Image sensor reading method and image sensor

Abstract

The present invention discloses an image sensor readout method and an image sensor. The method comprises: in a reset stage, a gate signal is controlled to be set to a low level, a reset signal and a transmission signal are both set to a high level, and a pixel unit is reset; thereafter, the reset signal and the transmission signal are controlled to switch from a high level to a low level, the gate signal is controlled to remain at a low level, and a photodiode starts to be exposed, and then the reset signal is switched from a low level to a high level, and a floating node is reset; in an exposure stage, the reset tube is disconnected, the transmission tube is turned on, and photoelectrons stored in a short exposure time in the photodiode are transferred to a floating node; the transmission tube is disconnected, and the photodiode starts a long exposure; in a signal reading stage, the gate switch is turned on, and a short exposure integral signal value stored on a floating node is read out; then the floating node is reset, and the reset signal is read out, and thereafter the transmission signal is switched from a low level to a high level, and a long exposure integral signal is read out.

Description

Technical Field

[0001] The present invention relates to the technical field of integrated circuit design, and in particular to an image sensor readout method and an image sensor. Background Art

[0002] At present, CMOS image sensors (CIS) have been widely used in imaging fields such as video, monitoring, industrial manufacturing, automobiles, and home appliances. With the continuous improvement of CIS application requirements, the brightness range of the image presented is required to be continuously expanded, which requires CIS to have a higher dynamic range. At present, the method of improving the dynamic range can be mainly achieved through dark current compensation, reducing pixel and readout circuit noise and other noise reduction methods, but this method has limited room for improving the dynamic range and still cannot meet actual needs. Combining short exposure and long exposure signals can improve the dynamic range. This method requires two frames of data to be read out separately and then synthesized. Before synthesis, the long frame or short frame data needs to be stored, which requires a large amount of on-chip memory to store the read data. Storing this part of data will generate a large chip area overhead.

[0003] Therefore, it is urgent to provide a new image sensor readout solution to improve the above problems. Summary of the invention

[0004] The embodiment of the present invention provides an image sensor readout method and an image sensor, which are used to adjust the exposure and readout timing control of pixels so that a frame can read out long exposure and short exposure data at the same time without on-chip storage, thereby effectively saving chip area.

[0005] In a first aspect, the present invention provides an image sensor readout method, which is applied to an image sensor. The method comprises: in a reset stage, controlling a strobe signal SEL to be set to a low level, and setting a reset signal RX and a transmission signal TX to a high level, so that a transmission tube Mtx, a reset tube Mrst and an amplifier tube Msf are turned on, and a pixel unit of the image sensor is reset; after the pixel unit is reset, controlling the reset signal and the transmission signal to switch from a high level to a low level, controlling the strobe signal SEL to remain at a low level, so that the transmission tube Mtx, the reset tube Mrst, the amplifier tube Msf and the strobe switch Msel are all turned off, and the photodiode PD starts to be exposed, and then the reset signal RX is switched from a low level to a high level, so that the reset tube Mrst is turned on, and the floating node FD is reset; in an exposure stage, controlling the strobe signal SEL to switch from a high level to a low level first, and controlling the transmission signal TX Switching from low level to high level, the reset tube Mrst is disconnected, the transmission tube Mtx is turned on, and the photoelectrons stored in the short exposure time of the photodiode PD are transferred to the floating node FD; the control transmission signal TX is switched from high level to low level, so that the transmission tube Mtx is disconnected, and the photodiode PD starts long exposure; in the signal reading stage, the control selection signal SEL is first switched from low level to high level, so that the selection switch Msel is turned on, and the short exposure integral signal value VSIG_S stored on the floating node FD is read out; then the reset signal RX is controlled to switch from low level to high level, so that the reset tube Mrst is turned on, and the floating node FD is reset, the reset signal RX is switched from high level to low level, and the reset signal VRST is read out, and then the transmission signal TX is switched from low level to high level, so that the selection switch Msel is turned on, and the long exposure integral signal VSIG_L is read out.

