WO2011016657A2 - Lamp signal generator using a lamp signal correcting function, and image sensor comprising same - Google Patents

Abstract

According to one embodiment of the present invention, a lamp signal generator for generating a lamp signal for analog-digital conversion comprises: a lamp signal generating unit which charges a prepared capacitive element using a predetermined reference power and outputs, as a lamp signal, a signal indicating an electrical discharge trend while discharging the capacitive element; and a lamp signal correcting unit which feeds back the corrected driving current to the lamp signal generating unit such that the lamp signal is corrected, in accordance with the change in the tilt of the lamp signal, and outputted. The present invention is advantageous in that the lamp signal can be corrected in a feedback manner, to thereby output an accurate lamp signal in accordance with a process and/or temperature.

Description

Lamp signal generator having a correction function of the lamp signal and an image sensor including the same

The present invention relates to a lamp signal generator for generating a ramp signal for analog-to-digital conversion and having a function of correcting the slope of the ramp signal through feedback and an image sensor including the same.

Today, with the development of the technology related to electric devices, image sensors formed by arranging a plurality of unit components (for example, pixels) that are sensitive to incident light in a line or matrix form are used in various fields. .

For example, in the field of imaging equipment, an image sensor of a CCD (Charge Coupled Device) type, a metal oxide semiconductor (MOS) type, or a complementary metal-oxide semiconductor (CMOS) type that detects incident light is used. They read the amount of incident light converted by the unit component (pixel) into an electric signal as an electric signal.

Such an image sensor, for example a CMOS image sensor, is a single tilt analog-to-digital converter (Analog / Digital Converter, hereinafter referred to as " ADC ") to read an analog electrical signal (photoelectric conversion signal) generated according to the light intensity. I'm using. In this case, the single slope ADC includes a ramp signal generator for generating a signal of a ramp waveform (hereinafter referred to as a “lamp signal”) as a reference signal for comparison with a photoelectric conversion signal.

On the other hand, the conventional lamp signal generator has been generally implemented in a method using a resistor DAC (Digital / Analog Converter) or a current steering DAC, there was a problem in generating a lamp signal containing switching noise in accordance with this switching operation. In order to solve this problem, the invention of generating a lamp signal in a non-switching method is described in the domestic application 10-2009-50465.

However, even when the lamp signal is output by the non-switching method, since the value of the current source and the capacitive element changes according to the process and / or the temperature change, it is necessary to correct the ramp signal slope.

SUMMARY An exemplary embodiment of the present invention provides a lamp signal generator having a function of correcting a ramp signal slope according to a process and / or temperature change, and an image sensor using the same.

In addition, an embodiment of the present invention is to provide a lamp signal generator and an image sensor using the same to be able to select the most appropriate correction time in a variety of situations by distinguishing the correction time of the lamp signal.

In order to achieve the above object, the lamp signal generator for generating a ramp signal for the analog-to-digital conversion according to an embodiment of the present invention charges a capacitive element prepared in advance using a predetermined reference power to discharge the capacitive element A ramp signal generator for outputting a signal indicating a discharge trend of charge as the ramp signal; And a lamp signal corrector for feeding back the corrected driving current to the lamp signal generator so that the lamp signal is corrected and output according to a change in the slope of the lamp signal.

The lamp signal generation unit may include the capacitive element charged using the reference power source; And a current source for discharging the charge charged in the capacitive element, wherein the ramp signal indicates a discharge trend of the charge while the charge is discharged according to the corrected driving current by the current source. Can be.

The ramp signal corrector may include: a first programmable current mirror set according to at least one of an amplitude of the ramp signal, a charge capacity of the capacitive element, and an output current value of the current source; And a second programmable current mirror set to correspond to a change in slope of the output ramp signal.

The ramp signal correcting unit may fix the maximum value of the ramp signal and feed back the driving current so that the amplitude of the ramp signal is corrected in response to a difference between the maximum value and the minimum value of the input pixel signal.

The ramp signal generator includes: a comparator for comparing the ramp signal with a comparison reference value; And a ramp signal measuring unit configured to count a case in which the ramp signal is higher than the comparison reference value according to the comparison of the comparator, and to measure a slope of the ramp signal using the count of the counter.

