Abstract:
An integrated circuit is disclosed having at least one digital input, which has a first circuit section that has a current-voltage characteristic and that in the absence of an input signal holds a voltage at the input at a defined value, and having a second circuit section that provides a signal that is internal to the circuit and whose state does not directly show itself at an output of the circuit. The first circuit section has a control input for a control signal and is designed to change its current-voltage characteristic when the control signal is present at the control input. In addition, a method for testing such a circuit is disclosed.

Description:
[0001]     This nonprovisional application claims priority under 35 U.S.C. § 119(a) on German Patent Application No. DE 102005052269, which was filed in Germany on Oct. 27, 2005, and which is herein incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an integrated circuit having at least one digital input and having a first circuit section that has a current-voltage characteristic and that in the absence of an input signal holds a voltage at the input at a defined value, and having a second circuit section that provides a signal that is internal to the circuit and whose state does not directly show itself at an output of the circuit. The invention further relates to a method for testing such an integrated circuit.  
         [0004]     2. Description of the Background Art  
         [0005]     In this regard, a digital input is understood to be an input with a threshold switch that does not forward an input signal to an internal signal processing unit until the input signal exceeds a predefined threshold. When no input signal is provided to the input, electromagnetic influences can in principle cause voltage variations at the input that exceed the threshold, this, however, is undesirable. The aforementioned circuit section serves to avoid the undesirable states. For example, the circuit section has a current sink that is able to draw a current with a predefined maximum value from the input. The threshold cannot be exceeded until a current flowing into the input is greater than the current of the current sink. Such a current sink, also known as a pull-down current source, holds the voltage at the input to a value below the threshold. In an analogous manner, a pull-up current source holds the voltage above the threshold.  
         [0006]     In the operation of integrated circuits, changes occur in the states of internal signals that are not directly reflected in the behavior of the input signals and output signals of the integrated circuit. The question of whether and, if applicable, at what point in time such a state change occurs can be important for functional testing at the end of a manufacturing process. In principle, an internal signal can be provided with its own terminal at which the internal signal can be tapped for test purposes. For a given circuit, however, the number of possible terminals is already limited by considerations of space. Moreover, each terminal constitutes an undesirable input for interfering influences such as ESD pulses.  
         [0007]     Another option for evaluating an internal signal resides in the analysis of the input/output signal behavior of the integrated circuit. However, it is problematic in this regard that internal signals may under some circumstances only be reflected in a regular output signal very indirectly and with a long time delay, so that internal signals can only be detected incompletely or with a large expenditure of time in measurement, which can result in a reduced production rate, in particular in the case of final testing on a production line.  
         [0008]     A circuit and a method are known from DE 100 64 478 A1, which corresponds to U.S. Pat. No. 6,937,048, which is incorporated herein by reference. This document addresses the problem that measurement pad areas within the circuit, and additional pins that are needed for external measurement of signals, occupy a substantial portion of the total circuit area, particularly for highly integrated circuits. Due to the large fraction of the overall chip area, the resulting fraction of the total cost of a circuit is substantial, which has an adverse economic impact. To remedy this, it is proposed therein for the signals generated by a circuit unit within an integrated circuit which are not measurable at the outputs in normal operation to be switched to the existing signal outputs as test signals for functional testing. To this end, while the supply voltage is present, a specific voltage is applied to at least one signal output of the integrated circuit, thus switching the integrated circuit into a test mode.  
         [0009]     However, in the conventional art, when an internal test signal is switched to an output, the regular output signal cannot be obtained at the same time. Tests in which the internal signal and the regular output signal are required can thus only be performed one after the other, increasing the test time.  
       SUMMARY OF THE INVENTION  
       [0010]     It is therefore an object of the present invention to provide an integrated circuit that permits testing of the states of internal signals without additional terminals and with a reduced expenditure of time in measurement.  
         [0011]     A first circuit section is provided that has a control input for a control signal and is designed to change its current-voltage characteristic when the control signal is present at the control input. In addition, an electrical test signal can be applied to the at least one input and a time behavior of the current-voltage characteristic is evaluated for diagnosis.  
