External bias pixel structure of light receiving stage circuit in dynamic vision sensor pixel

Abstract

The invention discloses an external bias pixel structure of a light receiving stage circuit in a dynamic vision sensor pixel. The external bias pixel structure comprises a photodiode, a logarithmic tube, an amplifier tube, a switch tube and a bias tube, the bias tube, the switch tube and the amplifier tube are sequentially connected in series, a source electrode of the bias tube is connected with a power supply, a drain electrode of the bias tube is connected with a drain electrode of the switch tube, a source electrode of the switch tube is connected with a drain electrode of the amplifier tube, and a source electrode of the amplifier tube is grounded; the grid electrode of the bias tube is in signal connection with Vbias, the grid electrode of the switch tube is in signal connection with Vcpr, the source electrode of the switch tube and the drain electrode of the amplifier tube are connected with the grid electrode of the logarithmic tube, the drain electrode of the logarithmic tube is connected with a power supply, the source electrode of the logarithmic tube is connected with the negative electrode of the photodiode, and the positive electrode of the photodiode is grounded. And a grid electrode of the amplifier tube is also connected between the source electrode of the logarithmic tube and the negative electrode of the photodiode.

Description

Technical Field

[0001] The invention relates to the field of dynamic vision sensors, and in particular to an external bias pixel structure of a light receiving level circuit in a dynamic vision sensor pixel. Background Art

[0002] With the continuous development of image sensing technology, image sensors have been widely used in various fields. Image sensors are instruments that use optical elements and imaging devices to obtain image information of the external environment. Image sensors in the prior art generally include: Active Pixel Sensor (APS) and Dynamic Vision Sensor (DVS).

[0003] DVS is different from traditional image sensors. Traditional image sensors convert absolute light intensity information into frame images, while DVS converts changes in light intensity into event information. Compared with traditional image sensors, DVS has the characteristics of low latency, low data volume, and low power consumption. Currently, DVS can be divided into two modes according to the readout method: synchronous readout and asynchronous readout. For large arrays of DVS pixels, when the event rate is high, synchronous readout has lower readout latency and more precise time resolution. Periodic synchronous readout causes the DVS pixel to read changes in light intensity only at the readout time, and continuously detect external light intensity information at non-readout times. Summary of the invention

[0004] The purpose of the present invention is to overcome the deficiencies in the prior art and to provide an external bias pixel structure of a light receiving level circuit in a dynamic vision sensor pixel, which only works at the pixel readout moment and can further reduce the pixel size based on the traditional dynamic vision sensor pixel structure.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] An external bias pixel structure of a light receiving level circuit in a dynamic vision sensor pixel, wherein the dynamic vision sensor pixel comprises a light receiving level circuit, a buffer level circuit, a sampling and holding circuit and an event generating circuit which are arranged in sequence, wherein the light receiving level circuit converts external light intensity changes into logarithmic voltage changes; the buffer level circuit electrically isolates the light receiving level circuit from the sampling and holding circuit; the sampling and holding circuit samples and stores the voltage generated by the light intensity at the current moment; the event generating circuit compares the voltage difference between the previous moment and the current moment with a set threshold value to determine whether an event is generated; the external bias pixel structure comprises a photodiode, a logarithmic tube, an amplifier tube, a switch tube and a bias tube;

[0007] The bias tube, switch tube and amplifier tube are connected in series in sequence, wherein the source of the bias tube is connected to the power supply, the drain of the bias tube is connected to the drain of the switch tube, the source of the switch tube is connected to the drain of the amplifier tube, and the source of the amplifier tube is grounded; the gate of the bias tube is connected to V bias Signal connection, V bias The signal is generated by the bias circuit outside the dynamic vision sensor pixel array, and the gate of the switch tube is connected to V cpr Signal connection, V cpr The signal is generated by a controller outside the pixel array of the dynamic vision sensor. The source of the switching tube and the drain of the amplifier tube are also connected to the gate of the logarithmic tube. The drain of the logarithmic tube is connected to the power supply. The source of the logarithmic tube is connected to the cathode of the photodiode. The positive electrode of the photodiode is grounded. The source of the logarithmic tube and the cathode of the photodiode are also connected to the gate of the amplifier tube.

[0008] Furthermore, in the dynamic vision sensor pixel array in the dynamic vision sensor, each column of pixels shares x bias tubes, and x is determined by the setup time from off to on of the light receiving stage circuit and the readout row cycle; V cpr The signal passes through each row of the pixel array and is connected to the control circuit, which controls the on and off of the switch tubes in the light receiving level circuit of each row of pixels to control the working conditions of the entire row of pixels.

