Abstract:
The semiconductor device includes: an A/D conversion circuit; a digital processing circuit for performing processing based on conversion results of the A/D conversion circuit; and an output terminal for testing for outputting the conversion results of the A/D conversion circuit externally. The output of the conversion results from the output terminal for testing is made at timing that is different from timing of other conversion operation of which conversion results are to be outputted later and is longer in cycle than timing of conversion operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a semiconductor device having an A/D conversion circuit, and more particularly to a semiconductor device that outputs conversion results of the A/D conversion circuit externally to enable testing of the A/D conversion circuit. 
         [0003]    2. Description of the Prior Art 
         [0004]    In recent years, semiconductor devices called system-on-chips, in which an A/D conversion circuit, a digital circuit for performing processing based on the conversion results and the like are mounted on one chip, have increasingly become mainstream. In some of such semiconductor devices having an A/D conversion circuit, a mode switch circuit is embedded so that AID conversion results can be directly outputted externally via a digital buffer, to thereby allow separate testing of the A/D conversion circuit. 
         [0005]    During the testing, however, a variation in power supply voltage and noise may occur because the digital buffer drives a comparatively large load testing apparatus (LSI tester). If influences of such a variation and noise become great, the A/D conversion precision may be degraded even though high-precision A/D conversion can be performed in actual use of the semiconductor devices, and thus proper testing may not be obtained. 
         [0006]    To deal with the above problem, a technology is proposed in which a digital signal outputted from an A/D conversion circuit is converted to an analog signal of which level is slowly changed and the resultant signal is outputted via an analog buffer, thereby reducing the influences of a variation in power supply voltage and noise to improve the testing precision (see Japanese Laid-Open Patent Publication No. 2002-246909, for example) 
         [0007]    However, the conventional semiconductor device described above has the following problems. Even though a digital signal is converted to an analog signal of which level is slowly changed, the influences of a variation in power supply voltage and noise will not necessarily be eliminated as long as a level shift occurs, depending on the relationship thereof with the magnitude of the load of the testing apparatus and the like. Moreover, the use of an analog buffer will cause an increase in circuit scale. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the present invention is providing a semiconductor device that can avoid influences of a variation in power supply voltage and noise occurring at the time of output of A/D conversion results to permit proper testing of an A/D conversion circuit to be performed easily, and also can reduce the circuit scale by adopting a simple configuration. 
         [0009]    The semiconductor device of the present invention includes: 
         [0010]    an A/D conversion circuit; 
         [0011]    a digital processing circuit for performing processing based on conversion results of the A/D conversion circuit; and 
         [0012]    an output terminal for testing for outputting the conversion results of the A/D conversion circuit externally, 
         [0013]    wherein the output of the conversion results from the output terminal for testing is made at timing that is different from timing of other conversion operation of which conversion results are to be outputted later and is longer in cycle than timing of conversion operation. 
         [0014]    The semiconductor device of the present invention includes: 
         [0015]    an A/D conversion circuit for performing A/D conversion every clock cycle of an operation clock signal; 
         [0016]    a digital processing circuit for performing processing based on conversion results of the A/D conversion circuit; and 
         [0017]    an output terminal for testing for outputting the conversion results of the A/D conversion circuit externally, 
         [0018]    wherein the device further comprises an output suppression circuit for suppressing output of the conversion results to the output terminal for testing at predetermined timing. 
         [0019]    By controlling the output timing of the conversion results during testing of the A/D conversion circuit as described above, conversion results converted at timing at which no conversion results are outputted are free from influences of noise and the like occurring with output of the conversion results. Thus, high-precision conversion results can be easily obtained with a simple configuration. 
         [0020]    In the case of controlling the output timing of the conversion results with an external timing control signal, also, substantially the same control can be given easily, and thus high-precision conversion results can be obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a circuit diagram showing a configuration of a main portion of a semiconductor device of Embodiment 1. 
           [0022]      FIG. 2  is a timing chart showing the operation of the semiconductor device of Embodiment 1 during testing. 
