Abstract:
An integrated circuit is provided comprising: a serial transmitter, a serial receiver and a serial connection providing communication between the serial transmitter and the serial receiver. The integrated circuit further comprises a duty cycle distortion circuit so that the integrated circuit can be stress tested by distorting the duty cycle of a signal within the integrated circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to stress testing integrated circuits. More particularly, this invention relates to stress testing integrated circuits in which serial data is communicated by altering the duty cycle of a signal within that integrated circuit. 
         [0003]    2. Description of the Prior Art 
         [0004]    It is known to perform tests on integrated circuits comprising serial receivers which receive serialized data from serial transmitters over high speed serial links. Such testing, for example shortly after manufacturing, involves artificially downgrading the quality of the signal sent over the serial link in order to test how well the serial receiver copes with that poor quality data and whether it can still recover the original data that was transmitted. A receiver&#39;s ability to cope with such distorted signals is referred to as its jitter tolerance. 
         [0005]    Typically, a external programmable signal generator is used to provide the source of the variation in the serial data. The distorted signal used for testing jitter tolerance is commonly referred to as a stressed eye. Such a stressed eye may comprise variation in the duty cycle of a transmitted signal. Currently, the primary method for testing jitter tolerance is to create a stressed eye off-chip using high speed test equipment. Such equipment may, for example, create a serial data stream to be received by a serial receiver in which the duty cycle is distorted. 
         [0006]    Such high speed test equipment is expensive and complex and hence it is desirable to provide an improved technique for testing integrated circuits. 
       SUMMARY OF THE INVENTION 
       [0007]    Viewed from a first aspect, there is provided an integrated circuit comprising: a serial transmitter; a serial receiver; a serial connection providing communication between said serial transmitter and said serial receiver; and a duty cycle distortion circuit configured to distort a duty cycle of a signal within said integrated circuit so as to stress test said integrated circuit. 
         [0008]    According to the techniques of the present invention, the creation of a stressed eye for stress testing an integrated circuit is enabled using components which are located on-chip, hence avoiding the need for costly off-chip testing equipment. A duty cycle distortion circuit distorts a duty cycle of a signal within the integrated circuit. This distortion of the duty cycle of the signal creates a stress with which the integrated circuit must cope, if it is to continue operating as it would if no distortion had been applied. The integrated circuit&#39;s ability to cope with imperfect signals is thus tested. Hence, a simpler arrangement for stress testing the integrated circuit is provided, which makes use of hardware that mostly already exists in an integrated circuit comprising a serial transmitter, serial receiver and serial connection between them. Thus, with a small amount of additional hardware a simple and easily programmable duty cycle distortion generation circuit for self-testing on-chip is provided. 
         [0009]    The distortion of the duty cycle of the signal may be used to test various different components of the integrated circuit, but in one embodiment said duty cycle distortion circuit distorts said duty cycle to degrade said communication so as to stress test said serial receiver. By degrading the communication over the serial connection in this manner, the serial receiver&#39;s ability to cope with real-world, less-than-ideal signals being transmitted over the connection is tested. 
         [0010]    In another embodiment said duty cycle distortion circuit distorts said duty cycle to stress test a duty cycle correction circuit. If the duty cycle of a signal exceeds the tolerances of components of the integrated circuit which receive that signal, then errors in the operation of the integrated circuit will arise. For this reason a duty cycle correction circuit may be provided to correct for departures from an ideal duty cycle, and the duty cycle distortion circuit of this technique can be used to test the performance of that duty cycle correction circuit. 
         [0011]    In order to stress test the integrated circuit the duty cycle distortion circuit may form part of various different components of the integrated circuit. In one embodiment the duty cycle distortion circuit forms part of said serial transmitter. In another embodiment the said duty cycle distortion circuit forms part of said serial receiver. In another embodiment the duty cycle distortion circuit forms part of a clock generation circuit. In another embodiment the duty cycle distortion circuit forms part of a buffer. 
         [0012]    In one embodiment where the duty distortion circuit forms part of a buffer, the buffer is a differential current mode logic buffer. 