[0009] The present invention also provides an imaging method of a dynamic vision sensor, comprising:

[0010] S1. Taking the pixels in the nth row and the n+1th row as an example, when the pixels in the nth row are within the t1 time, the switch tube is turned off, the light receiving stage circuit is closed, and the external light intensity information is no longer sensed;

[0011] S2. When the nth row of pixels is in the time period of t2 to t3, the switch tube is turned on, and the light receiving stage circuit is in the working state in the time period of t2 to t3, and the time of t2 is set according to the establishment time of the light receiving stage circuit;

[0012] S3. During the time t2, the light receiving stage circuit starts to be established, and the photodiode in the pixel of the dynamic vision sensor converts the light signal into photocurrent, and the logarithmic tube converts the current signal into the voltage signal at the current moment and outputs it to the buffer stage circuit;

[0013] S4. The current voltage signal is output to the sample and hold circuit through the buffer stage circuit;

[0014] S5. The voltage signal at the current moment passes through the sampling and holding circuit, and is subtracted from the voltage at the previous moment within the time t3 to obtain the voltage difference caused by the change in light intensity at the two moments, which is then amplified and input to the event generation circuit;

[0015] S6. The event generation circuit compares the voltage difference signal amplified at two moments with the set threshold value to determine whether an event occurs, and if an event occurs, it is output to the column bus;

[0016] S7. At time t4, the reading of the n+1th row begins, and steps S5 to S6 are repeated.

[0017] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0018] 1. Reduced pixel area: Since a PMOS bias tube needs to be separately configured in a traditional DVS pixel, and the PMOS bias tube must be placed in an N-well, the layout area is increased. The present invention moves the PMOS bias tube to the outside of the pixel array, and by sharing the bias tube between each column of pixels, circuit reuse is achieved, the layout area inside the pixel is reduced, and the space utilization rate of the pixel array is improved.

[0019] 2. Reduced manufacturing costs: Reducing the pixel layout area allows more pixels to be accommodated within a unit chip area, which improves the pixel integration density and helps achieve higher resolution with the same chip area, thereby reducing the manufacturing cost per pixel.

[0020] 3. Power consumption optimization: By designing the switch tube to control the working state of the light receiving stage circuit, each row of pixels only turns on the light receiving stage during the setup time before readout and during readout, and turns off the light receiving stage at other times, thus avoiding unnecessary energy consumption. Compared with the traditional DVS structure, the present invention further optimizes the overall power consumption while ensuring the response speed.

[0021] 4. Improved dynamic response efficiency: This invention adopts a synchronous readout method, which can quickly respond to changes in light intensity through switch control, reduce the delay of event information, and help improve the response accuracy and time resolution of image sensors in fast-motion scenes. It is especially suitable for dynamic scenes with high event rates.

[0022] 5. Improved application adaptability: Since the pixel area is reduced or more layout resources are reserved under the same area, the present invention can provide more layout flexibility for subsequent circuit design and adapt to a wider range of dynamic visual application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of a typical DVS pixel structure with synchronous readout;

[0024] Figure 2 It is a schematic diagram of the circuit structure of the traditional DVS optical receiving stage;

[0025] Figure 3 is a schematic diagram of an external bias pixel structure of a light receiving stage circuit in the present invention;

[0026] Figure 4 Schematic diagram of the connection relationship of the external bias pixel array of the light receiving stage (x=1);

[0027] Figure 5 This is a timing diagram of the external bias pixel at the light receiving stage. DETAILED DESCRIPTION

[0028] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] In this embodiment, the bias tube in the light receiving stage circuit of the DVS pixel is placed outside the pixel array, which can further reduce the pixel size under the same DVS pixel structure, or reserve more layout resources for use under the same pixel size.

[0030] The schematic diagram of the structure of a typical DVS pixel with synchronous readout is as follows: Figure 1 As shown. The DVS pixel includes a light receiving stage circuit, a buffer stage circuit, a sampling and holding circuit, and an event generating circuit. The light receiving stage circuit converts the external light intensity change into a logarithmic voltage change; the buffer stage circuit electrically isolates the light receiving stage circuit from the sampling and holding circuit; the sampling and holding circuit samples and stores the voltage generated by the light intensity at the current moment; the event generating circuit compares the voltage difference between the previous moment and the current moment with the set threshold to determine whether an event is generated.