In this case, when the ramp signal generator is used for an n-bit single gradient ADC (Analog-Digital Converter), the ramp signal corrector may be represented by the following equation (in one embodiment, n is 10 bits and the second programmable current mirror). The control bit for setting the drive current is determined by the control bit of 7 bits), and the counter counts n + 1 bits.

Control bit = [median-((Delcount) _decimal / 8) _{binary integer} ]

Delcount: difference between 2 ⁿ and the counter output

The lamp signal correcting unit may perform correction of the lamp signal from a time when power is supplied to the lamp signal generator until the output lamp signal reads a signal from a pixel.

In addition, the ramp signal corrector may be configured to correct the ramp signal during a blanking section of the image frame.

The ramp signal correction unit may add a correction pixel column for tilt correction to correct the ramp signal according to the correction pixel for each row of pixels.

In addition, the ramp signal correction unit may be characterized in that to correct the output signal of the analog-to-digital conversion in real time according to the correction amount of the output ramp signal slope.

In this case, the ramp signal correction unit may repeatedly correct for each correction time, and may correct the ramp signal with an average of the repeating results.

The digital signal may further include a digital low pass filer for removing noise added when the ramp signal slope is corrected.

The lamp signal generator may further include a constant current generator for converting a voltage of a bandgap circuit for setting the reference power into a current.

In order to achieve the above object, an image sensor according to an embodiment of the present invention includes a pixel array for converting the incident light into an electrical signal; After charging the capacitive element prepared in advance using a predetermined reference power source, the capacitive element is discharged, and a lamp signal generator for outputting a signal indicating a discharge trend of charge as the lamp signal and a change in the slope of the lamp signal A lamp signal generator including a lamp signal corrector for feeding back the corrected driving current to the lamp signal generator so that the lamp signal is corrected and outputted; A comparator that receives the pixel signal and the ramp signal from the pixel array, compares the two input signals and outputs a comparison signal; A counter for counting the input clock signal; And a latch for receiving the comparison signal and storing and outputting a counter value of the counter according to the comparison signal.

Therefore, the present invention has the advantage that the correct ramp signal can be output even in the process and / or temperature change by correcting the slope of the generated lamp signal in a feedback manner.

In addition, the present invention has the advantage that the user can select the most appropriate correction method in various situations.

1 is a block diagram of a lamp signal generator according to an embodiment of the present invention.

2 is a circuit diagram for measuring a lamp signal according to an embodiment of the present invention.

3 and 4 are exemplary diagrams for explaining the lamp signal correction timing according to an embodiment of the present invention.

<Description of the symbols for the main parts of the drawings>

110: lamp signal generation unit 120: lamp signal correction unit

130: lamp signal measuring unit 140: constant current generating unit

210: signal to start counting 220: ramp signal (Vramp_out)

230: comparison reference value (Vrefn) 240: clock signal

310: blanking section 410: pixel column for correction

As the invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

Terms including ordinal numbers, such as second and first, may be used to describe various components, but the components are not limited by the terms. The terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present invention, the second component may be referred to as the first component, and similarly, the first component may also be referred to as the second component. The term and / or includes a combination of a plurality of related items or any item of a plurality of related items.

When a component is referred to as being "connected" or "connected" to another component, it may be directly connected to or connected to that other component, but it may be understood that other components may be present in between. Should be. On the other hand, when a component is said to be "directly connected" or "directly connected" to another component, it should be understood that there is no other component in between.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art and shall not be construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or corresponding components will be denoted by the same reference numerals regardless of the reference numerals and redundant description thereof will be omitted.

The present invention relates to a lamp signal generator for generating a lamp signal in a non-switching manner and an image sensor using the same. The image sensor may be a CCD, a MOS, or a CMOS, but for convenience of understanding and explanation, the present disclosure describes a CMOS image sensor as a representative example, but is not limited thereto. A lamp signal generator for generating a lamp signal using a non-switching method is used. Includes all image sensors being.