           [0023]      FIG. 3  is a circuit diagram of an alteration of Embodiment 1. 
           [0024]      FIG. 4  is a circuit diagram showing a configuration of a main portion of a semiconductor device of Embodiment 2. 
           [0025]      FIG. 5  is a timing chart showing the operation of the semiconductor device of Embodiment 2 during testing. 
           [0026]      FIG. 6  is a circuit diagram showing a configuration of a main portion of a semiconductor device of Embodiment 3. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that in these embodiments, components having substantially the same functions are denoted by the same reference numerals, and the description thereof will not be repeated. 
       Embodiment 1 
       [0028]    As shown in  FIG. 1 , a semiconductor device of Embodiment 1 includes an AID conversion circuit  11  and a digital processing circuit  12 . 
         [0029]    The A/D conversion circuit  11  converts an analog input signal received from outside the semiconductor device, for example, into a 4-bit digital signal every clock cycle of an A/D conversion clock supplied inside or outside the semiconductor device. 
         [0030]    The digital processing circuit  12  performs predetermined processing using conversion results of the A/D conversion circuit  11  (in  FIG. 1 , only one-bit output of the AID conversion circuit  11  is representatively shown). A signal from the digital processing circuit  12  is output outside the semiconductor device via a selector  13 , a buffer  14  and an external output terminal (output terminal for normal operation and output terminal for testing) as a digital output signal. 
         [0031]    The conversion results of the A/D conversion circuit  11  are also input into a flipflop  15  to be held therein and then sent to the selector  13 . The hold operation of the flipflop  15  is made in synchronization with a frequency-divided clock obtained from the A/D conversion clock being frequency-divided by two by a frequency divider  16 . 
         [0032]    In the semiconductor device having the configuration described above, during normal operation, the selector  13  selects the output of the digital processing circuit  12  according a selection signal, and thus results of the predetermined processing by the digital processing circuit  12  are outputted outside the semiconductor device. 
         [0033]    During testing of the semiconductor device, a predetermined voltage is supplied as the analog input signal from outside the semiconductor device, and the selector  13  selects the output of the flipflop  15  according to the selection signal. Thus, the conversion results of the A/D conversion circuit  11 , held in the flipflop  15 , are outputted outside the semiconductor device. In this way, the conversion results can be checked by a testing apparatus not shown. 
         [0034]    The hold operation of the flipflop  15  of holding the conversion results will be described in detail. The AID conversion circuit  11  performs A/D conversion during each of periods A to H shown by hatching in  FIG. 2  in synchronization with the rising timing of the A/D clock. However, since the flipflop  15  holds the conversion results at the rising timing of the divide-by-2 clock from the frequency divider  16 , only the conversion results during the periods A, C, E and G are held and output via the selector  13  and the buffer  14 . In this case, the actual A/D conversion rate at which the conversion results are outputted is ½, but this does not especially causes a problem in testing of the A/D conversion precision and the like. 
         [0035]    At the timing of performing the above hold and output operation, a comparatively large load of a testing apparatus or the like (for example, about ten times that of the digital processing circuit  12 ) will be driven with the buffer  14 . Therefore, compared with driving during normal operation, a variation in power supply voltage, superimposition of noise on the analog input signal via a parasitic capacitance existing inside/outside the semiconductor device and the like will be great. However, influences of such noise and the like will arise only at the timing of conversion during the periods B, D, F and H. The hold and output operation is suppressed at the timing of conversion during the periods A, C, E and G, and thus the conversion during these periods will not be influenced by such noise and the like. Thus, high-precision AID conversion results can be obtained easily. 
         [0036]    As described above, by suppressing output of part of the A/D conversion results and setting the output timing of the remaining conversion results to be outputted to have a longer cycle than the timing of the A/D conversion operation, the output timing can be easily differentiated from the conversion operation timing. In this way, even in the case that the output result is composed of a lot of bits making the same level shift, for example, it is possible to easily prevent degradation of the conversion precision caused by driving of a large load of a testing apparatus and the like. 