[0031] Traditional optical receiving circuits such as Figure 2 As shown, it includes a photodiode (PD), a logarithmic tube M1, an amplifier tube M2, and a bias tube M3. PD senses the external light intensity, directs the photogenerated electrons in the circuit path, and generates a photocurrent proportional to the current light intensity. The logarithmic tube M1 operates in the subthreshold region, converting the photocurrent change generated by the PD into a logarithmic voltage change, thereby increasing the dynamic range of the pixel. The gate of the amplifier tube M2 is connected to the negative electrode of the PD to clamp the voltage across the PD. The bias tube M3 is a PMOS transistor, which is connected to the reference voltage V bias To adjust the current flowing through M2 and M3.

[0032] Since the light receiving circuit part of each pixel of DVS requires a PMOS bias tube, the PMOS bias tube needs to be placed in the N-well, which requires occupying more layout area in the pixel.

[0033] The external bias pixel structure of the light receiving stage circuit proposed in this embodiment is as follows Figure 3 shown. Figure 3Take a single pixel in DVS as an example. The light receiving circuit includes a photodiode PD, a logarithmic tube M1, an amplifier tube M2, a switch tube M3, and a bias tube M4.

[0034] The bias tube M4, the switch tube M3 and the amplifier tube M2 are connected in series in sequence, wherein the source of the bias tube M4 is connected to the power supply, the drain of the bias tube M4 is connected to the drain of the switch tube M3, the source of the switch tube M3 is connected to the drain of the amplifier tube M2, and the source of the amplifier tube M2 is grounded; the gate of the bias tube M4 is connected to V bias Signal connection, the gate of switch tube M3 is connected to V cpr Signal connection, the source of the switch tube M3 and the drain of the amplifier tube M2 are also connected to the gate of the logarithmic tube M1, the drain of the logarithmic tube M1 is connected to the power supply, the source of the logarithmic tube M1 is connected to the cathode of the photodiode PD, the anode of the photodiode PD is grounded, and the source of the logarithmic tube M1 and the cathode of the photodiode PD are also connected to the gate of the amplifier tube M2.

[0035] The external bias pixel structure of the light receiving stage circuit introduces an additional switch tube M3, which is generated by the controller. cpr The signal controls the on and off of the switch tube M3. If the switch tube M3 is turned on, the current flows from the power supply VDD through the bias tube M4, the switch tube M3, and the amplifier tube M2, and the light receiving stage circuit works normally; if the switch tube M3 is turned off, the bias tube M4, the switch tube M3, and the amplifier tube M2 have no current, and the light receiving stage circuit is closed. By moving the bias tube M4 to the outside of the pixel array, the position of the PMOS bias tube in the original pixel layout can be replaced with the NMOS switch tube M3. The NMOS tube does not need to be placed on the N well, saving the layout area.

[0036] Preferably, in the external bias pixel structure of the light receiving stage circuit proposed in this embodiment, in the DVS pixel array, each column of pixels shares x PMOS bias tubes, and x can be determined according to the establishment time from the DVS light receiving stage from off to on and the row cycle of the readout. In this embodiment, x=1 is taken as an example, that is, each column of DVS pixels shares one PMOS bias tube, and the connection relationship of the DVS pixel array with a×b resolution is as follows: Figure 4 As shown, V cpr The signal passes through each row of the pixel array and is connected to the control circuit, and the control circuit controls the on and off of the switch tube M3 in the light receiving stage of each row of pixels to control the working conditions of the entire row of pixels. Figure 4 Only the connection relationship between the switch tube M3 and the bias tube M4 in the pixel is schematically represented. The buffer stage, sample and hold circuit, and event generation circuit in the DVS pixel are represented by other circuits, and other signals are not drawn.

[0037] Specifically, the imaging process of the dynamic vision sensor based on the above external bias pixel structure is as follows:

[0038] S1. Taking the pixels in the nth row and the n+1th row as an example, the timing diagram is as follows Figure 5 As shown, when the pixels in the nth row are in the time t1, the switch tube M3 is turned off, the light receiving stage is closed, and the external light intensity information is no longer sensed.

[0039] S2. When the pixels in the nth row are in the time period of t2 to t3, the switch tube M3 is turned on. At this time, the light receiving stage is in the working state in the time period of t2 to t3. The time of t2 needs to be set according to the establishment time of the light receiving stage.

[0040] S3. During the time t2, the light receiving stage begins to be established, and the light signal is converted into photocurrent through the photodiode in the pixel. The logarithmic tube M1 converts the current signal into the voltage signal at the current moment and outputs it to the buffer stage.

[0041] S4. The voltage signal at the current moment is output to the sample and hold circuit through the buffer stage.

