in node memory 40. DEL.sub.-- CAL continues for each path until the leaf node is reached, adding each node the changes logic state to L.sub.-- SET [.tau.+CUMULATIVE DELAY]. Timing unit 16 then increments time .tau. and returns control back to solve unit 12 for the next Bryant algebra step.

The hardware switch level simulator is faster than software simulators executed on a general purpose computer. Typically, the hardware simulator of the present invention averages 13,200 gate evaluations per second (GEPS) in unit delay and 9500 GEPS in timing mode. By comparison, the software MOSSIM II on a VAX 11-780 computer runs at about 500 GEPS. Further, the simulator accurately simulates the behavior of circuits using pass transistors and the delay calculations by timing simulation are generally within 20% to 40% of those calculated by the software SPICE. Because dynamic RAM is used, the simulator can accommodate a circuit of up to 262,144 MOS devices. Larger amounts of memory can be used to handle larger circuits.