         [0037]    In the suppression of part of output of the conversion results described above, the conversion operation for the part of conversion results that are not outputted may be omitted, or the timing may be shifted. In general, however, continuous conversion operation at the same timing as that during normal operation of the semiconductor device is preferred because with this operation proper testing results can be easily obtained. 
         [0038]    The A/D clock is not necessarily frequency-divided by two, but may be divided by three or more depending on the time for which the influence of noise lasts and the like, for example. The frequency division ratio may be made programmably changeable. To suppress output of part of the conversion results, any means other than frequency-dividing the A/D clock as described above may be adopted, such as masking clock pulses partially to thin the pulses. 
         [0039]    The output timing of the conversion results is not limited to the timing of rising edge of the divide-by-2 clock of the A/D clock, but may be timing delayed by a predetermined time (time in a predetermined proportion of the cycle of the A/D clock) given by a delay element  21  placed downstream (and/or upstream) of the frequency divider  16  as shown in  FIG. 3 , for example. That is, the output timing of the conversion results (more specifically, the period for which the influence of this output exceeds an allowable degree) may just be shifted from both or one of the timing at which the input voltage is held in a sample-and-hold circuit of the A/D conversion circuit  11  and the timing at which comparison operation by a comparison circuit is completed. 
         [0040]    The output terminal for digital output signals is used both for the output of the A/D conversion results and the output of the digital processing circuit  12  as described above, so that the number of terminals can be reduced. Alternatively, exclusive output terminals may be provided. 
         [0041]    Likewise, the input terminal for the analog input signal may be made to serve also as an input/output terminal for other signals. 
       Embodiment 2 
       [0042]    In the above embodiment, the conversion results of the AID conversion circuit  11  were held in the flipflop  15  at predetermined timing. In this embodiment, as shown in  FIG. 4 , an AND circuit  25  and an output control signal generation circuit  26  are provided, to give timing, as shown in  FIG. 5 , with which the output of the conversion results (start, stop and their transient influential periods of the output) will not influence the precision of the A/D conversion. To state more specifically, the output control signal generation circuit  26  outputs an output control signal having every other clock pulse of the A/D clock or pulses synchronizing with the every other clock pulse. Thus, since the level of the signal outputted outside the semiconductor device is not changed during the periods other than those corresponding to the clock pulses of the output control signal, it is also possible in this embodiment to avoid influences of noise and the like at the timing at which conversion during the periods A, C, E and G is performed. 
       Embodiment 3 
       [0043]    In Embodiments 1 and 2, the divide-by-2 clock and the output control signal were generated inside the semiconductor device. Such a clock and signal may otherwise be supplied from outside the semiconductor device (directly or indirectly) as shown in  FIG. 6 , for example. In this case, the output timing and frequency of the conversion results can be easily set according to the degree to which the output of the conversion results influences the conversion precision, the conversion precision that is to be checked in the testing, and the like, and this can increase the degree of freedom of the testing. 
         [0044]    In the embodiments described above, the A/D conversion circuit  11  performed conversion every clock cycle of the AID clock. A similar configuration can also be adopted for the case of performing one unit of A/D conversion every plurality of clock cycles. For example, assume that a pipeline type A/D conversion circuit having ten stages performs one unit of A/D conversion operation in 5.5 clock cycles and the respective stages operate in parallel with one another to output conversion results every clock cycle. In this case, the A/D clock may be frequency-divided by six at least for control of the output timing of the conversion results. That is, conversion results of conversion at any given stages performed in a clock cycle in which the conversion results are output may possibly have been influenced by noise and the like. However, the conversion results of conversion at all of the ten stages performed in the subsequent six clock cycles during the period over which the output operation is suppressed can be outputted in the sixth clock cycle. In other words, high-precision conversion results can be easily obtained by arranging the conversion results to be outputted every six or more clock cycles. 
         [0045]    While the present invention has been described in preferred embodiments, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than that specifically set out and described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention.