Abstract:
A method, module, and program product for detecting signal strengths in a hardware description language, such as Verilog, that does not provide for such detection. The method includes the steps of creating a wired net configuration that provides for a data input signal and a controlled reference signal; varying the controlled reference signal based on a desired signal strength to be detected; and comparing the input signal with the controlled reference signal to determine if the desired signal strength has been detected.

Description:
BACKGROUND OF INVENTION 
   1. Technical Field 
   The present invention is related to hardware description languages. More particularly, the present invention provides multi-level or single strength signal detection in a hardware description language (HDL), such as Verilog, that does not easily provide for such detection. 
   2. Related Art 
   The current Verilog core standard does not provide any way to test for the specific strength of a signal. One solution to this problem is the use of the Verilog Programming Language Interface (PLI) standard. As known in the art, PLI provides a mechanism for interfacing Verilog programs with programs written in the C language, and for accessing the internal databases of a Verilog simulator from a C program. PLI is used for implementing system calls which would have been hard to do otherwise (or impossible) using the Verilog syntax. To this extent, PLI provides a way to extend the functionality of a given Verilog simulator that supports the PLI (as described in the IEEE 1364-1995 Verilog standards document). 
   A major drawback to using the PLI is that it is very cumbersome. To test the specific strength of a signal, for example, a C compiler is needed, and a custom C function must be written to perform the signal strength detection. The custom C function must then be linked (statically or dynamically) with a user&#39;s specific Verilog simulation tool executable, and then the user&#39;s Verilog HDL must be coded to make use of the newly written PLI routine. 
   There is a need, therefore, for a reusable Verilog module that is capable of performing signal strength detection. 
   SUMMARY OF INVENTION 
   The present invention provides a reusable Verilog module that is capable of performing signal strength detection. Specifically, use is made of Verilog&#39;s standard strength resolution capabilities as they relate to wired net configurations to “compare” a given input signal with a controlled reference signal of fixed strength. The controlled reference signal will vary depending on the desired signal strength that a user would like to detect. 
   One advantage of the present invention is ease of use and simplicity as compared to the cumbersome and far more complicated PLI method. The present invention simply requires that a user incorporate a predefined module into their Verilog code when they require signal strength detection functionality. This is no different than any other call to a hierarchical “building block” function that may be written for a given design. Thus, the present invention is much more convenient and user-friendly than the prior art PLI solution. 
   A first aspect of the present invention is directed to a method for detecting signal strengths in a hardware description language that does not provide for such detection, comprising: creating a wired net configuration that provides for a data input signal and a controlled reference signal; varying the controlled reference signal based on a desired signal strength to be detected; and comparing the input signal with the controlled reference signal to determine if the desired signal strength has been detected. 
   A second aspect of the present invention is directed to a module for detecting signal strengths in a hardware description language that does not provide for such detection, the module comprising: a first wired net for receiving an input signal and a controlled reference signal, wherein the hardware description language resolves the first wired net to obtain a resolved value; a scalar net for receiving a signal corresponding to the input signal; and a comparison system for comparing the resolved value on the first wired net with the signal corresponding to the input signal on the scalar net to determine if the desired signal strength has been detected. 
   A third aspect of the present invention is directed to a program product stored on a recordable medium for detecting signal strengths, comprising: program code for providing a first wired net for receiving an input signal and a controlled reference signal and for resolving the first wired net to obtain a resolved value; program code for providing a scalar net for receiving a signal corresponding to the input signal; and program code for comparing the resolved value on the first wired net with the signal corresponding to the input signal on the scalar net to determine if the desired signal strength has been detected. 