         [0012]     The circuit section is coupled to an input which is present in any case. Its current-voltage characteristic is thus in principle detectable through the input. By changing the current-voltage characteristic as a function of the internal signal, the change of state of the internal signal can be reflected in a signal behavior at the input and can be read out without additional terminals.  
         [0013]     The signal internal to the circuit can be the control signal. This is especially simple to implement in circuitry. Accordingly, the corresponding method is simple to carry out.  
         [0014]     The control signal can be the result of an AND combination of an enable signal with the signal internal to the circuit. As a result of such an enable signal, it is possible to determine when the change in the current-voltage characteristic is permitted. This is helpful when the change could affect normal operation.  
         [0015]     Another embodiment provides that the digital input can have a first pull-down current source that draws a first current from an input of a threshold switch, and for the circuit section to have a switchable pull-down current source that is switched by the signal internal to the circuit and draws a second current from the input of the threshold switch.  
         [0016]     Alternatively, the digital input can have a first pull-up current source that supplies a first current to an input of a threshold switch, and the circuit section can have a switchable pull-up current source that is switched by the signal internal to the circuit and supplies a second current to the input of the threshold switch.  
         [0017]     Additional alternatives provide for the digital input to have a first pull-down resistor (pull-up resistor) that is located between an input of a threshold switch and a first (second) reference voltage, and for the circuit section to have a switchable pull-down resistor (pull-up resistor) that is switched by the signal internal to the circuit and is located between the input of the threshold switch and the first (second) reference voltage.  
         [0018]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitive of the present invention, and wherein:  
         [0020]      FIG. 1  illustrates an integrated circuit according to an embodiment of the invention;  
         [0021]      FIG. 2  shows a timing diagram of signals from the embodiment illustrated in  FIG. 1 ;  
         [0022]      FIG. 3  illustrates an embodiment of the circuit which works with an enable signal; and  
         [0023]      FIG. 4  illustrates an embodiment of a first circuit section with pull-down resistors. 
     
    
     DETAILED DESCRIPTION  
       [0024]      FIG. 1  shows a portion of an integrated circuit  10 , with a digital input  12 . Of course, in general the integrated circuit  10  has additional digital inputs. The digital input  12  has an input pad  14  through which signals are fed into the integrated circuit  10 , a threshold switch  16 , and a pull-down current source  18 . The threshold switch  16  is generally implemented as a Schmitt trigger. Input signals present at the input pad  14  that exceed the threshold of the threshold switch  16  are transmitted to a digital section  20  of the integrated circuit  10 . The digital section  20  processes the input signals into one or more digital output signals while taking into account other signals. Such a digital output signal can be converted into an analog signal by means of the analog-to-digital converter  22 , and can be transmitted to an output pad  24 .  
         [0025]     In the embodiment shown in  FIG. 1 , the digital output signal is formed while, in particular, taking into account the output signal of the threshold switch  16  and a signal S_i internal to the circuit that is output by an analog section  26 . The analog section represents an example of a second circuit section. The signal S_i internal to the circuit is an example of a signal that arises in the interior of the integrated circuit  10 , but is not directly obtainable at one of the input pads or output pads of the integrated circuit  10 . In the embodiment shown in  FIG. 1 , the signal S_i internal to the circuit is, for example, then formed with a certain time delay when a predefined signal occurs at an analog input  28  of the integrated circuit  10 . The integrated circuit  10  also has a switchable pull-down current source  30 .  
         [0026]     While the pull-down current source  18  continuously draws a first current from a node  32  of the digital input  12 , a second current that the second pull-down current source  30  can draw is only drawn from the node  32  when the pull-down current source  30  is switched on. Taken together, the two pull-down current sources  18  and  30  connected together through the node  32  constitute a first circuit section  34  with a variable current-voltage characteristic that holds a voltage at the input  12 , which is to say at the input pad  14  or node  32 , at a defined value in the absence of an input signal.  