The image sensor includes a pixel array, a ramp signal generator, a comparator, a latch, a counter, a timing controller. The image sensor of the present invention converts the accumulated charge into an electrical signal according to the light incident on the pixel array. The image sensor compares and reads the converted electrical signal and the reference signal (lamp signal).

However, in the present specification, for the convenience of clear understanding and explanation, description of the general configuration of the image sensor will be omitted. The characteristic lamp signal generator of the present invention will be described below, but the configuration of the lamp signal generator will be described separately by function. Therefore, although the implementation of the actual lamp signal generator may be different from that described separately in this specification, it is clear that the scope of the present invention is not limited only to this division.

Hereinafter, a configuration and operation of a lamp signal generator according to an exemplary embodiment of the present invention will be described in detail with reference to FIG. 1.

1 is a block diagram of a lamp signal generator according to an embodiment of the present invention. 1, a lamp signal generator according to an exemplary embodiment of the present invention includes a lamp signal generator 110, a lamp signal corrector 120, a lamp signal measurer 130, and a constant current generator 140. do.

As illustrated, the ramp signal generator 110 may be implemented as a bandgap circuit, a resistor, a buffer, a switch, a capacitive element, and a third programmable current mirror (variable current source).

In the present specification, the operation of the lamp signal generator 110 will be described as a representative example of a lamp signal generator that generates a lamp signal in a non-switching manner.

In the ramp signal generator 110, a reference voltage is set at both ends of the resistor part due to a bandgap circuit, and a reference voltage set to close the first switch S1 is applied to both ends of the capacitive element so that charge is applied to the capacitive element. Is charged. After the first switch S1 is opened and the second switch S2 is closed, the charge charged in the capacitive element by the third programmable current mirror is discharged, so that the applied voltage across the capacitive element is constantly reduced to a constant slope. The signal in the form of a lamp (lamp signal) is generated.

The lamp signal corrector 120 corrects the lamp signal generated / output by the lamp signal generator 110. In detail, the lamp signal corrector 120 applies the driving current to the lamp signal so that the lamp signal is corrected and output by the lamp signal generator 110 according to a change in the slope of the lamp signal measured by the lamp signal measurer 130, which will be described later. Transfer to the generator 110.

In the present invention, when the slope of the lamp signal is changed in accordance with the process and / or temperature changes, the lamp signal correction unit 120 serves to output the lamp signal by reflecting this.

As shown in FIG. 1, the ramp signal correction unit 120 according to an embodiment of the present invention calculates correction data through a ramp signal slope correction algorithm using the slope data measured by the lamp signal measurement unit 130. The driving current is transferred to the lamp signal generator 110 according to the calculated correction data. Here, the transmission of the drive current is implemented using a programmable current mirror. According to an embodiment of the present invention, the programmable current mirror may be installed in two stages.

The first programmable current mirror is set so that the slope of the ramp signal conforms to the normal value. Specifically, the first programmable current mirror may be set according to one or more of the amplitude of the ramp signal, the charging capacity of the capacitive element, and the output current value of the current source. Therefore, the first programmable current mirror is a predetermined value and is set to a normal value under the assumption that there is no change in the lamp signal according to the process and temperature.

The second programmable current mirror is for correcting the gradient change of the ramp signal with process and / or temperature. The second programmable current mirror transfers an appropriate driving current to the ramp signal generator 110 according to the data whose slope is to be corrected.

Meanwhile, when a ramp signal generator is used in a 10-bit single slope analog-to-digital converter (ADC), the first programmable current mirror may be designed as 4 bits, and the second programmable current mirror may be designed as 7 bits. Because the first programmable current mirror is the number of bits in consideration of the difference between the maximum value (Vpixelmax) of the pixel signal and the minimum value (Vpixelmin) of the pixel signal, the second programmable current mirror is the amount of change in the slope and the ramp caused by the process and / or temperature changes The process of adjusting the signal slope is considered.

An algorithm for correcting the slope of the ramp signal is as follows. First, the gain of the ramp signal generator 110 is fixed to one. The amplitude Vramp of the ramp signal is adjusted while the maximum value Vrefp of the ramp signal is fixed. Specifically, the amplitude of the ramp signal is adjusted by Equation 1 below.