   A fourth aspect of the present invention is directed to a reusable module for detecting signal strengths in Verilog, the reusable module comprising: a first wired net for receiving an input signal and a controlled reference signal, wherein the hardware description language resolves the first wired net to obtain a resolved value; a scalar net for receiving a signal corresponding to the input signal; and a comparison system for comparing the resolved value on the first wired net with the signal corresponding to the input signal on the scalar net to determine if the desired signal strength has been detected, wherein the desired signal strength is selected from the group consisting of: supply 0 , strong 0 , pull 0 , large 0 , weak 0 , medium 0 , small 0 , supply 1 , strong 1 , pull 1 , large 1 , weak 1 , medium 1 , small 1 , and ranges of signal strengths. 
   The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
       FIG. 1  illustrates an LDETECT circuit for detecting a data input signal DATA_IN having strength L in accordance with the present invention, wherein L is a logic 0 signal having a signal strength in the range of “small 0 ” up to “pull 0 .” 
       FIG. 2  is a truth table illustrating the output IS_L of the LDETECT circuit of  FIG. 1 , for all possible input values of the data input signal DATA_IN. 
       FIG. 3  illustrates a reusable Verilog module corresponding to the LDTETCT circuit of  FIG. 1 . 
       FIG. 4  illustrates an HDETECT circuit for detecting a data input signal DATA_IN having strength H in accordance with the present invention, wherein H is a logic 1 signal having a range of signal strengths from “small 1 ” up to “pull 1 .” 
       FIG. 5  is a truth table illustrating the output IS_H of the HDETECT circuit of  FIG. 4 , for all possible input values of the data input signal DATA_IN. 
       FIG. 6  illustrates a reusable Verilog module corresponding to the HDTETCT circuit of  FIG. 4 . 
       FIG. 7  illustrates a PULLODETECT circuit for detecting a data input signal DATA_IN having strength “pull 0 ” in accordance with the present invention. 
       FIG. 8  is a truth table illustrating the output IS_PULL 0  of the PULL 0 DETECT circuit of  FIG. 7 , for all possible input values of the data input signal DATA_IN. 
       FIG. 9  illustrates a reusable Verilog module corresponding to the PULL 0 DTETCT circuit of  FIG. 7 . 
       FIG. 10  illustrates a PULL 1 DETECT circuit for detecting a data input signal DATA_IN having strength “pull 1 ” in accordance with the present invention. 
       FIG. 11  is a truth table illustrating the output IS_PULL 1  of the PULL 1 DETECT circuit of  FIG. 10 , for all possible input values of the data input signal DATA_IN. 
       FIG. 12  illustrates a reusable Verilog module corresponding to the PULL 1 DTETCT circuit of  FIG. 10 . 
       FIG. 13  is a driver truth table for a differential I/O. 
       FIG. 14  is a receiver truth table for the differential I/O. 
       FIG. 15  illustrates the use of a PULL 0 DETECT module to model the receiver portion of the differential I/O described in  FIG. 14 . 
   

   DETAILED DESCRIPTION 
   There are only 4 logic values in Verilog, including: 0, 1, X, and Z (also known as hi-Z). Each of these values (except for Z which, by definition, is a signal that has zero strength) has a specific signal strength associated with it. The default strength is “strong,” in the absence of a signal strength being specified. Logic 0 and logic 1, as well as X (indeterminate) can have any of the following strengths, in ascending strength order: highz (no strength, or Z), small, medium, weak, large, pull, strong, and supply. For example, a logic 0 signal that is of “weak” strength would be referenced as “weak 0 ,” while a logic 1 signal that is of “strong” strength would be referenced as “strong 1 .” In a first embodiment of the present invention, a logic 0 signal having a signal strength in the range of “small 0 ” up to “pull 0 ” is detected. In this embodiment, as illustrated in  FIGS. 1–3 , a logic 0 signal having a range of signal strengths from “small 0 ” up to “pull 0 ” is referenced as L. A circuit  10  (LDETECT) for detecting a data input signal DATA_IN having a signal strength L is illustrated in  FIG. 1 . A reusable Verilog module  30  corresponding to the LDETECT circuit  10  of  FIG. 1  is shown in  FIG. 3 . A truth table  20  illustrating the output IS_L of the LDETECT circuit  10  for all possible input values of the data input signal DATA_IN is shown in  FIG. 2 . Also shown in the truth table  20  are the resultant values of the a_out and b_out nets in the LDETECT circuit  10 . 