[0042] S5. The voltage signal at the current moment passes through the sampling and holding circuit and is subtracted from the voltage at the previous moment within the time t3 to obtain the voltage difference caused by the change in light intensity at the two moments, which is amplified and input into the event generation circuit.

[0043] S6. The event generation circuit compares the voltage difference signal amplified at two moments with the set threshold value to determine whether an event occurs. If an event occurs, it is output to the column bus.

[0044] S7. At time t4, the reading of the n+1th row begins, and processes 5 to 6 are repeated.

[0045] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solution of the present invention. The above specific embodiments are merely illustrative and not restrictive. Without departing from the scope of the present invention and the scope of protection of the claims, a person of ordinary skill in the art can also make many forms of specific changes under the guidance of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. An external bias pixel structure of a light receiving level circuit in a dynamic vision sensor pixel, wherein the dynamic vision sensor pixel comprises a light receiving level circuit, a buffer level circuit, a sampling and holding circuit and an event generating circuit arranged in sequence, wherein the light receiving level circuit converts the external light intensity change into a logarithmic voltage change; the buffer level circuit electrically isolates the light receiving level circuit from the sampling and holding circuit; the sampling and holding circuit samples and stores the voltage generated by the light intensity at the current moment; the event generating circuit compares the voltage difference between the previous moment and the current moment with a set threshold value to determine whether an event is generated; characterized in that: The external bias pixel structure includes a photodiode (PD), a logarithmic tube (M1), an amplifier tube (M2), a switch tube (M3), and a bias tube (M4); The bias tube (M4), the switch tube (M3) and the amplifier tube (M2) are connected in series in sequence, wherein the source of the bias tube (M4) is connected to the power supply, the drain of the bias tube (M4) is connected to the drain of the switch tube (M3), the source of the switch tube (M3) is connected to the drain of the amplifier tube (M2), and the source of the amplifier tube (M2) is grounded; the gate of the bias tube (M4) is connected to V bias Signal connection, the gate of the switch tube (M3) and V cpr Signal connection, the source of the switch tube (M3) and the drain of the amplifier tube (M2) are also connected to the gate of the logarithmic tube (M1), the drain of the logarithmic tube (M1) is connected to the power supply, the source of the logarithmic tube (M1) is connected to the negative electrode of the photodiode (PD), the positive electrode of the photodiode (PD) is grounded, and the source of the logarithmic tube (M1) and the negative electrode of the photodiode (PD) are also connected to the gate of the amplifier tube (M2).

2. The external bias pixel structure of the light receiving stage circuit in the pixel of a dynamic vision sensor according to claim 1, characterized in that: In the dynamic vision sensor pixel array in the dynamic vision sensor, each column of pixels shares x bias tubes (M4), and x is determined by the setup time from off to on of the light receiving stage circuit and the readout row cycle; V cpr The signal passes through each row of the pixel array and is connected to the control circuit, and the control circuit controls the on and off of the switch tube (M3) in the light receiving stage circuit of each row of pixels to control the working conditions of the entire row of pixels.

3. An imaging method of a dynamic vision sensor, based on the external bias pixel structure of the light receiving stage circuit in the pixel of the dynamic vision sensor according to any one of claims 1-2, characterized in that: include: S1. Taking the pixels in the nth row and the n+1th row as an example, when the pixels in the nth row are within the t1 time, the switch tube (M3) is turned off, the light receiving stage circuit is closed, and the external light intensity information is no longer sensed; S2. When the pixels in the nth row are in the time period of t2 to t3, the switch tube (M3) is turned on, and the light receiving stage circuit is in the working state in the time period of t2 to t3, and the time of t2 is set according to the establishment time of the light receiving stage circuit; S3. During the time t2, the light receiving stage circuit starts to be established, and the photodiode in the pixel of the dynamic vision sensor converts the light signal into photocurrent, and the logarithmic tube (M1) converts the current signal into the voltage signal at the current moment and outputs it to the buffer stage circuit; S4. The current voltage signal is output to the sample and hold circuit through the buffer stage circuit; S5. The voltage signal at the current moment passes through the sampling and holding circuit, and is subtracted from the voltage at the previous moment within the time t3 to obtain the voltage difference caused by the change in light intensity at the two moments, which is then amplified and input to the event generation circuit; S6. The event generation circuit compares the voltage difference signal amplified at two moments with the set threshold value to determine whether an event occurs, and if an event occurs, it is output to the column bus; S7. At time t4, the reading of the n+1th row begins, and steps S5 to S6 are repeated.

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