[Equation 1]

Vramp = Vrefp-Vrefn

      = Vpixelmax-Vpixelmin

For example, consider the case of a 10-bit single slope ADC. The first programmable current mirror adjusts the amount of current that discharges the capacitive element when the amplitude of the ramp signal needs to be changed according to the range of the pixel signal, thereby ramping the ramp signal after 1024 (= 210) clock pulses while the ramp signal remains at its maximum value. Try to reach the lowest value (Vrefn).

At this time, the output current I of the first programmable current mirror is determined by Equation 2 below.

[Equation 2]

I = C * Vramp / T

C: charge capacity of capacitive elements

T: Operating time of the lamp signal: (1 / Clock Pluse Frequency) * 1024

I: current to discharge the charged charge from the capacitive element

Then, by the second programmable current mirror, the slope of the lamp signal is corrected by the process and / or the temperature change.

The second programmable current mirror sets the value of the control bit to the median value. When the second switch S2 shown in FIG. 1 is closed, a ramp signal lowered by a constant slope is output from Vrefp as described above. The lamp signal measuring unit 130 receives the output lamp signal as an input signal.

Here, the ramp signal measuring unit 130 includes a comparator and a counter. A detailed configuration (circuit diagram) for implementing the ramp signal measurement unit 130 will be described in detail later with reference to FIG. 2.

The ramp signal measuring unit 130 compares the ramp signal with a comparison reference value (preferably set to Vrefn). In addition, the ramp signal measuring unit 130 counts using a counter until a time when the ramp signal that is constantly lower than the comparison reference value.

The second programmable current mirror may output (deliver) a driving current whose slope change according to a process and / or temperature is corrected using the count value described above.

For example, in the case of a 10-bit ADC, the second programmable current mirror is set to a control bit value calculated through Equations 3 and 4 below for 7 bits.

[Equation 3]

Control bit = [0111111-((Delcount) decimal / 8) binary integer]

[Equation 4]

Delcount = 1024-Counter_read

Counter_read: Counter output

As described above, the lamp signal generator performs a process of measuring the lamp signal, correcting the ramp signal slope based on the measured data, and generating / outputting an appropriate lamp signal again. In this way, the lamp signal generator of the present invention can output an accurate lamp signal despite the change in process and / or temperature.

The constant current generator 140 converts the voltage of the bandgap circuit of the ramp signal generator 110 into a current. As shown in FIG. 1, one side of the constant current generator 140 is connected between the bandgap circuit and the resistor, and the other side thereof is connected to the lamp signal corrector 120. The constant current generator 140 contributes a constant current to the lamp signal corrector 120. The introduced current is used as the reference current of the first programmable current mirror.

So far, the configuration of the lamp signal generator according to the exemplary embodiment of the present invention has been described with reference to FIG. 1.

Hereinafter, a configuration of a circuit for measuring a lamp signal will be described in detail with reference to FIG. 2.

2 is a circuit diagram for measuring a lamp signal according to an embodiment of the present invention. The lamp signal generator of the present invention includes a lamp signal measuring unit 130 as described above. The ramp signal measuring unit 130 compares the ramp signal 220 and the comparison reference value 230 (preferably set to Vrefn) when the start of the output of the ramp signal 210 is input. The ramp signal measuring unit 130 counts the case where the ramp signal 220 is higher than the comparison reference value 230 according to the comparison of the two signals 220 and 230.

Referring to FIG. 2, when the ramp signal is output, a signal 210 indicating the start of counting is input to the ramp signal measuring unit 130. Thereafter, the ramp signal 220 and the comparison reference value 230 are input to the comparator. The comparator outputs a comparison result of both signals 220 and 230.

The counter counts using the input clock signal 240 until the ramp signal 220 is smaller than the comparison reference value.

As a result, the ramp signal measuring unit 130 may measure how many times the ramp signal reaches the reference value.

The data measured by the lamp signal measuring unit 130 is transmitted to the lamp signal correction unit 120. The lamp signal corrector 120 sets the appropriate control bit of the second programmable current mirror through the same process as described in FIG. 1, and transmits a driving current to the lamp signal generator 110 to correct the lamp signal. To be printed.