   As shown in  FIG. 1 , the data input signal DATA_IN is fed into a unidirectional, isolating, nmos gate nm 1 , whose control input (i.e., gate) is tied to Vdd (i.e., logic 1). The nmos gate nm 1  does not alter the strength of the data input signal DATA_IN in any way. A controlled reference signal  12  having a “strong 1 ” signal strength is generated via a standard Verilog buffer gate (instance b 1 ) whose input is tied to Vdd. The output of buffer b 1  has a “strong” value because, as known in the art, the output of any logic gate in Verilog has a “strong” value. The controlled reference signal  12  (i.e., “strong 1 ”) and the output  14  of the nmos gate nm 1  (i.e., the data input signal DATA_IN) are driven into the a_out net. 
   The a_out net is a standard “wired net” configuration in Verilog. As known in the art, Verilog uses a standard set of rules for signal strength resolution to resolve the value that it places on a wired net, such as the a_out net. These strength resolution rules are documented clearly in most any Verilog reference manual, including the IEEE 1364-1995 Verilog standards specification, and will not be described in detail herein. 
   Referring to  FIG. 2 , the resultant, “resolved” values on the a_out net of  FIG. 1  are listed in the truth table  20  for all possible input values of the data input signal DATA_IN. For example, as shown in row 1 of the truth table  20 , the value on the a_out net is equal to logic 1 when a data input signal DATA_IN having a signal strength L is driven into the a_out net along with a controlled reference signal  12  having a signal strength of “strong 1 .” This occurs because the signal strength “strong 1 ” is greater than the signal strength L in the absolute value sense. 
   The b_out net is derived simply by buffering the data input signal DATA_IN using another standard Verilog buffer gate (instance b 2 ). If the data input signal DATA_IN is a logic 0 signal of any strength (i.e., L, “strong 0 ”, or “supply 0 ), then b_out=0. As detailed above, this occurs because the output of any logic gate in Verilog is a “strong” value. In general, b_out is a “strong” version of the data input signal DATA_IN. The a_out and b_out nets are then compared to a predetermined vector value (“key combination”) in a vector comparison block  16 . 
     FIG. 3  shows an example of a reusable Verilog module  30  corresponding to the LDETECT circuit  10  of  FIG. 1 . In this example, the vector comparison block  16  is coded with the following statement in Verilog: 
   wire preIS_L=({a_out,b_out}===2′b 10 ). 
   This is a standard “continuous assignment” statement in Verilog that returns one of only two possible values: 0 (false) or 1 (true). It returns 1 (true) if and only if the combined signals (as a vector) of {a_out,b_out} are equal to “10.” This is an efficient way of determining the case where a_out=1 and b_out=0 (i.e., the signal strength of the data input signal DATA_IN=L). 
   The truth table  20  in  FIG. 2  shows all possible input values for the data input signal DATA_IN (both logic values and associated strengths), and the values of the resultant a_out/b_out nets from the LDETECT circuit  10  of  FIG. 1 . It can be seen from the truth table  20  that there is only one unique combination of a_out/b_out (i.e., “10”) that describes, and hence uniquely detects, a data input signal DATA_IN having a signal strength L. 
   The output of the above-described “continuous assignment” statement is subsequently buffered using another standard Verilog buffer gate (instance b 3 ) to drive the output signal IS_L. The output buffer b 3  is used in case Verilog needs to assign interconnect delays to the reusable Verilog module  30 . In such a case, a Verilog gate, such as a buffer, is required on the output stage of the module  30 . 
