Abstract:
The present invention is capable of registering and reading out a logical element for which the state of the output pin changes. The system includes an input side reading out circuit for reading out the kind of logical element and the states of all the input pins thereof, a decision circuit for deciding the presence of the output pin that the status change is produced on when a logical operation is carried out according to the kind of logical element, an output side reading out circuit for reading out the information related to the logical element of the output pin producing the status change, and an exchange sending circuit for sending each information read out from the output side reading out circuit to the desired registering and reading out circuit for precise high speed logic simulation of a large scale logic circuit containing MOS-type logical elements.

Description:
BACKGROUND OF THE INVENTION 
     1.  Field of the Invention 
     The present invention relates to a logic circuit simulator, particularly, to a special purpose processor suitable for high speed simulation of a large scale gate level circuit. 
     2. Background of the Invention 
     Documents describing special purpose processors for logic simulation of one gate level (G. F. Pfister: The Yorktown Simulation engine, 19th Design Automation Conference) and one functional level [Sasaki: Summary of the High Speed Simulator (HAL), The 26th National Conference of Information Processing] have been published. A well-known example of the former which is similar to the present invention has the disadvantage of being unable to precisely effect logic simulation of a MOS element circuit, because only unit delay or three-value (0, 1, indefinite) simulation can be carried out thereby. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide a special purpose processor for high speed and precise simulation of a large scale logic circuit containing MOS elements. 
     Logic simulation is divided into four processes, (1) registration and read out of activated logical elements, (2) read out of the type of elements (AND, OR, etc) and the state the input pins, (3) logical operation of the elements and decision of output status change, (4) read out of the output side elements and of a delay time. These are effected by special purpose hardware, and data or operating instructions are circulated according to the sequence of (1)→(2)→(3)→(4)→(1)--for high speed logic simulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a structural drawing of an embodiment of a logic simulator based on the present invention; 
     FIG. 2 is a block diagram of circuits 1 to 3 shown in FIG. 1; 
     FIG. 3 is a block diagram of circuits 4 to 6 shown in FIG. 1; 
     FIG. 4 is a block diagram of circuits 7 to 9 shown in FIG. 1; 
     FIG. 5 is a block diagram of circuits 13 to 15 shown in FIG. 1; and 
     FIG. 6 is an operating explanatory drawing of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In an embodiment shown in FIG. 1, an overall operating processor is composed of a triple loop. 
     The first (or the second or the third, read the numbers in this order in parentheses hereinafter) loop 17 (18, 19) is composed of a registration and read out CKT 1 (2, 3) for registering and reading out the activated element; an element input side read out CKT 4 (5, 6) for reading out the kind of the element and the state of the input pin; a decision CKT 7 (8, 9) for a logical operation of the element and for deciding the presence of output status change; a FIFO data buffer 10 (11, 12); an element output side read out CKT 13 (14, 15) for reading out the element to be output and the delay time; and an exchange network 16. 
     It is assummed that a logic simulation circuit is divided into as many sub-circuits as the number of loops. The data for the kind of element, the connecting structure, and the delay time are divided corresponding to the subgraph, assigned to each one of the loops, and stored in the processor of the corresponding loop. 
     Next, the structure of each one of the circuits is described. Each of the circuits 1 to 3 is composed of an event processor 20 and an event memory 21, as shown in FIG. 2. The event memory 21 comprises a time wheel 22 which is individually accessible and an event list 23. In the time wheel 22, a head pointer H and a tail pointer T of the event list is stored, corresponding to each time. In the event list 23, the number of the activated element G, the input pin number N showing the status change of the element, and a status value S of the input pin are connected by a pointer P to be stored. If the four values [Driving, Resistive, High Impedance, X (indefinite)] are considered as a strength besides the level 0, 1, and X (indefinite) to provide 3×4=12 values-for the status value S, a sufficient precision for processing MOS can be obtained. 
     The circuits 4 to 6 are composed of a fan in processor 30 and an element memory 31, as shown in FIG. 3. The element memory 31 is composed of an activated element list 32, which is individually accessible, an element kind table 33, and an input value table 34. The element number G is stored in the activated element list, the element kind OP and the pointer P of the input table are stored in the element kind table, and the status of the input pins of all the element S 1 , S 2 , . . . are stored in the input value table. 
     The circuits 7 to 9 are composed of an evaluation unit 40 and a status memory 44, as shown in FIG. 4. The evaluation unit 40 is composed of a data compression CKT for compressing the data of the input signal, a memory 42 for storing a true value for each element, and a decision circuit 43 for deciding the presence of the status change of output. The data compression CKT is provided for excluding a combination of a redundancy or meaningless input and for decreasing the memory capacity required for the true value table. The status of the output pin of the element is stored in the status memory 44, corresponding to the element number. 
