Patent Publication Number: US-2023138651-A1

Title: Integrated test circuit, test assembly and method for testing an integrated circuit

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application 10 2021 128 331.0, filed on Oct. 29, 2021. The contents of the above-referenced Patent Application is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     Exemplary embodiments relate generally to integrated circuits, test assemblies and methods for testing integrated circuits. 
     SUMMARY 
     Integrated circuits, for example microcontrollers (MCUs) such as for vehicles, for instance, have to meet high quality standards. Because of this, they are tested extensively. One area of testing is performance screening. The performance of an integrated circuit is in this case the maximum clock frequency of the integrated circuit in the worst case (that is to say worst-case conditions). A circuit (for example a chip) that does not pass the performance screening is typically eliminated. 
     However, the performance, for example of a microcontroller, depends on many device parameters and environmental conditions. For a direct performance measurement, a comprehensive test at system level would be required in order to test each case of application in which the microcontroller is used. However, microcontrollers are mass-produced products with a high cost pressure, while the test system level is associated with a high degree of outlay and huge test costs. 
     Therefore, test structures are conventionally used to measure the performance indirectly. If an indirect measurement is used to determine a parameter (such as the performance here), the accuracy of the measurement depends greatly on the quality of the test structures. A type of test structure that can be used to achieve a high degree of accuracy is a ring oscillator (ROs), in particular a functional ring oscillator that is formed from gates present in the integrated circuit for the normal functioning thereof. In this case, suitable side inputs for the gates of scan flip-flops are supplied, the side inputs being provided in the integrated circuit (for example to carry out other test and for normal operation) in order that the gates form a logic path. 
     However, the implementation of ring oscillators (even functional ring oscillators) in high numbers in an integrated circuit typically leads to considerable overheads, in particular routing outlay. Therefore, more efficient approaches to testing the performance of integrated circuits are desirable. 
     One exemplary embodiment provides an integrated circuit, comprising at least one ring oscillator circuit, comprising a plurality of logic paths, wherein each logic path comprises a path input connection, a path output connection and an input multiplexer, which has an output connection that is connected to the path input connection of the logic path, wherein each logic path of the logic paths, beginning with a first logic path, is assigned a respective subsequent logic path by virtue of the path output connection of the logic path being connected to a data input connection of the input multiplexer of the subsequent logic path, wherein a last logic path of the logic paths is assigned the first logic path as subsequent logic path and wherein, for each logic path, the multiplexer is set up in such a way that, when a control signal that indicates a test mode is fed thereto, it connects the data input connection of the input multiplexer to the path input connection of the logic path. 
     Further embodiments provide a test assembly and a method for testing an integrated circuit as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The figures do not represent the actual scales but are intended to used to illustrate the principles of the various exemplary embodiments. Various exemplary embodiments are described in detail below with reference to the following figures. 
         FIG.  1    shows an integrated circuit (or a chip) according to one embodiment. 
         FIG.  2    shows a ring oscillator. 
         FIG.  3    shows a circuit for forming a functional ring oscillator in an integrated circuit. 
         FIG.  4    shows the routing outlay on a chip when implementing three functional ring oscillators from three logic paths. 
         FIG.  5    shows an example of a subset of the logic paths of a chip that are sensitized by a single path delay pattern. 
         FIG.  6    shows a natural loop ring oscillator according to one embodiment. 
         FIG.  7    shows the reduction in the routing outlay in the example of  FIG.  4   . 
         FIG.  8    shows an integrated circuit according to one embodiment. 
     
    
    
     DESCRIPTION 
     The following detailed description relates to the appended figures, the details and exemplary embodiments. These exemplary embodiments are described in detail so that a person skilled in the art can execute the invention. Other embodiments are also possible and the exemplary embodiments can be changed in structural, logic and electrical terms without departing from the subject matter of the invention. The various exemplary embodiments are not necessarily mutually exclusive but various embodiments can be combined with one another to produce new embodiments. Within the scope of this description, the terms “linked,” “connected,” and “coupled” are used to describe both a direct and an indirect link, a direct or indirect connection, and direct or indirect coupling. 
       FIG.  1    shows an integrated circuit (or a chip)  100  according to one embodiment. 
     By way of example, the integrated circuit  100  is a microcontroller, for example for an ECU (electronic control unit) in a vehicle or else a chip card module for a chip card of any form factor. 