         [0013]    It will be appreciated by those skilled in this art that the duty cycle distortion circuit could operate in a variety of ways, but in one embodiment the duty cycle distortion circuit is controlled by a duty cycle distortion control circuit. This provides an advantageously simple manner for the operation of the duty cycle distortion circuit to be programmable, such that the operator can stress test the integrated circuit as fully as the application requires. 
         [0014]    When the duty cycle distortion circuit is controlled by a duty cycle distortion control circuit, it will be recognized that there are various ways that the control can be effected, but in one embodiment the duty cycle distortion control circuit controls said duty cycle distortion circuit by voltage control. Thus, by the simple mechanism of setting a voltage level the degree of distortion applied to the duty cycle can be determined. In another embodiment the duty cycle distortion control circuit controls said duty cycle distortion circuit by a digital control signal. This represents an advantageously simple mechanism for transmitting a range of discrete control signals, depending on the particular encoding of the digital control signal. 
         [0015]    In some embodiments where the duty cycle distortion control circuit controls said duty cycle distortion circuit by a digital control signal the duty cycle distortion circuit comprises a digital to analog converter. This enables the duty cycle correction circuit to derive an analog signal, such as a particular current on a control line with which to effect the duty cycle distortion. 
         [0016]    It will be recognized that the particular manner in which the duty cycle distortion is applied to the signal could take a number of forms, but in one embodiment said duty cycle distortion circuit adds an offset current to said signal. By adding an offset current to the signal, the signal is shifted with respect to a given threshold, and due to the finite slew rate of the signal, an asymmetric duty cycle will results where that signal is received. Whilst the offset current could be added in a unilateral manner, in one embodiment the offset current is added differentially to said signal. 
         [0017]    According to one embodiment the communication is self-timed serial data communication. The accurate transmission and reception of this variety of communication is dependent on well timed duty cycles, and stress testing of an integrated circuit in which self-timed serial data communication occurs is advantageously carried out by distorting the duty cycle of a signal within that integrated circuit. 
         [0018]    Viewed from a second aspect there is provided a method of stress testing an integrated circuit, comprising the steps of: 
         [0019]    providing communication between a serial transmitter and a serial receiver on said integrated circuit over a serial connection; and 
         [0020]    distorting within said integrated circuit a duty cycle of a signal to stress test said integrated circuit. 
         [0021]    Viewed from a third aspect there is provided a integrated circuit comprising: 
         [0022]    a serial transmitter means for transmitting serial data; 
         [0023]    a serial receiver means for receiving serial data; 
         [0024]    a serial connection means for providing communication between said serial transmitter means and said serial receiver means; and 
         [0025]    a duty cycle distortion means for distorting a duty cycle of a signal within said integrated circuit to stress test said integrated circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The present invention will be described further, by way of example only, with reference to embodiments thereof as illustrated in the accompanying drawings, in which: 
           [0027]      FIG. 1  schematically illustrates an integrated circuit according to one example embodiment of the present invention; 
           [0028]      FIG. 2  schematically illustrates a differential current mode logic buffer with duty cycle distortion circuit; and 
           [0029]      FIG. 3  schematically illustrates imposing an asymmetric duty cycle on a signal by adding an offset current. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  schematically illustrates an integrated circuit  100 , comprising a serial transmitter  110 , a serial receiver  120  and a serial link  130 . Serial link  130  connects serial transmitter  110  to serial receiver  120 . Serial transmitter  110  receives parallel data and serializes it with serializer  140  for transmission over serial link  130 . In this embodiment serializer  140  generates self-timed serial data. This is done with reference to a clock signal received from clock generation circuit  170 . On the transmission path leading out of serializer  140  is a data buffer  150 , through which the serialized data passes on its way to serial link  130 . Also connected to data buffer  150  is duty cycle distortion control unit  160 . Within data buffer  150  is a duty cycle distortion circuit configured to distort a duty cycle of the serialized data passing though the buffer. This distortion is carried out under the control of duty cycle distortion control unit  160  by means of a duty cycle control signal. 