     The circuits 13 to 15 are composed of a fan-out processor 50 and a connecting structure memory 51, as shown in FIG. 5. The connecting structure memory 51 is composed of a pointer table 52, which is individually accessible, and a table to be output 53. The pointer P of the table to be outputted is stored in the pointer table and the number of elements to be output, the input pin number of the element, and the delay time d are stored in the table to be output. Furthermore, propagation delay time is also considered as the delay time to provide a value such that the propagation delay time is added to the delay time of the element to be output. The output is complicated, and termination of the reading out of the table to be output is decided by an END flag in the stored data. 
     The exchange network 16 distributes the input data according to the address in a header part, and is composed of a multiple step network or a crossbar switch. 
     The operation of each one of the above-described parts is divided into two phases, as shown in FIG. 6. In phase I, only the event processor and the fan-in processor are operated and the evaluation unit and the fan-out processor are in an idling state. In phase II, all the processors operate at the same time. 
     The operations in phase I will be described according to FIGS. 2 and 3. First, the event processor 20 in FIG. 2 reads out the number of element G activated at the current time t, the input pin number N, and the input pin state S and feeds out the combination of (G,. N, S) to the fan-in processor. The fan-in processor 30 receives the combination of (G, N, S) and writes G in the activated element list 32 and S in the input value table 34. When this operation terminates for all the input pins of the activated element, phase I is complete. 
     Next, the operations in the phase II will be described according to FIGS. 2 to 5. The fan-in processor 30 reads out the element number G for the element registered in the activated element list, the kind OP, and the status of all the input pins of the element S 1 , S 2 , . . . from the element memory 31 and feeds out the combination of (G, OP, S 1 , S 2 , . . .). When there are many input points, it is preferable to provide a function that enables sending them separately. The evaluation unit 40 receives the combination of (G, OP, S 1 , S 2 , . . . ) and inputs them to the data compression circuit 41. The values read out from the status memory 44 are simultaneously inputted when the internal states are required for logical operations such as a flip-flop. The output of the data compression circuit 41 becomes the address of the true value table memory 42 and the value read out from the memory 42 becomes a new output of the element. Accordance or discordance of the present output and the previous output is checked by the decision circuit 43, and if there is a change, a new output value S is stored in the status memory 44, while the combination of (G, S) is sent to the data buffer. The fan-out processor 50 of FIG. 5 fetches the combination of (G, S) then reads out the number of the element to be output G, the pin number N, and the delay time d from the connection structure memory 51, and sends out the combination of (G, N, S, d) to the exchange network 16. When all data has been outputted, the next combination of (G, S) is fetched from the data buffer. 
     The exchange network 16 decodes the element number G and sends out the combination of (G, N, S, d) to any one of the three event processors. That is to say, the data is sent out to the event processor connected to the event memory which stores the information related to G. The event processor 20 of FIG. 2 receives the combination of (G, N, S, d), calculates the estimated time of activation t+d from the present time t and the delay time d, and then registers (G, N, S) with the event list of this time. 
     The above-described operations are effected by a pipe-line method along a loop. That is to say, in phase I, the event processor and the fan-in processor, and in phase II, the event processor, the fan-in processor, the evaluation unit, the fan-out processor, and the exchange network simultaneously conduct the operation for the different elements. 
     The above operations are described with regard to one loop, but calculations proceed in parallel with the exchange of data through the exchange network. If more parallel processing is desired, the number of loops can be increased without a major modification of the structure. 
     Not shown in FIG. 1, the calculation device is connected to a host computer, for example, (a micro-computer) which loads data into memory, reads out results, and controls exchanges between phases I and II. 
     The present invention can resolve the problems inherent in parallel processing to effect maximum calculation so that it can effect high speed logic simulation. For example, a large computer corresponding to the M200-H class effects simulation at 5×10 4  elements/ second. In contrast, the present invention can effect simulation at 1.2×10 7  element/second four loops and at 4.8×10 7  element/second 16 loops, provided that the machine cycle is 100 ns. 
     As compared with the well-known special purpose processor of logic simulation (G. F. Pfister: The Yorktown Simulation Engine, described before), the well-known example can process only a unit delay, whereas the present invention can process a standard delay containing a propagation delay. Furthermore, precise simulation becomes possible because the present invention can process many statuses. 
     According to the present invention, less hardware is required due to its limited purpose, and the degree of parallel processing is easily improved.