[0006] In a possible embodiment, after the long exposure integration signal VSIG_L is read out, the method further includes:

[0007] The analog-to-digital conversion unit of the image sensor converts the reset signal VRST and the short exposure integration signal value VSIG_S into digital quantities and performs a subtraction operation, and outputs the short exposure digital quantity of the pixel unit;

[0008] The analog-to-digital conversion unit of the image sensor converts the reset signal VRST and the long exposure integration signal VSIG_L into digital quantities and performs a subtraction operation, and outputs the short exposure digital quantity of the pixel unit.

[0009] In a second aspect, the present invention further provides an image sensor, comprising a pixel array, an analog-to-digital conversion unit, a reference signal generator, a timing controller, a row selection decoding driver and an output signal processor, wherein the timing controller is used to control the image sensor to execute the method provided in any embodiment of the first aspect.

[0010] The beneficial effect of the image sensor readout method provided by the present invention is that: based on the traditional 4T pixel structure, by adjusting the exposure and readout timing control of the pixels, short exposure and long exposure are performed simultaneously, and then the short and long exposure data are read out successively, so that one frame can read out long and short exposure data at the same time without on-chip storage, which can effectively save chip area while improving the dynamic range of the image. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0012] Figure 1 A schematic diagram of the circuit structure of a CIS standard four-tube pixel unit provided by the prior art;

[0013] Figure 2 A schematic flow chart of a timing control method for a four-pixel pixel unit provided in the prior art;

[0014] Figure 3 A schematic diagram of a CIS readout circuit architecture provided by the prior art;

[0015] Figure 4 A schematic diagram of the operation timing of a CIS readout circuit provided by the prior art;

[0016] Figure 5 A schematic diagram of a double exposure CIS data output timing provided by the prior art;

[0017] Figure 6 A schematic diagram of the change of the signal-to-noise ratio with the light intensity under a double exposure combination provided by the prior art;

[0018] Figure 7 A schematic diagram of a method for an image sensor readout circuit provided by an embodiment of the present invention;

[0019] Figure 8 A schematic diagram of an improved pixel exposure and reading operation timing diagram provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0020] Figure 1It is the current CIS standard four-tube pixel unit circuit structure, which is widely used in row exposure CIS. It consists of a photodiode PD, a charge transfer tube Mtg, a reset tube Mrst, an amplifier tube Msf, and a gate switch Msel. The photodiode PD is sensitive to light and generates photoelectrons proportional to the light intensity. The function of Mtg is to transfer the photoelectrons in the photodiode PD. When the transmission signal TX is a high voltage, the charge transfer tube Mtg is turned on, and the photoelectrons in the photodiode PD are transferred to the floating node FD. The function of the reset tube Mrst is to reset the pixel unit when the reset signal RX is a high potential. The amplifier tube Msf is an amplifier tube. When the selection signal SEL is a high potential and the gate switch Msel is turned on, the amplifier tube Msf, the gate switch Msel and the current source to the ground form a path. At this time, the amplifier tube Msf is essentially a source follower, following the change in the potential of the floating node FD and finally output by PIX_OUT.

[0021] Figure 2 The operation timing of the four-tube pixel unit is divided into reset (Rst), exposure (Exp), and signal reading (Read). In the reset stage, the transmission signal TX and the reset signal RX are both high, the charge transfer tube Mtg and the reset tube Mrst are both turned on, the pixel unit is reset and its potential is pulled up to the power supply voltage VDD. After that, the transmission signal TX and the reset signal RX are both low, entering the exposure (Exp) stage, and the photodiode PD is sensitive to light and accumulates electrons. Entering the signal reading (Read) stage, the selection signal SEL is high, the reset signal RX is first high to reset the pixel unit, and the reset signal RX is pulled to a low level, and the transmission signal TX remains low. At this time, the amplifier tube Msf is controlled by the pixel unit potential and outputs the reset signal VRST through PIX_OUT. After that, the transmission signal TX switches to a high level, transferring the electrons on the photodiode PD to the floating node FD. At this time, the amplifier tube Msf is controlled by the floating node FD potential and outputs the integral signal VSIG through PIX_OUT. The reset signal VRST and the integral signal VSIG are converted into digital quantities by the subsequent analog-to-digital converter (ADC) circuit and subtracted to obtain the digital quantity corresponding to the photoelectrons on the photodiode PD. If the bit width of the ADC is 12 bits and the reference voltage range of the ADC is VREF, the final output is DOUT = (VRST-VSIG) × 2 12 / VREF.