   In a second embodiment of the present invention, a logic 1 signal having a signal strength in the range of “small 1 ” up to “pull 1 ” is detected. In this embodiment, as illustrated in  FIGS. 4–6 , a logic 1 signal having a range of signal strengths from “small 1 ” up to “pull 1 ” is referenced as H. A circuit  40  (HDETECT) for detecting a data input signal DATA_IN having strength H is illustrated in  FIG. 4 . A reusable Verilog module  60  corresponding to the HDE-TECT circuit  40  of  FIG. 4  is shown in  FIG. 6 . A truth table  50  illustrating the output IS_H of the HDETECT circuit  40  for all possible input values of the data input signal DATA_IN is shown in  FIG. 5 . Also shown in the truth table  50  are the resultant values of the a_out and b_out nets in the HDETECT circuit  40 . 
   As shown in  FIG. 4 , the data input signal DATA_IN is fed into a unidirectional, isolating, nmos gate nm 1  whose control input is tied to Vdd. As detailed above, the nmos gate nm 1  does not alter the signal strength of the data input signal DATA_IN in any way. A controlled reference signal  42  having a “strong 0 ” signal strength is generated via a standard Verilog buffer gate (instance b 1 ), whose input is tied to ground. The controlled reference signal  42  (i.e., “strong 0 ”) and the output  44  of the nmos gate nm 1  (i.e., the data input signal DATA_IN) are driven into the a_out net. 
   Referring to  FIG. 5 , the resultant values on the a_out net of  FIG. 4  are listed in the truth table  50  for all possible input values of the data input signal DATA_IN. For example, as shown in row  2  of the truth table  50 , the value on the a_out net is equal to 0 when a data input signal DATA_IN having a signal strength H is driven into the a_out net along with a “strong 0 .” This occurs because the signal strength “strong 0 ” is greater the signal strength H in the absolute value sense. 
   The b_out net is derived by buffering the data input signal DATA_IN using another standard Verilog buffer gate (instance b 2 ). If the data input signal DATA_IN is a logic 1 signal of any strength (i.e., H, “strong 1 ”, or “supply 1 ), then b_out=1. In general, b_out is a “strong” version of the data input signal DATA_IN. The a_out and b_out nets are then compared to a predetermined vector value in a vector comparison block  46 . 
     FIG. 6  shows an example of a reusable Verilog module  60  corresponding to the HDETECT circuit  10  of  FIG. 4 . In this example, the vector comparison block  46  is coded with the following statement in Verilog: 
   wire preIS_H=({a_out,b_out}===2′b 01 ). 
   This “continuous assignment” statement returns 1 if and only if the combined signals (as a vector) of {a_out,b_out} are equal to “01.” This is an efficient way of determining the case where a_out=0 and b_out=1 (i.e., the signal strength of the data input signal DATA_IN=H). 
   The truth table  50  in  FIG. 5  shows all possible input values for the data input signal DATA_IN (both logic values and associated strengths), and the values of the resultant a_out/b_out nets from the HDETECT circuit  40  of  FIG. 4 . It can be seen from the truth table  50  that there is only one unique combination of a_out/b_out (i.e., “01”) that describes, and hence uniquely detects, a data input signal DATA_IN having a signal strength H. The output of the “continuous assignment” statement is subsequently buffered using another standard Verilog buffer gate (instance b 3 ) to drive the output signal IS_H. Comparing  FIGS. 1 and 4 , it should be noted that the only differences between the LDETECT and HDETECT circuits  10 ,  40 , are the values of the controlled reference signals  12 ,  42  (“strong 1 ” vs. “strong 0 ” and the resultant {a_out,b_out} key combinations (“10” vs. “01”). In  FIG. 1 , for example, a data input signal DATA_IN having a signal strength L is detected using a controlled reference signal  12  having a signal strength “strong 1 .” In  FIG. 4 , a data input signal DATA_IN having a signal strength H is detected using a controlled reference signal  42  having a signal strength “strong 0 .” In these two cases, the controlled reference signals  12 ,  42 , each comprise a signal having a “strong” signal strength (“strong 1 ” or “strong 0 ) that is “logically opposite” to the range of signals strength (L or H) to be detected. 