     As is typically the case, the integrated circuit  100  comprises a multiplicity of logic gates  101  (AND gates, NOR gates, exclusive OR gates, inverters, etc.) that are connected to one another via connecting lines. The logic gates  101  are cells from a chip design library and they may also be more complex circuits (for example complex gates). 
     The integrated circuit also comprises flip-flops  103  that are connected to the logic gates  101 . At least a portion of the flip-flops  103  are provided as scan flip-flops in order to be able to load test patterns for testing the integrated circuit into the scan flip-flops. A scan flip-flop is a D flip-flop with a multiplexer added at the input connection, wherein an input connection of the multiplexer functions as functional input connection D and the other input connection is used as scan-in input connection (SI). The test pattern is pushed into the flip-flops  103  (in each case via the scan-in input connection) for example by way of one or more test input pins  102 . A scan enable signal (or test enable signal, not shown in  FIG.  1   ) switches the multiplexer of the scan flip-flop for the testing from the data input connection (D) to the scan-in input connection (SI). The scan enable signal is a general signal that is the same for all scan flip-flops (it therefore does not need to be routed separately). 
     One possibility for measuring the performance is to use a chain of logic gates  101  (or generally cells) that are already present in the integrated circuit  100  to form a ring oscillator. 
       FIG.  2    shows a ring oscillator  200 . 
     The ring oscillator  200  comprises a chain of logic gates  201 ,  202 ,  203  (generally cells) connected in series. Each logic gate  201 ,  202 ,  203  comprises an input connection and an output connection using which said gate is interconnected in the chain, wherein the output connection of the last logic gate  203  of the chain is connected to the input connection of the first logic gate  201  via a feedback line (or feedback connection)  204 . The other input connections of the logic gates  201 ,  202 ,  203  (for example the second input connection of a NAND gate or the second input connection of a NOR gate), subsequently referred to as side input connections, are set to a fixed value so that each logic gate  201 ,  202 ,  203  forms an inverter with respect to the input connection and output connection using which said gate is interconnected in the chain. If the number of logic gates N is uneven, the chain thus has an overall inverting effect and the loop formed by the feedback line  204  together with the chain oscillates. 
     The inputs for the side input connections of the logic gates  201 ,  202 ,  203  that cause them to function as inverters are referred to as side inputs. The side inputs together form a side input pattern. The side inputs are supplied by scan flip-flops  103  that are suitably loaded (through storage of a suitable test pattern that contains the side input pattern in the scan flip-flops). If it is not possible for a gate with a suitable side input to be made to function as an inverter (for example an AND gate), the side input is selected so that it has a non-inverting effect (that is to say simply as a buffer) and either the number of logic gates  201 ,  202 ,  203  is adjusted or an inverter is provided in the feedback line, with the result that an inverting response results again overall and the loop oscillates. 
     The frequency of this oscillation, that is to say the oscillation frequency of the ring oscillator formed in this way, can be observed and used to test the performance of the integrated circuit  100 . The quality of the testing depends on the information content of the oscillation frequency of the ring oscillator  100 , that is to say it must represent the timing behavior (and thus the performance) of the entire chip as far as possible. However, the oscillation frequency typically correlates well with the performance of the integrated circuit, in particular when the chain of logic gates that is formed corresponds well to the design of the chip. Nevertheless, a high number of ring oscillators is typically required. 
     The ring oscillator described with reference to  FIG.  2    is what is known as a functional ring oscillator, that is to say a functional path ring oscillator, that is to say it is formed as mentioned above from logic gates  101  that are present in the integrated circuit (for the normal functioning thereof, that is to say normal operation in contrast to test operation). This prevents the ring oscillators leading to additional outlay in terms of area and energy, as is the case when they are formed from additional logic gates (that is to say not from logic gates that are present anyway). 
       FIG.  3    shows a circuit for forming a functional ring oscillator in an integrated circuit. 
     As is described with reference to  FIG.  2   , a combinational logic path  302  (that is to say a functional path) is formed by a chain of logic gates that are suitably supplied with side inputs. The feedback is effected via a feedback line  303  (which is provided with an inverter if the combinational logic path  302  is not self-inverting) that is connected to an input connection of an (input) multiplexer  304 . 
