Abstract:
An image converter includes: a pixel array having multiple imager pixels for outputting imager pixel signals, and a readout and processing device for reading out the pixel array and for receiving and processing the imager pixel signals. The pixel array has multiple reference pixels for outputting reference pixel signals, and at least one reference current device for outputting reference currents to the reference pixels for simulating illumination intensities. The readout and processing is adapted to jointly read out, receive, and process the reference pixel signals and the imager pixel signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to Application No. 10 2010 001 918.6, filed in the Federal Republic of Germany on Feb. 15, 2010, which is expressly incorporated herein in its entirety by reference thereto. 
     FIELD OF THE INVENTION 
     In cameras, such as those used in the automotive field, for example, an image converter is generally provided behind the camera lens which receives the incident radiation and provides an image which is resolved in two dimensions. The image converter is manufactured using CMOS or CCD technology, for example, and has a pixel array having multiple imager pixels from which the image signals are output for the subsequent image evaluation and, for example, representation of the image on a display device. An analog processing device, for example for filtering, and an analog-digital converter are generally provided at the pixel array, and digital processing and outputting to an external control device, for example a control module such as a microcontroller or FPGA, are subsequently carried out. This control device is able to adjust image control parameters, for example the integration time, the dynamic reset level, subaddress regions, in particular so-called AOIs, and the A/D converter characteristics, via a control interface. 
     In general, the self-monitoring of the image converter or image sensor is problematic. Thus, it is difficult or impossible for the external control device to recognize when the image converter is defective or is transmitting erroneous image data. In particular, image data which do not correspond to the image control parameters adjusted via the control interface, or which do not correspond to the visual setting in terms of the image refresh rate, must be regarded as erroneous. In order to detect the dark currents in the pixels which are not caused by the external incidence of light, dark pixels are sometimes provided which are shielded from external light incidence, for example by the metal mask provided in the pixel array, also between the imager pixels, and which therefore output only the dark currents caused, for example, by thermal excitations of the photodiode. As the result of the imager pixels thus being read out together with the dark pixels, it is possible to recognize dark currents, and on this basis to adjust the image control parameters. It is also conventional to read out and monitor register queries and parity checks of the transmitted data via the control interface. 
     However, these control mechanisms or signal monitoring systems are not able to check in particular for errors or inaccuracies in the processing of the pixel signals in the analog range, including the analog-digital conversion as well as the subsequent digital processing, and, if necessary, to check for errors in the timing or the synchronization of the image converter. 
     SUMMARY 
     According to example embodiments of the present invention, in addition to the imager pixels, reference pixels are provided in the pixel array which are acted on by reference currents. The reference currents may be output in particular by reference current sources which are connected in parallel to the photodiode, together with a parasitic capacitor, which is formed in each pixel. Thus, the reference currents simulate the photocurrent which is output by the photodiode and which charges the parasitic capacitor, which in turn is read out by the pixel architecture and the readout circuit. 
     Unlike dark pixels which are known per se, and which according to example embodiments of the present invention may in particular be additionally provided and which are exposed only to thermal noise and other effects, the reference pixels are thus acted on by the reference currents in a defined manner in order to reproduce or simulate appropriate illuminations or grayscale values. According to example embodiments of the present invention, with regard to the level of effort very simple checking is made possible which is carried out substantially by comparing the setpoint grayscale values, expected on the basis of the adjusted reference currents, to the grayscale values ascertained in the image signals of the reference pixels. 
     According to example embodiments of the present invention, the multiple reference pixels may be acted on in particular by different reference currents. An expected grayscale value range or intensity range may be varied between a minimum value, which may correspond to the dark current, for example, and a maximum expected brightness value, it being possible to cover this range linearly or logarithmically, for example, as a result of the different reference current values. 
     Several advantages are thus achieved. The entire signal path between the pixels of the pixel array and the subsequent signal processing in the image converter may be checked in particular for continuity and integrity. The known and adjusted reference current values may be compared to the expected grayscale values of the image signals generated in the image converter. It is thus possible to directly check for proper signal transmission or, if necessary, for errors. 
     The reference current sources may be fixed, or may be configured by the external control device, for example. Due to the fact that the reference currents are adjustable, in each case a test pattern or reference pattern may be suitably modified, for example also as a function of the particular use, for example also as a function of the expected brightness values and optionally color values. The image signal which is output by the reference pixels may be compared to the setpoint value, taking the set conversion characteristic curve into account. A match of the two values indicates an intact readout path in the analog and the digital ranges, as well as an absence of errors in the control interface and in the timing for the control and readout of the image converter. Thus, the entire readout path of the image converter may also be monitored by the external control device, of which use may be made in particular in safety-relevant applications without redundant sensors. 