   The LDETECT circuit  10  and HDETECT circuit  40  are each used to detect a signal having a range of signal strengths at a given logic value. The present invention, however, may also be used to detect a signal having a specific logic value and signal strength. 
   Referring now to  FIG. 7 , there is illustrated a circuit  70  (PULL 0 DETECT) for detecting a data input signal DATA_IN having a logic 0 value with a “pull 0 ” signal strength. A reusable Verilog module  90  corresponding to the PULL 0 DETECT circuit  70  of  FIG. 7  is shown in  FIG. 9 . A truth table  80  illustrating the output IS_PULL 0  of the PULL 0 DETECT circuit  70  for all possible input values of the data input signal DATA_IN is shown in  FIG. 8 . Also shown in the truth table  80  are the resultant values of the a_out and b_out nets in the PULL 0 DETECT circuit  70 . In this embodiment of the present invention, buffer b 1  outputs a controlled reference signal  72  having a logic value and signal strength (i.e., “pull 1 ”) that is exactly opposite to the signal strength to be detected (i.e., “pull 0 ”). This is achieved by tying the input of buffer b 1  to Vdd and, in the Verilog module  90 , assigning a “pull 1 ” strength to the output of buffer b 1  using the Verilog “buf” logic gate with drive strength specifications: 
   buf (highz 0 ,pull 1 ) b 1  (a_out, 1 ′b 1 ). 
   The data input signal DATA_IN is fed into a unidirectional, isolating, nmos gate nm 1  whose control input is tied to Vdd. The controlled reference signal  72  (i.e., “pull 1 ”) and the output  74  of the nmos gate nm 1  (i.e., the data input signal DATA_IN) are driven into the a_out net. The resultant values on the a_out net of  FIG. 7  are listed in the truth table  80  for all possible input values of the data input signal DATA_IN. For example, as shown in row  2  of the truth table  80 , the value on the a_out net is equal to X (indeterminate) when a data input signal DATA_IN having a signal strength “pull 0 ” is driven into the a_out net along with a controlled reference signal  72  having a signal strength of “pull 1 .” The X logic value is generated because the “pull 0 ” signal strength of the data input signal DATA_IN is equal to the “pull 1 ” signal strength of the controlled reference signal  72  in the absolute value sense. 
   The b_out net is derived by buffering the data input signal DATA_IN using another standard Verilog buffer gate (instance b 2 ). The a_out and b_out nets are then compared to a predetermined vector value in a vector comparison block  76 . 
     FIG. 9  shows an example of a reusable Verilog module  90  corresponding to the PULL 0 DETECT circuit  70  of  FIG. 7 . In this example, the vector comparison block  76  is coded with the following statement in Verilog: 
   wire preIS_PULL 0 =({a_out,b_out}===2′bx 0 ). 
   This “continuous assignment” statement returns  1  if and only if the combined signals (as a vector) of {a_out,b_out} are equal to “X 0 .” The truth table  80  in  FIG. 8  shows all possible input values for the data input signal DATA_IN (both logic values and associated strengths), and the values of the resultant a_out/b_out nets from the PULL 0 DETECT circuit  70  of  FIG. 7 . It can be seen from the truth table  80  that there is only one unique combination of a_out/b_out (i.e., “X 0 ”) that describes, and hence uniquely detects, a data input signal DATA_IN having a signal strength “pull 0 ”. The output of the “continuous assignment” statement is subsequently buffered using another standard Verilog buffer gate (instance b 3 ) to drive the output signal IS_PULL 0 . 
   The analogous case of a circuit  100  (PULL 1 DETECT) for detecting a data input signal DATA_IN having a logic 1 value with a “pull 1 ” signal strength is illustrated in  FIG. 10 . The corresponding truth table  110  and a reusable Verilog module  120  are shown in  FIGS. 11 and 12 , respectively. In this embodiment of the present invention, the controlled reference signal  102  has a signal strength of “pull 0 .” The “pull 0 ” value is provided by tying the input of buffer b 1  to ground and, in the Verilog module  120 , assigning a “pull 0 ” signal strength to the output of buffer b 1  using the Verilog “buf” logic gate with drive strength specifications: 
   buf (pull 0 ,highz 1 ) b 1 (a_out, 1 ′b 0 ). 