     As explained above, the basic idea of a functional ring oscillator can be considered that of using a functional combinational logic path  302  that is present for the normal functioning of the integrated circuit (that is to say as per design) in order to form the ring oscillator. 
     The multiplexer  304  at the input connection of the path makes it possible to switch over from the functional mode (that is to say the use of the logic path  302  for normal operation, in this case input “0” of the multiplexer) to the oscillation mode in which the multiplexer  304  feeds the signal of the feedback line to the logic path  302  (in this case input “1” of the multiplexer  304 ). For normal operation, the multiplexer  304  connects for example an input signal from an input-side flip-flop  306  (that may also be from a memory or register) through to the logic path  302 . The output of the logic path  302  goes to an output-side flip-flop  307  (for example of a memory or register). The input-side flip-flop  306  is also referred to as a launch flip-flop. 
     The oscillation frequency of the ring oscillator can be observed via a measurement terminal  305 . 
     As explained above, suitable side inputs are applied to the logic gates that form the logic path  302  in order to form the logic path  302 . To this end, an industrial automatic test pattern generation (ATPG) tool can be used in path delay mode. The ATPG tool is executed on a test apparatus (that is to say a test computer) and provides test patterns to the integrated circuit via the test pin  102 . The ATPG tool thus sensitizes the logic path  302  by setting all side inputs to stable values. 
     In this way, the testing by means of functional ring oscillators can be easily integrated into a conventional industrial test procedure by applying design for testing (DFT) methods. 
     The functional ring oscillators represent the actual chip behavior well without producing much overhead in terms of area. Only the multiplexer  304  and the feedback line  303  are additionally required to form the ring oscillator. 
     Each functional ring oscillator on the chip can be activated by an individual, central control signal  308  (“enable” signal) that is fed in the integrated circuit to the multiplexer  304 . This control signal is individual for the functional ring oscillator since not all of the ring oscillators are to be activated at the same time during testing. There is therefore a high degree of routing outlay for the lines for the control signal  308  in this procedure. This can be prevented by an architecture in which the ring oscillators activate themselves. In this case, a portion of the scan flip-flops present in a chip is used to activate ring oscillators, wherein different scan flip-flops for activation are used for different ring oscillators. This removes the need for a control connection to a central control device on the chip. 
     However, further routing outlay is produced by virtue of the fact that the observation signal has to be routed from an output connection  305  for the frequency measurement at a GPIO pin via the chip to the GPIO pin and for the feedback line  303  that connects the end point (output connection) to the starting point (input connection) of the logic path  302 . 
       FIG.  4    illustrates the routing outlay on a chip  400  when implementing three functional ring oscillators from three logic paths  401 ,  402 ,  403  through the lines for the enable signal  404 , the lines for the observation signals  405  and the feedback lines  406 . The multiplexers at the input connections of the logic paths  401 ,  402 ,  403 , to which the lines for the enable signal  404  and the feedback lines  406  lead, are not illustrated for the sake of simplicity. 
     In this example, the lines for the enable signal  404  and the lines for the observation signals  405  are both connected to a central control unit  407 . The lines for the observation signals  405  can also lead to an output connection pin, which is arranged at a different location to the central control unit  407 . However, the routing outlay remains similar in this case. 
     In the case of few ring oscillators on a chip, the routing outlay plays a small role. On account of the process variation, in particular the variation within a chip, in modern CMOS technologies, however, a lot of test structures that are distributed over the chip are typically required in order to cover the fluctuations due to the process variations and to detect the overall performance of the chip. However, the addition of more, for example hundreds, of ring oscillators on a chip results in a high routing outlay. Furthermore, the test duration has to be short in order to reduce the test costs. 
     Ultimately, there is a compromise between the number of test structures (that is to say in this case the ring oscillators) and the high routing outlay for the test structures. 
     In particular, the feedback lines  406  produce a considerable amount of routing outlay, particularly for logic paths  401 ,  402 ,  403  that extend over long distances to the chip  400 . In this case, furthermore, additional buffers are typically required in order to achieve suitable edge gradients. Depending on the length of the feedback lines, this leads to additional required logic circuits. 
     Various embodiments provide a procedure that reduces the routing outlay of functional ring oscillators. 