     By varying the test pattern via the position of its supply point, i.e., its address, and by varying over the complete valid or allowed signal range, it is also possible to discover artifacts in the signal path which have resulted in previously unnoticed image distortions due to thermal or mechanical strain or electrically controlled signal conduction. 
     Another advantage is that this checking is possible with little outlay of hardware; it is necessary only to expand the pixel array by a few pixels, for example an upper and a lower row of reference pixels, and to provide the reference current sources. The pixel signals of the reference pixels are subsequently read out together with the pixel signals of the imager pixels and optionally the dark pixels and processed, thus requiring no further effort. For the high integration densities of the pixel arrays which are already currently achievable, additionally providing one or two rows, or diminishing the matrix range of the imager pixels by one to two rows, is generally not a problem. The reference pixels may be provided in particular outside of or at the edge of the imager range, and therefore do not interfere with the image recording and image signal quality. The reference current devices may be designed with relatively little complexity, and in terms of circuitry may be connected in parallel very easily to the photodiode and its parasitic capacitor formed by the pixel. The reference pixels as well as the dark pixels may be shielded using the metal layer which is already present. 
     The different current values may be achieved digitally, for example using a chargeable register with subsequent digital-analog conversion, in order to systematically cover the expected grayscale value range of the image. According to example embodiments of the present invention, the test pattern supply structures may be expanded to multiple structures, it being possible to make separate use of cell structures above and below the array and of row and column structures, in order to check the data integrity to a very high degree. 
     According to example embodiments of the present invention, a device composed of the image converter and the connected control device is also provided, the control device being able to program or adjust the reference current values; it is thus possible, for example, to provide better checking of expected brightness values or also spectral values according to various requirements, or, for detecting an error, to carry out further checks, for example in the relevant grayscale value range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a pixel array of an image converter according to an example embodiment of the present invention. 
         FIG. 2  shows an image converter. 
         FIG. 3  shows a readout circuit for reading out the pixel array. 
         FIG. 4  shows the pixel control system from  FIG. 3  in an enlarged illustration. 
         FIG. 5  shows illustrations of the grayscale values of the lower and upper reference pixel rows. 
         FIG. 6  shows (a) the output signal of the reference pixels in one row as a function of the number of pixels, together with two analog reference signals, (b) a histogram of a correctly converting analog-digital converter as a diagram of the frequency of occurrence of the digital output values, and (c) a corresponding histogram of an incorrectly converting ADC. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows an image converter  1 , which, as shown in greater detail in  FIG. 1 , has a pixel array  2  having a matrix system of individual pixels  5 ,  6 ,  7 , in the present case having eight rows, for example—that is, the number of rows i=1 through 8, and having six columns, i.e., j=1 through 6. The dashed lines in  FIG. 1  are provided solely to illustrate the individual types of pixels. The middle four rows, i.e., having row number i=3 through 6, are designed as imager pixels  5  for detecting an image, and thus form a 4×6 submatrix  2 - 1  of pixel array  2 . A row  2 - 2  composed of six dark pixels  6  is respectively situated above and below imager pixel submatrix  2 - 1 . Dark pixels  6  are provided in the substrate similarly as for imager pixels  5 , except that they are “shielded,” i.e., covered. Imager pixels  5  output imager pixel signals S 1 , which are read out line by line, for example.  FIG. 1  shows an example of such an imager pixel signal S 1 . Dark pixels  6  are likewise read out line by line, and they output dark pixel signals S 2  which are used in the image processing for evaluating imager pixel signals S 1  in order to subtract the dark current. 
     According to example embodiments of the present invention, reference pixels  7  (monitor pixels) are also provided in pixel array  2 , and are situated in two reference pixel rows  2 - 3  which according to  FIG. 1  are respectively located below lower dark pixel row  2 - 2  and above upper dark pixel row  2 - 3 . Reference pixels  7  are acted on by current via current sources, with respect to which an upper and a lower reference current device  8   a ,  8   b  are shown in  FIG. 1 , as described in greater detail below with reference to  FIGS. 4 and 5 . Reference current devices  8   a  and  8   b  preferably form direct current sources, and supply different reference current values Iref to each of individual reference pixels  7 , preferably as direct current values which are constant during a readout operation. Rows  2 - 2  and  2 - 3  may also be interchanged. 