So far, the lamp signal measuring unit 130 according to the exemplary embodiment of the present invention has been described with reference to FIG. 2.

Hereinafter, a lamp signal correction method will be described in detail with reference to FIGS. 3 and 4.

3 and 4 are exemplary views illustrating a lamp signal correction method according to an embodiment of the present invention.

The present invention will be described by dividing the method of correcting the lamp signal into four embodiments.

The lamp signal generator according to an embodiment of the present invention corrects a value after the power is supplied to the chip until the pixel signal is read from immediately after the power is supplied to the chip until the power is cut off or the user corrects it as necessary. Keep it.

In this case, the ramp signal generator continues to generate and output a ramp signal based on the corrected slope. This embodiment is effective in the case where the change of the slope is small during the operation of the chip, so that the correction is continuously unnecessary.

In general, the output current of the bandgap circuit is quite stable with temperature, and the charge capacitance of the capacitive element is also not temperature sensitive. Therefore, the lamp signal generator may perform an excellent operation within an error range of 2 to 3% only by correcting the slope of the lamp signal immediately after the power is applied.

On the other hand, it is preferable that the ramp signal generator performs the correction operation as many times as possible during the above-described correction time to use the average value. The reason is that if the slope correction is performed only once due to various noises in the chip, it may be inaccurate.

The ramp signal generator according to another embodiment of the present invention performs correction for each image frame. That is, the ramp signal generator may perform correction of the ramp signal during the blanking section of the image frame when the output ramp signal is used for analog-to-digital conversion.

Referring to FIG. 3, there is a blanking section 310 in which each image frame does not read a signal from a pixel. The ramp signal generator of the present embodiment performs tilt correction of the ramp signal using the blanking period 310. Also, in the present embodiment, the ramp signal generator preferably uses the average value by performing as many correction operations as possible in the blanking section 310.

On the other hand, in the present embodiment, the correction after the first frame does not change much. Therefore, the sharp change in the correction value after the correction of the first frame is mostly caused by the influence of noise. Therefore, the ramp signal generator uses a digital low pass filer to prevent the change in the correction value. It is more efficient to remove.

In the present embodiment, the second programmable current mirror is set to the center value in the first frame, but the second programmable current mirror can be set using the result value (center value + correction value) corrected in the previous frame from the second frame. have.

The ramp signal generator according to another embodiment of the present invention may perform correction for each row of pixels.

At this time, referring to FIG. 4, since there is no extra time such as the blanking section described above for every row correction, the ramp signal generator adds a correction pixel column 410 for tilt correction. The ramp signal generator may perform the correction of the ramp signal according to the correction pixel for each row of the pixels.

In addition, in the present embodiment, the image sensor changes the gain according to the brightness of the surrounding light. Therefore, when the slope correction of the ramp signal is performed within the low operation time of each pixel, the gain cannot be fixed to 1. Therefore, the lamp signal generator according to the present embodiment sets the reference value (Vrefp-Vrefn) for the slope correction first by fixing the value of Vrefp and changing the value of Vrefn, and then measuring the slope of the lamp signal based on this to measure the amplitude of the lamp signal. The slope must be corrected to equal the value (Vrefp-Vrefn).

In addition, since the ramp signal generator of this embodiment does not have a large change in correction for every row, it is efficient to remove noise by using a digital low pass filer.

Another embodiment of the present invention is a method in which a ramp signal generator measures a slope of a ramp signal and uses the value to correct an output signal value in real time. In each row, the counter value M of the ramp signal measuring unit 130 is obtained, and a value for signal correction is calculated through Equation 5 below.

[Equation 5]

a = 1024 / M

Subsequently, the ramp signal generator may obtain an output signal S by correcting the signal value Sp of the pixel using Equation 6 below.

[Equation 6]

S = a * Sp

As such, the lamp signal generator of the present invention can correct the lamp signal by the four methods described above. Each embodiment has advantages and disadvantages such as accuracy, operation time, and chip price, so that the user can select according to the situation.