     According to various exemplary embodiments, this is achieved by virtue of two or more logic paths  401 ,  402 ,  403  being combined so that a functional ring oscillator is formed without a long feedback line being required. This clearly takes place by virtue of (at least a large part of) a feedback line being replaced by a logic path and thus not being additionally required. That is to say, logic paths clearly form both the outgoing and return direction in such a ring oscillator, which is also referred to in the following text as “natural loop” ring oscillator, that is to say as a ring oscillator that is formed by a loop whose component parts are “natural”, that is to say are present on the chip anyway according to the function provided for the chip. The routing outlay can be further reduced by this approach being combined so that self-activation of the functional ring oscillators is provided. 
     As mentioned above, a logic path  302  is sensitized by means of an ATPG tool by virtue of all of the side inputs of the logic gates that form the logic path  302  being set to a static value, with the result that an oscillation signal can be produced in or can pass through the ring oscillator that is formed by the logic path. This sensitization is carried out by means of a robust path delay pattern, which is loaded (for example shifted) into a set of scan flip-flops by the ATPG tool. 
     The range of such ATPG path delay patterns is typically large enough so that sufficient degrees of freedom are present so that all of the side inputs for logic paths that are used to form functional ring oscillators can be selected suitably (and do not contradict one another for example for a ring oscillator). An ATPG path delay pattern that is selected to sensitize a logic path for a ring oscillator on a chip typically sensitizes several logic paths on the chip. It is then possible to make a selection from all of these sensitized logic paths, in particular in order to form a natural loop, that is to say to replace at least a portion of the feedback for the logic path for the sensitization of which the ATPG path delay pattern has actually been selected. 
       FIG.  5    shows an example  500  of a subset of the logic paths of a chip that are sensitized by a single ATPG path delay pattern. 
     As can be seen in the example of  FIG.  5   , the logic paths from which a selection can be made are distributed over the chip and have different lengths. 
     In accordance with one embodiment, in the simplest case, use is made of two logic paths that are sensitized by the same ATPG path delay pattern and that run in a different direction to the one illustrated in  FIG.  6   . 
       FIG.  6    shows a natural loop ring oscillator  600  according to one embodiment. 
     The natural loop ring oscillator  600  has a first (combinational) logic path  601  and a second (combinational) logic path  602 , these each being formed by a chain of logic gates that are suitably supplied with side inputs (that is to say so that the logic paths  601 ,  602  are sensitized). 
     A respective multiplexer  603 ,  604  is provided on the input side for each logic path  601 ,  602 , said multiplexer making it possible to switch over from the function mode (that is to say the use of the logic path for normal operation, in this case input “0” of the multiplexer) to the oscillation mode. 
     For normal operation, the multiplexer  603 ,  604  connects an input signal from a respective input-side (launch) flip-flop  605 ,  606  (that may also be from a memory or register) through to the logic path  601 ,  602 . The output of each logic path  601 ,  602  goes to a respective output-side flip-flop  607 ,  608  (for example of a memory or register). 
     The output connection of the first logic path  601  is connected to the data input connection of the input multiplexer  604  of the second logic path  602  that the multiplexer  604  connects through (in this case input “1” of the multiplexer  604 ) in the oscillation mode (that is to say in the test mode). Analogously, the output connection of the second logic path  602  is connected to the data input connection of the input multiplexer  603  of the first logic path  601  that the multiplexer  603  connects through (in this case input “1” of the multiplexer  603 ) in the oscillation mode. A loop is thus formed in the oscillation mode, wherein the two directions of the loop are formed by the two logic paths  601 ,  602 . 
     The formation of ring oscillators by means of natural loops can significantly reduce the routing outlay. 
       FIG.  7    shows the reduction in the routing outlay in the example of  FIG.  4   . 
     As is described with reference to  FIG.  6   , a natural loop ring oscillator composed of two logic paths  701 ,  702  on a chip  700  is formed (the two logic paths corresponding to the logic paths  401 ,  402  of  FIG.  4    on the chip  400 ). 
     Instead of the feedback lines  406  for the two logic paths  401 ,  402 , only short lines  706  are required to connect the output connection of the first logic path  701  to the input multiplexer of the second logic path  702  and to connect the output connection of the second logic path  702  to the input multiplexer of the first logic path  701 . As in  FIG.  4   , the multiplexers at the input connections of the logic paths  701 ,  702 ,  703  are not illustrated for the sake of simplicity. 