   As can be inferred from the above embodiments, the present invention can be easily extended to create “detectors” for any given range of strength values, or any given single, specific logic strength value. There are many possible uses for this invention. One example use is in the area of analog-to-digital (A/D) or digital-to-analog (D/A) conversion in the digital (e.g., Verilog) realm. Intermediate voltages could be represented as varying signal strengths in Verilog, and signal strength detection modules could be used to help encode (or decode) the varying voltage levels which represent an analog signal. Another example of usage of this invention in real-world applications is in the modeling of digital ASIC input/output (I/O) cells.  FIGS. 13–15  illustrate such an application. In this example, a differential I/O having pull down resistors on each of the bidirectional ports to the cell (PAD and PADN) is to be modeled in Verilog. The I/O has both a driver and a receiver section. The driver truth table  130 , illustrated in  FIG. 13 , shows that the pull down resistors are enabled on PAD and PADN when the driver is off (rows a and b in  FIG. 13 ).  FIG. 14 , which illustrates the receiver truth table  140 , highlights a unique requirement in rows g and h. In particular, row g specifies that the receiver output (Z) is “X” when PAD=PADN=0. A different requirement for Z is shown in row h, where Z is 0 when PAD=PADN=L, where L in this case is “pull 0 .” This requires detection of “pull 0 ” signals in the Verilog I/O model. 
     FIG. 15  shows an implementation  150  of this requirement that uses two “PULL 0 DETECT” modules, which were described above with regard to  FIG. 9 . The use of the PULL 0 DECTECT modules greatly simplifies an otherwise moderately complex differential I/O. The PULL 0 DECTECT modules simply return a True/False result indicating whether their input is “L” or not (PAD and PADN in this case). Using this information, the crux of the model is contained in a 4:1 multiplexer (MUX 41 ). The outputs from the PULL 0 DETECT modules driven by PAD and PADN are then fed into the MUX 41  select lines as shown in line  152  of  FIG. 15 , which decode the entries in rows d l of the truth table in  FIG. 14 . All of the possible select line combinations are described below: 
   00: neither PAD nor PADN are “L” the muxOut 1  signal is used which models the logic in rows d through g in  FIG. 14 . 
   01, 10: only one of either PAD or PADN is “L,” but not both at the same time. Rows i through l in  FIG. 14  are covered by these two select line combinations. A constant “X” ( 1 ′bx in the Verilog language) logic value is driven into the data lines to the 4:1 Mux for the 01 and 10 combinations to reflect the fact that the “Z” output is shown to go to “X” in rows i l. 
   11: PAD=PADN=L corresponding to row h in  FIG. 14 . The truth table  140  dictates the “Z” output in this case should be logic 0, hence the 1′b 0  value for the data line corresponding to the “11” position in the MUX 41 . This is in contrast to row g which dictates that when PAD=PADN=0, the Z output should go to X. 
   It is understood that the systems, functions, mechanisms, methods, and modules described herein can be implemented in hardware, software, or a combination of hardware and software. They may be implemented by any type of computer system or other apparatus adapted for carrying out the methods described herein. A typical combination of hardware and software could be a general-purpose computer system with a computer program that, when loaded and executed, controls the computer system such that it carries out the methods described herein. Alternatively, a specific use computer, containing specialized hardware for carrying out one or more of the functional tasks of the invention could be utilized. The present invention can also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods and functions described herein, and which—when loaded in a computer system—is able to carry out these methods and functions. Computer program, software program, program, program product, or software, in the present context mean any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: (a) conversion to another language, code or notation; and/or (b) reproduction in a different material form. 
   While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.