[0022] Figure 3The structure diagram of a typical CIS readout circuit includes a pixel array, an analog-to-digital converter (ADC), a reference signal generator, a timing controller, a row selection decoder driver, and an output signal processor. Figure 3 The pixel unit "P" is composed of the above. The pixel array is read out row by row, and the specific order is ROW(0), ROW(1), ... ROW(k-1), ROW(k). Each column of the pixel array has an output bus, which are PIX_OUT(0), PIX_OUT(1), ... PIX_OUT(n-1), PIX_OUT(n). The PIX_OUT output is connected to the ADC. The column-parallel ADC uses a unified reference signal as a reference, converts the above VRST-VSIG difference into a digital quantity, and then outputs it.

[0023] Figure 4 Control timing for common CIS output operations, from Figure 4 As can be seen, the pixels are reset row by row, then exposed row by row, and finally read out row by row. "TN" is the exposure time, which is determined by the automatic exposure control logic according to the current image brightness. The length of a frame is the time required for the CIS to output a full-frame image, which is defined as the interval from the first row read time T0 of the current frame to the first row read time T1 of the next frame.

[0024] Figure 5 (a) is a schematic diagram of the short-exposure CIS data output timing. Figure 5 (b) is a schematic diagram of the long exposure CIS data output timing. Figure 5 (c) in the figure is the double exposure CIS data output timing, which adds a short exposure output frame based on the original long exposure timing. Figure 5 After the short exposure frame and the long exposure frame are combined in (c), one frame will contain information of both long and short exposure. Figure 4 Compared with the normal operation control sequence shown in the figure, the dynamic range can be significantly improved. Assume that the long exposure time is TL and the short exposure time is TS. The dynamic range can be written as DR = 20log (Im / Is), where Im and Is are the maximum and minimum photocurrents of the photodiode, Im = Qm / TS, Is = Qs / TL, where Qm and Qs are the maximum and minimum charges that can be generated in the photodiode, respectively. Then DR = 20log [(Qm / Qs) · (TL / TS)], so the dynamic range of the image synthesized by double exposure can be improved by 20log (TL / TS), if TL / TS = 256, the dynamic range can be improved by 48dB.

[0025] exist Figure 5In the synthesis method of short exposure frame and long exposure frame shown in FIG, when the current generated by the photodiode light exceeds Qm / TL, the extended signal range will be extended by the signal of the short exposure time. At the signal boundary, the charge will drop from the maximum value Qm to Qm(TS / TL). Since the signal-to-noise ratio is determined by the ratio of the effective photogenerated signal to the shot noise corresponding to the actual exposure time, the signal-to-noise ratio will drop at this boundary, as shown in FIG. Figure 6 As shown in the figure, under the double exposure combination, the intersection point between short exposure and long exposure is K, and the signal-to-noise ratio drops by ΔSNR. In addition, Figure 4 The problem with the two-frame synthesis method shown is that the long exposure frame needs to store a certain number of rows of long exposure values. Figure 4 As shown by the horizontal dotted line, only when the first line of the short exposure frame is read, the value of the first line of long exposure will be synthesized with the value of the first line of short exposure. Due to the time difference, the long exposure frame has read the 200th line at this time. This means that the long exposure frame needs to store 200 lines of values. (Assuming that there are 1000 lines in total per frame, the first line of the short exposure frame is read, and the 200th line of the long exposure frame is just read). If the column ADC accuracy is 10 bits, the full frame has a total of 1000 columns, and the SRAM required to store these 200 lines of data is 244KB. In actual applications, there may be frequency deviations and flexible adjustments in the application, which are generally higher than the minimum storage capacity, such as 300~500KB, which will occupy a large chip area.