     Furthermore, in this example, the observation line  405  from the output connection of the first logic path  401 ,  701  to the central control unit  407 ,  707  is omitted. The observation line  705  can be provided at a suitable point of the entire ring oscillator, for example so that the routing outlay for this remains low. The lines for the enable signal  704  to the central control unit  707  can be avoided by switching the input multiplexer by means of self-activation. 
     The procedure of forming functional ring oscillators by means of natural loops that is described above is advantageous particularly for long logic paths, which would require a long feedback line and many buffers along the feedback line. It should be noted that a natural loop can also be formed from more than two logic paths when it is advantageous, from a routing outlay point of view, to connect three or more logic paths. 
     The formation of functional ring oscillators by means of natural loops also makes it possible to average the arising voltage drop, which may distort the measurement. The oscillation frequency is measured specifically in a test mode in which it is possible for an additional voltage drop to arise between chip partitions that are active only in the test mode. The natural loops average the influence of the voltage drop and reduce the errors caused thereby. 
     Furthermore, the oscillation frequency is reduced when two or more logic paths are combined with one another. There is a maximum frequency that the chip physically supports as per design. The oscillation frequency of ring oscillators should accordingly lie below these physical limits. When logic paths having a short delay are used, the oscillation frequency is very high, however. Since the use of natural loops means that the ring oscillators have a longer logic path (in the example of  FIG.  6   , the first logic path  601  plus the second logic path  602 ), the oscillation frequency is reduced and also short logic paths that are part of a natural loop can be monitored. 
     The procedure of forming natural loops on logic paths is also able to be applied to bus connections of a data bus on the chip when the bus has a unidirectional line in both directions. A data bus transmits data signals via the chip. These data bus signals can also dominate the maximum performance of the chip. A bus with unidirectional lines has an outgoing line (transmission line) and a return line (reception line). Therefore, a natural line ring oscillator can be formed by virtue of the two lines being connected in accordance with the procedure described above. In this case, the transmission line and the reception line (that consist of chains of buffers, for example) are used to form a loop. 
     In summary, according to various embodiments, an integrated circuit (for example a chip) is provided, as is illustrated in  FIG.  8   . 
       FIG.  8    shows an integrated circuit  800 . 
     The integrated circuit comprises at least one ring oscillator circuit  801 . 
     The ring oscillator circuit  801  comprises a plurality of logic paths  802 , wherein each logic path comprises a path input connection  803 , a path output connection  804  and an input multiplexer  805 , which has an output connection  806  that is connected to the path input connection  803  of the logic path. 
     Each logic path of the logic paths  802 , beginning with a first logic path, is assigned a respective subsequent logic path by virtue of the path output connection  804  of the logic path being connected to a data input connection  807  of the input multiplexer  805  of the subsequent logic path, wherein a last logic path of the logic paths is assigned the first logic path as subsequent logic path. 
     The input multiplexer  805  is set up in such a way that, when a control signal  808  that indicates a test mode is fed to it (at the control input connection thereof), it connects the data input connection  807  to the path input connection  803  of the logic path (with which it is associated). 
     In other words, a ring oscillator is formed by virtue of a plurality of logic paths being connected one behind the other in the test mode so that a loop is produced (and thus no feedback line is required). 
     In the normal operating mode (that is to say the non-test mode), the logic paths that are connected one behind the other in the test mode are decoupled, for example. In other words, logic paths that are not (at least not directly) connected in the normal operating mode, that is to say do not (directly) interact, are connected one behind the other for the testing, that is to say none of the logic paths obtains a result of a processing of another logic path. The logic paths are bit-logic paths according to various embodiments. 
     As explained above, the procedure of  FIG.  8    makes it possible to reduce the routing outlay (which is important in particular for a high number of ring oscillators and thus improves the scalability, that is to say enables a higher number of test structures), to average the voltage drop and to keep the arising oscillation frequencies low, with the result that more functional logic paths can be monitored. 
     Various exemplary embodiments are stated below. 
     Exemplary embodiment 1 is an integrated circuit, as described with reference to  FIG.  8   . 
     Exemplary embodiment 2 is an integrated circuit according to exemplary embodiment 1, wherein each logic path comprises a chain of a plurality of logic gates connected one behind the other and/or comprises a data bus line. 
     Exemplary embodiment 3 is an integrated circuit according to exemplary embodiment 1 or 2, wherein the input multiplexer comprises another data input connection and is set up so that, when a control signal that indicates a normal operating mode is fed thereto, it connects the other data input connection to the path input connection of the logic path. 