         [0027]     If an input signal with a constant voltage u is applied to the input pad  14 , the signal current from the input pad  14  to the node  32  changes as a function of whether the switchable current source  30  is switched on or off. In this regard, the first circuit section  34  possesses the aforementioned variable current-voltage characteristic. The first circuit section  34  has a control input  36  for a control signal with which the switchable pull-down current source  30  can be switched on. In the embodiment shown in  FIG. 1 , the aforementioned signal S_i internal to the circuit serves as the control signal for the first circuit section  34 .  
         [0028]      FIG. 2   a  shows a time behavior of a voltage signal  38  at the input pad  14 , such as can be produced by connecting a constant voltage source to the input pad  14 . At time t_ 0 , the constant voltage source is switched on. Then a current i flows from the constant voltage source into the digital input  12  to the node  32 . The current i exactly replaces the flows into the threshold switches  16  and pull-down current source  18 . The switchable pull-down current source  30  is initially switched off here because no control signal is yet present at the control input  36  of the first circuit section  34 . The current i then assumes a constant value, for example, such as is qualitatively represented in  FIG. 2   c  between the times t_ 0  and t_ 1 .  
         [0029]     Then, at time t_ 1 , the signal S_i internal to the circuit appears within the integrated circuit  10 . In the embodiment from  FIG. 1 , the signal S_i is output by the analog section  26 . However, the signal S_i can also be a signal that is formed within the digital section  20  of the integrated circuit  10 . As an internal signal, this signal S_i does not appear at inputs or outputs of the integrated circuit  10 , so it cannot be detected directly. In order to make detection possible nevertheless, the internal signal S_i is used in the embodiment from  FIG. 1  to switch on the switchable pull-down current source  30  by the control input  36  of the first circuit section  34 . Consequently, the switchable pull-down current source  30  is switched on at time t_ 1  when the signal S_i internal to the circuit occurs. The occurrence of the internal signal S_i within the circuit at time t_ 1  is shown in  FIG. 2   c.    
         [0030]     The switching on of the switchable pull-down current source  30  causes the voltage characteristic of the first circuit section  34  to change. The switchable pull-down current source  30  draws an additional current from the node  32  of the digital input  12 . The constant voltage source connected to the input pad  14  (has to) provide this additional current flowing out of the node  32  in order to keep the voltage at the input pad  14  constant. The constant voltage source must therefore supply a higher current starting at the time t_ 1  than in the period between t_ 0  and t_ 1 . This higher current flow through the input pad  14  is measurable and is qualitatively represented in  FIG. 2   c  through the step increase  40  at the time t_ 1 . The occurrence of the step in the current signal i thus indicates the occurrence of the signal S_i internal to the integrated circuit.  
         [0031]     The first circuit section  34  can thus also be viewed as a circuit section to aid in testing. The variation of the input current resulting from switching on of the switchable pull-down current source  30  is not normally problematic as long as the specified values for the circuit are maintained. In an embodiment from  FIG. 1 , sections of which are shown in  FIG. 3 , the variation of the input current can additionally be influenced by an enable signal S_f. In the embodiment from  FIG. 3 , a control signal S that switches on the switchable pull-down current source  30  is only output when the signal S_i from the analog section  26  that is present at the control input  36  and the enable signal S_f that, e.g., is output from the digital section  20 , are present at the same time. In  FIG. 3 , this is represented by the AND combination  42 .  
         [0032]     It is a matter of course that the first circuit section  34  can also have pull-up current sources in place of pull-down current sources  18  and  30 . Another alternative is shown in  FIG. 4 .  FIG. 4  shows an alternative first circuit section  34  with a pull-down resistor  44 , which replaces the current source  18  from  FIG. 1 . In this case, the switchable pull-down current source  30  from  FIG. 1  is replaced by a switchable pull-down resistor  46 , as is also shown in  FIG. 4 . The pull-down resistors  44 ,  46  can also be replaced by pull-up resistors.  
         [0033]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.