[0026] To this end, this embodiment provides an image sensor readout method, which adjusts the exposure and readout timing of pixels so that long exposure data and short exposure data can be read out simultaneously in one frame without on-chip storage. This can effectively save chip area while improving the image dynamic range.

[0027] To make the content of the present invention more clear and understandable, the content of the present invention is further described below in conjunction with the accompanying drawings of the specification. Of course, the present invention is not limited to this specific embodiment, and general replacements known to those skilled in the art are also covered within the protection scope of the present invention.

[0028] It should be noted that in the following specific embodiments, when describing the embodiments of the present invention in detail, in order to clearly represent the structure of the present invention for the convenience of explanation, the structures in the accompanying drawings are not drawn according to general proportions, and are partially enlarged, deformed and simplified. Therefore, it should be avoided to understand this as a limitation of the present invention.

[0029] As attached Figure 7 As shown, the present invention provides a method for an image sensor readout circuit, which can be applied to the above-mentioned image sensor and may specifically include the following steps:

[0030] S701, in the reset stage, the selection signal SEL is controlled to be set to a low level, and the reset signal RX and the transmission signal TX are both set to a high level, so that the transmission tube Mtx, the reset tube Mrst and the amplifier tube Msf are turned on, and the pixel unit is reset.

[0031] S702, when the pixel unit is reset, the reset signal RX and the transmission signal TX are controlled to switch from high level to low level, and the selection signal SEL is controlled to remain at a low level, so that the transmission tube Mtx, the reset tube Mrst, the amplifier tube Msf and the selection switch Msel are all disconnected, and the photodiode PD begins to be exposed. After that, the reset signal RX is switched from low level to high level, so that the reset tube Mrst is turned on, and the floating node FD is reset.

[0032] S703, in the exposure stage, the control selection signal SEL is first switched from a high level to a low level, and the control transmission signal TX is switched from a low level to a high level, so that the reset tube Mrst is disconnected and the transmission tube Mtx is turned on, and the photoelectrons stored in the short exposure time in the photodiode PD are transferred to the floating node FD.

[0033] S704, controlling the transmission signal TX to switch from a high level to a low level, so that the transmission tube Mtx is disconnected, and the photodiode PD starts long exposure.

[0034] S705, in the signal reading stage, the selection signal SEL is controlled to switch from a low level to a high level, so that the selection switch Msel is turned on, and the short exposure integration signal value VSIG_S stored on the floating node FD is read out.

[0035] S706, then control the reset signal RX to switch from low level to high level, so that the reset tube Mrst is turned on, the floating node FD is reset, the reset signal RX is switched from high level to low level, and the reset signal VRST is read out.

[0036] S707, then the transmission signal TX is controlled to switch from a low level to a high level, so that the transmission tube Mtx is turned on, and the long exposure integration signal VSIG_L is read out.