         [0031]    Duty cycle distortion control unit  160  is programmed to provide a number of duty cycle distortion tests by varying the distortion applied to the serialized data passing through data buffer  150 . These may range from simply applying a constant duty cycle distortion, to periodically varying the duty cycle, to applying a pseudo-random pattern to the variation of the duty cycle. 
         [0032]    The self-timed serialized data, with its duty cycle thus distorted is transmitted over serial link  130  to serial receiver  120 . Serial receiver  120  comprises duty cycle correction unit  180  and clock data recovery unit  190 . Duty cycle correction unit attempts to correct for any duty cycle distortions in the signals received over serial link  130  and passes the corrected signal on to clock data recovery unit  190 . Clock data recovery unit attempts to unfold the clock signal encoded in the serialized data to enable the transmitted data to be sampled and recovered. Distortion in the duty cycle of the serialized data makes these tasks more difficult and varying the degree of duty cycle distortion applied at data buffer  150  enables the serial receiver to be stress tested. 
         [0033]    In other embodiments the techniques of the present invention may be applied to stress test the integrated circuit in other ways. For example the duty cycle distortion circuit may be provided within the serial receiver instead, to apply the duty cycle distortion after the serialized data has been transmitted over the serial link  130 . In another example the duty cycle distortion circuit is provided in clock generation circuit  170  to cause it to generate a clock signal for use by the serializer  140  with a distorted duty cycle. 
         [0034]      FIG. 2  schematically illustrates a buffer such as data buffer  150  in  FIG. 1 . This differential current mode logic buffer has data inputs IN 1  and IN 2  are connected to transistors  200  and  210  respectively. Outputs OUT 1  and OUT 2  are connected to points  220  and  230  respectively. Points  220  and  230  are connected via variable resistors  240  and  250  respectively to V DD . Both transistors  200  and  210  are each connected via variable current source  260  to ground. 
         [0035]    When operating without duty cycle distortion being applied, the input data stream applied to inputs IN 1  and IN 2  alternately switches the left and right branches such that points  220  and  230  are alternately driven high (towards V DD ) and low (towards ground). The absolute levels of the outputs OUT 1  and OUT 2  are set via the particular setting of variable resistors  240  and  250 , and of variable current source  260 . 
         [0036]    When duty cycle distortion is to be applied, this is done via variable current sources  270  and  280 . By controlling the level of each of these current sources, a differential bias is applied across the points  220  and  230 , and hence across OUT 1  and OUT 2 . This current offset shifts the absolute level of the output and results in a duty cycle distortion of the signal passing through the buffer, as will be further explained with reference to  FIG. 3 . The control of variable current sources  270  and  280  comes from the duty cycle distortion control unit  160  and may in some embodiments be a voltage control. In the illustrated embodiment the duty cycle distortion control unit  160  issues a digital control signal, which passed through digital to analog converter  290  to provide the steering signal for each of the variable current sources  270  and  280 . 
         [0037]      FIG. 3  schematically illustrates the distortion of a signal duty cycle by offsetting a signal. In  FIG. 3A  the upper waveform oscillates symmetrically around the level V 0 , resulting in a symmetrical duty cycle (equal periods of high (H) and low (L)) as illustrated by the lower waveform. Once an offset has been applied, as illustrated in  FIG. 3B , this results in an asymmetrical duty cycle (unequal periods of high (H) and low (L)). This is due to the slew rate of the upper waveform, which therefore means that it takes a finite amount of time for, say, the rising edge to cross the level V 0  from the illustrated starting point. Thus by varying the offset applied to the signal any chosen distorted duty cycle may be applied to the signal to stress test the components receiving that signal. 
         [0038]    Thus, according to the present technique, a programmable duty cycle distortion generation circuit is provided, embodied on-chip, thus avoiding the need to use external off-chip testing facilities to create a stressed eye to stress test components of the integrated circuit. 
         [0039]    Although particular embodiments of the invention have been described herein, it will be apparent that the invention is not limited thereto, and that many modifications and additions may be made within the scope of the invention. For example, various combinations of the features of the following dependent could be made with the features of the independent claims without departing from the scope of the present invention.