While the above has been shown and described with respect to preferred embodiments of the present invention, the present invention is not limited to the specific embodiments described above, it is usually in the technical field to which the invention belongs without departing from the spirit of the invention claimed in the claims. Various modifications can be made by those skilled in the art, and these modifications should not be individually understood from the technical spirit or the prospect of the present invention.

Claims (14)

1. In the ramp signal generator for generating a ramp signal for analog-to-digital conversion,

   A lamp signal generator for charging a capacitive element prepared in advance using a predetermined reference power source and discharging the capacitive element and outputting a signal indicating a discharge trend of charge as the lamp signal; And

   And a lamp signal corrector for feeding back the corrected driving current to the lamp signal generator so that the lamp signal is corrected and output according to a change in the slope of the lamp signal.
2. The method of claim 1,

   The lamp signal generation unit

   The capacitive element charged using the reference power source; And

   A current source for discharging the charge charged in the capacitive element,

   And the ramp signal is indicative of the discharge trend of the charge while the charge is discharged in accordance with the corrected drive current by the current source.
3. The method of claim 2,

   The lamp signal correction unit

   A first programmable current mirror set according to at least one of an amplitude of the ramp signal, a charge capacity of the capacitive element, and an output current value of the current source; And

   And a second programmable current mirror set to correspond to a change in slope of the output ramp signal.
4. The method of claim 1,

   The lamp signal correction unit

   And the maximum value of the ramp signal is fixed and feeds back the driving current so that the amplitude of the ramp signal is corrected in response to the difference between the maximum value and the minimum value of the input pixel signal.
5. The method of claim 1,

   A comparator for comparing the ramp signal with a comparison reference value; And

   And a counter for counting a case where the ramp signal is higher than the comparison reference value according to the comparison of the comparator, and further comprising a ramp signal measuring unit for measuring a slope of the ramp signal using the count of the counter. Signal generator.
6. The method of claim 5,

   The lamp signal correction unit

   When the ramp signal generator is used for a n-bit single slope analog-to-digital converter (ADC), a control bit for setting the driving current is determined by the following equation, and the counter is n + 1. Ramp signal generator, characterized in that it counts in bits.

   Control bit = [median-((Delcount) _decimal / 8) _{binary integer} ]

   Delcount: difference between 2 ⁿ and the counter output
7. The method of claim 1,

   The lamp signal correction unit

   And ramping the ramp signal from a time when power is supplied to the ramp signal generator until reading the signal from a pixel.
8. The method of claim 1,

   The lamp signal correction unit

   And ramp correction of the ramp signal during a blanking period of an image frame.
9. The method of claim 1,

   The lamp signal correction unit

   And a ramp signal generator for correcting the ramp signal according to the correction pixel for each row of pixels by adding a correction pixel column for tilt correction.
10. The method of claim 1,

    The lamp signal correction unit

    The ramp signal generator, characterized in that for correcting the output signal of the analog-to-digital conversion in real time according to the correction amount of the ramp signal slope.
11. The method according to any one of claims 7 to 9,

    The lamp signal correction unit

    And correcting the ramp signal as an average of the repeating results by repeatedly performing correction for each correction time.
12. The method according to claim 8 or 9,

    And a digital low pass filer (Digital Low Pass Filer) to remove noise added when the ramp signal slope is corrected.
13. The method of claim 1,

    And a constant current generator for converting a voltage of a bandgap circuit for setting the reference power into a current to the lamp signal generator.
14. In the image sensor,

    A pixel array for converting incident light into an electrical signal;

    After charging the capacitive element prepared in advance using a predetermined reference power source, the capacitive element is discharged, and a lamp signal generator for outputting a signal indicating a discharge trend of charge as the lamp signal and a change in the slope of the lamp signal A lamp signal generator including a lamp signal corrector for feeding back the corrected driving current to the lamp signal generator so that the lamp signal is corrected and outputted;

    A comparator that receives the pixel signal and the ramp signal from the pixel array, compares the two input signals and outputs a comparison signal;

    A counter for counting the input clock signal; And

    And a latch configured to receive the comparison signal and store and output a counter value of the counter according to the comparison signal.

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