[0037] Combination Figure 8For the improved pixel exposure and reading operation timing diagram shown in the figure, in the reset stage (Rst), the selection signal SEL is low, the reset signal RX and the transmission signal TX are high, thereby resetting the photodiode PD and the floating node FD. After the reset is completed, the reset signal RX and the transmission signal TX are both low, the selection signal SEL remains low, the photodiode PD begins to be exposed, and the reset signal RX changes from low to high to reset the floating node FD. After that, it enters the exposure stage (Exp), the reset signal RX is pulled to a low level again, and the transmission signal TX changes from a low level to a high level, thereby transferring the photoelectrons stored in the short exposure time of the photodiode PD to the floating node FD (the length of the short exposure time is TS). After the transmission signal TX switches from a high level to a low level, the photodiode PD enters the long exposure stage (the length of the exposure time is TL). In the signal reading phase (Read), the selection signal SEL switches from low level to high level, and the short exposure integration signal VSIG_S previously stored on the floating node FD is read out, and then the reset signal RX switches from low level to high level to reset the floating node FD, and then switches from high level to low level to read the reset signal VRST, and then the transmission signal TX changes from low level to high level to read out the long exposure integration signal VSIG_L. The column ADC transmits the above reset signal VRST, short exposure integration signal VSIG_S, and long exposure integration signal VSIG_L to the subsequent system, and the system converts the reset signal VRST and the short exposure integration signal value VSIG_S into digital quantities and performs a subtraction operation to obtain VRST-VSIG_S, and converts the reset signal VRST and the long exposure integration signal VSIG_L into digital quantities and performs a subtraction operation to obtain VRST-VSIG_L, and outputs the short exposure digital quantity of the pixel unit and the long exposure digital quantity of the pixel unit respectively. The above process shows that based on the traditional 4T pixel structure, by adjusting the exposure and readout timing control of the pixels, short exposure and long exposure can be performed simultaneously, and then the short and long exposure data are read out successively, so that one frame can read out long and short exposure data at the same time without on-chip storage, thus avoiding the overhead of storage resources occupied by the traditional double exposure synthesis that requires temporary storage of long frame data of some rows.

[0038] The above-described embodiments are only preferred embodiments of the present invention, and the embodiments are not intended to limit the patent protection scope of the present invention. Therefore, any equivalent structural changes made using the description and drawings of the present invention should also be included in the protection scope of the present invention.

[0039] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention.

[0040] Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An image sensor readout method, applied to an image sensor, characterized in that: include: In the reset stage, the control selection signal SEL is set to a low level, the reset signal RX and the transmission signal TX are both set to a high level, so that the transmission tube Mtx, the reset tube Mrst and the amplifier tube Msf are turned on, and the pixel unit of the image sensor is reset; When the pixel unit is reset, the reset signal RX and the transmission signal TX are controlled to switch from a high level to a low level, and the selection signal SEL is controlled to remain at a low level, so that the transmission tube Mtx, the reset tube Mrst, the amplifier tube Msf and the selection switch Msel are all disconnected, and the photodiode PD begins to be exposed. After that, the reset signal RX is switched from a low level to a high level, so that the reset tube Mrst is turned on, and the floating node FD is reset; In the exposure stage, the control selection signal SEL is first switched from high level to low level, and the control transmission signal TX is switched from low level to high level, so that the reset tube Mrst is disconnected and the transmission tube Mtx is turned on, and the photoelectrons stored in the short exposure time in the photodiode PD are transferred to the floating node FD; Control the transmission signal TX to switch from high level to low level, so that the transmission tube Mtx is disconnected and the photodiode PD starts long exposure; In the signal reading stage, the control selection signal SEL is first switched from a low level to a high level, so that the selection switch Msel is turned on, and the short exposure integral signal value VSIG_S stored on the floating node FD is read out; Then, the reset signal RX is controlled to switch from a low level to a high level, so that the reset tube Mrst is turned on, the floating node FD is reset, the reset signal RX is switched from a high level to a low level, and the reset signal VRST is read out; Then, the transmission signal TX is controlled to switch from a low level to a high level, so that the transmission tube Mtx is turned on and the long exposure integration signal VSIG_L is read out.

2. The method according to claim 1, characterized in that After reading out the long exposure integration signal VSIG_L, it also includes: The analog-to-digital conversion unit of the image sensor converts the reset signal VRST and the short exposure integration signal value VSIG_S into digital quantities and performs a subtraction operation, and outputs the short exposure digital quantity of the pixel unit; The analog-to-digital conversion unit of the image sensor converts the reset signal VRST and the long exposure integration signal VSIG_L into digital quantities and performs a subtraction operation, and outputs the long exposure digital quantity of the pixel unit.

3. An image sensor, characterized in that: The image sensor comprises a pixel array, an analog-to-digital conversion unit, a reference signal generator, a timing controller, a row selection decoding driver and an output signal processor, wherein the timing controller is used to control the image sensor to execute the method described in any one of claims 1 to 2.

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