     Pixel array  2  together with all pixels  5 ,  6 , and  7  is initially provided uniformly, i.e., under identical production conditions, dark pixels  6  and reference pixels  7  being covered, preferably using the aluminum mask, which also covers webs  8  remaining between imager pixels  5 . Imager pixels  5  as well as dark pixels  6  and reference pixels  7  are controlled and read out in a similar manner. 
     In addition to pixel array  2 , image converter  1  has a device  10  for analog data processing and AD conversion which receives analog signals S 1 , S 2 , and S 3  of pixel array  2 , subsequently carries out analog processing of the signals, for example by filtering, and carries out an analog-to-digital conversion, so that the device outputs digital signals S 4  to a device  12  for digital processing. Image converter  1  also has a control interface  14  and a device  16  for synchronization and timing (“timing and control”) which receives a clock signal S 7 . Device  12  for digital processing outputs, via control interface  14 , digital image signals S 5  which contain information concerning pixel signals S 1 , S 2 , S 3 . Control interface  14  also receives control signals S 6  and S 9 , S 10  from an external control device  40  (only indicated here), which may be an FPGA or μC, for example. Control signals S 6  may be supplied to all devices  10 ,  12 ,  16  via control interface  14 . Devices  10 ,  12 ,  14 ,  16  thus form a readout and processing device whose proper operation is checked. 
     Reference current devices  8   a  and  8   b  supply individual reference pixels  7  in reference pixel rows  2 - 3  with different current values Iref, values Iref being a function of column number j; i.e., the two reference current devices  8   a  and  8   b  each represent multiple reference current sources  8   a - j ,  8   b - j  in order to output reference current values Iref (j) (i.e., Iref as a function of j) for individual reference pixels  7  having a different column number j. In  FIG. 4 , the lower circuit region is provided as a pixel architecture, i.e., a readout circuit  26 , which is identical for all pixels  5 ,  6 , and  7  and is basically known per se. Each pixel  5 ,  6 , and  7  has a photodiode  20  for receiving incident light  21  (for example, in the visible or IR range), and a (parasitic) capacitor  22  which is provided in the semiconductor material or its border layers. The control is carried out via transistors, for example MOSFETs  23 ,  24 , and  25 . Transistor  23  is controlled via a reset control signal Vrst; Vrst controls transistor  23  via a positive potential in order to discharge parasitic capacitor  22  multiple times during a readout operation, i.e., to switch between ground V 0  and applied potential VRT. Multiple resetting during a readout operation is used to ascertain even higher illuminations more accurately, since the pixel is less sensitive to higher illuminations; i.e., for the same intensity the photovoltage which is output decreases only slightly for higher illuminations or longer time periods. Transistor  24  is used as a source follower for transistor  23 , and the drain of the former is connected to reference voltage Vdd. Transistor  25  is used as a readout amplifier, which is controlled by a control voltage VRead as readout control voltage, so that, similarly as for an output signal, a voltage Vout is read out which forms signal S 3  of reference pixel  7 . 
     For each reference pixel  7 , the particular current source  8   a - j ,  8   b - j  is also connected in parallel to its photodiode  20  and its parasitic capacitor  22 . Thus, corresponding to its output reference current Iref (j), current source  8   a - j ,  8   b - j , which is adjustable for each reference pixel  7 , charges parasitic capacitor  22 . Since reference pixels  20  are covered, no appreciable light  21  strikes photodiode  20 , so that in any event, for upper left reference pixel  7  in which j=1, which is supplied with Iref=0, the dark current is at best output by photodiode  20 , the same as for dark pixels  6 . Thus, reference current Iref of reference current devices  8   a  and  8   b  is used as a “substitute” for incident light  21  or as a defined reference grayscale value for reference radiation, in order to subsequently check the transmission characteristics or transmission errors via elements  10 ,  12 ,  14 ,  16  of  FIG. 3 . The output grayscale value of signal S 3  of upper left reference pixel  7 , in which Iref=0, should therefore correspond to the grayscale value of dark pixels  6 . 
     According to  FIG. 2 , current sources  8   a ,  8   b  may be configured by external control device  40  via control interface  14 , with the aid of control signals S 9 , S 10 , in order to adjust the particular current values for the individual reference pixels  7 . Furthermore, reference current devices  8   a  and  8   b  may also be preset during manufacture, and thus may output fixed current values Iref (j) for each pixel  7  of a reference pixel row  2 - 3 . 