     Exemplary embodiment 4 is an integrated circuit according to one of exemplary embodiments 1 to 3, wherein the input multiplexer is a 2-to-1 multiplexer. 
     Exemplary embodiment 5 is an integrated circuit according to one of exemplary embodiments 1 to 4, wherein the at least one ring oscillator comprises a test output connection and the integrated circuit comprises a test terminal to which the test output connection is connected. 
     Exemplary embodiment 6 is an integrated circuit according to exemplary embodiment 5, wherein the test output connection is arranged between one of the logic paths and the following logic path. 
     Exemplary embodiment 7 is an integrated circuit according to one of exemplary embodiments 1 to 6, wherein each logic path of the plurality of logic paths comprises a chain of a plurality of logic gates connected one behind the other and wherein the at least one ring oscillator circuit is assigned one or more scan flip-flops, which are connected to the input connections of at least a portion of the logic gates of the chains of logic gates of the ring oscillator circuit in such a way that, when they store a predefined side input pattern, for each logic path of the ring oscillator circuit, the logic gates of the chain of the logic path form a serial 1-bit logic path from an input connection of the first logic gate of the chain of the logic path to the output connection of the last logic gate of the chain of the logic path. 
     Exemplary embodiment 8 is an integrated circuit according to one of exemplary embodiments claims  1  to  7 , wherein each logic path is assigned a scan flip-flop group, which contains at least one scan flip-flop, wherein the input multiplexer of the logic path is controller in a manner depending on a bit stored by the at least one scan flip-flop of the scan flip-flop group assigned to the logic path in such a way that it connects the data input connection of the input multiplexer to the path input connection of the logic path. 
     Exemplary embodiment 9 is an integrated circuit according to one of exemplary embodiments 1 to 8, comprising a multiplicity of ring oscillator circuits, wherein each ring oscillator circuit comprises: a plurality of logic paths, wherein each logic path comprises a path input connection, a path output connection and an input multiplexer, which has an output connection that is connected to the path input connection of the logic path, wherein each logic path of the logic paths, beginning with a first logic path, is assigned a respective subsequent logic path by virtue of the path output connection of the logic path being connected to a data input connection of the input multiplexer of the subsequent logic path, wherein a last logic path of the logic paths is assigned the first logic path as subsequent logic path, wherein, for each logic path, the multiplexer is set up in such a way that, when a control signal that indicates the test mode is fed thereto, it connects the data input connection of the input multiplexer to the path input connection of the logic path. 
     Exemplary embodiment 10 is an integrated circuit according to exemplary embodiment 9, wherein each logic path of each ring oscillator circuit is assigned a scan flip-flop group, which contains at least one scan flip-flop, wherein the input multiplexer of the logic path is controller in a manner depending on a bit stored by the at least one scan flip-flop of the scan flip-flop group assigned to the logic path in such a way that it connects the data input connection of the input multiplexer to the path input connection of the logic path, wherein the logic paths of different ring oscillator circuits are assigned different scan flip-flop groups. 
     Exemplary embodiment 11 is a test assembly for testing an integrated circuit according to exemplary embodiment 9 or 10, which is set up, for each ring oscillator circuit and each logic path, to feed the control signal that indicates the test mode to the input multiplexer of the logic path of the ring oscillator circuit and to receive measurement signals generated by the ring oscillator circuits in the test mode. 
     Exemplary embodiment 12 is a test assembly according to exemplary embodiment 11, comprising an analysis device that is set up to ascertain a performance of the integrated circuit from the oscillation frequencies of the measurement signals received. 
     Exemplary embodiment 13 is a method for testing an integrated circuit according to exemplary embodiment 9 or 10, comprising: for each ring oscillator circuit and each logic path, feeding the control signal that indicates the test mode to the input multiplexer of the logic path of the ring oscillator circuit, and receiving measurement signals generated by the ring oscillator circuits in the test mode. 
     Although the invention has been shown and described primarily with reference to specific embodiments, it should be understood by those familiar with the technical field that numerous modifications can be made with regard to configuration and details thereof, without departing from the essence and scope of the invention as defined by the claims hereinafter. The scope of the invention is therefore determined by the appended claims, and the intention is for all modifications to be encompassed which come under the literal meaning or the scope of equivalence of the claims.