     According to  FIG. 5 , in the upper current pixel diagram of upper reference pixel row  2 - 3 , for example, reference current values Iref may increase to the right with increasing column number j, in particular incrementally, for example linearly or also logarithmically, until at last reference pixel  7 , i.e., j=6 in this case, Iref corresponding to a maximum light incidence. Upper and lower reference pixel rows  2 - 3  may each be controlled pixel by pixel, using the same reference current values Iref. In addition, according to  FIG. 5  lower reference pixel row  2 - 3  may be set exactly oppositely from upper reference pixel row  2 - 3 , so that set reference current values Iref or the grayscale value decrease(s) with increasing j, similarly as for a linear conversion characteristic curve, for example. Thus, the analog output signals may be simply added column by column, in which the analog output signals of the two reference pixels  7  having the same column number j are added in each case, and a check is made to determine whether this sum is constant over j, deviations being easily identifiable. The evaluation may be easily carried out on the chip, for example, using an adder and a comparator, or also by external control device  40 . Similarly, other advantageous adjustments of the current values are also possible. Thus, for the same control on a column basis, i.e., control using the same reference current values, for example a column-based subtraction may be carried out in which analog reference pixel signals S 3  which are output are subtracted from reference pixels  7  having the same column number j. 
     According to example embodiments of the present invention, variable, chargeable, or adjustable test patterns may be used for reference currents Iref for individual reference pixels  7 , which therefore represent effective monitoring of the sensor function, and which in particular may be adapted to the particular circumstances, for example also as a function of an overall identified incident intensity, for example as a function of daytime or nighttime functions. 
     In the known adjustment of reference currents Iref, the result of reference pixels  7  may be predicted and computed as a function of the setting or addressing of the readout and control path of image converter  1 , which proceeds via output signals S 1 , S 2  and S 3  and analog processing device  10  to device  12  for digital processing. Taking into account the conversion characteristic curve which is set via control interface  14 , the result of reference pixels  7  may be computed, and the actual result may be compared to the setpoint value. A match of the two values indicates an intact readout path in the analog and the digital ranges. Conclusions may also be drawn concerning an absence of errors in control interface  14  and in the timing of device  16  for controlling and reading out the image converter. Reference current devices  8   a ,  8   b  shown in  FIGS. 3 and 4  may, for example, be formed by a register having a connected digital-analog converter DAC, thus allowing the reference values to be dynamically changed by appropriate charging of the register. 
     In addition to the pixel architecture,  FIG. 3  shows the subsequent readout circuit in greater detail: voltage value Vout output by readout transistor  25  is read out by a correlated double sampling (CDS) readout circuit  30  and supplied to an analog-digital converter  32 , which in the present example has an amplifier  32 - 1 , a comparator  32 - 2 , and a buffer  32 - 3 ; the comparative voltage is also provided to comparator  32 - 2  by a counter  33  via a digital-analog converter  34 . This additional circuit is basically conventional. 
     According to  FIG. 6   a , analog reference current Iref along reference pixel row  2 - 3  may be adjusted in a linearly increasing manner from the minimum current value, in particular near the dark current, to a maximum expected intensity corresponding to a photocurrent, forming a steplike progression due to the discrete number of pixels, unlike the idealized illustration of  FIG. 6   a . In addition, according to example embodiments of the present invention repetitions in the curve progression are possible, for example a saw-toothed progression successively following multiple linear progressions. Since the entire expected intensity range is thus reproduced by reference current values, the correct behavior of the connected analog-digital converters in device  10  may be tested. Any brightness value to be converted may be simulated during readout of a complete reference pixel row  2 - 3 .  FIG. 6   b  shows a histogram of the frequency of occurrence H of digital output values d, i.e., illustrated in a simplified manner by d=0, 1, 2, 3, 4, . . . . Thus, when devices  10 ,  12 ,  14 ,  16  are operating correctly, the constant progression for which H=1 in  FIG. 6   b  is expected.  FIG. 6   c  shows an example of a histogram for an incorrectly converting ADC in which two digital output values d 1  and d 2  are absent, resulting in, for example, twice the frequency of occurrence of the subsequent digital output value; thus, for example, digital output values d 1  and d 2  have been output slightly in excess due to an error in the ADC.