Direct time of flight distance measuring system with in-pixel histogram circuit incorporating LFSR shift register based photon counting accumulator
A novel and useful direct time of flight distance measuring device that incorporates smart pixel architecture, real-time in-pixel histogram and intensity acquisition, optical power control, and variable resolution. The device avoids the use of time to digital (TDC) converters by generating in-pixel histograms based on a global high-speed clock that implements per pixel time bin single photon event accumulation using circuitry located under or near the SPAD pixels. Laser events are processed within a smart pixel architecture increasing substantially the data conversion throughput while reducing data movement in the device to achieve high spatial resolution, low power operation, and fast reliable depth capture. The device uses a compact split pseudo-random counter. Each smart pixel operates independently and concurrently, allowing the sensor to operate in global shutter while monitoring the SNR and determining the optimal exposure time for transmitter and receiver power savings. This enables full control of the pixel operation, enable/disable, and optimization.
1 . A time of flight distance measuring system, comprising:
an array of pixel photon detectors for detecting photons reflected off one or more targets in response to a series of laser events;
in-pixel histogram circuits disposed under said plurality of pixel photon detectors, each in-pixel histogram circuit incorporating an accumulator per time bin for counting photons, each accumulator including a linear feedback shift register (LFSR) circuit, wherein each LFSR circuit is divided into multiple LFSR circuits coupled by one or more carry bits, said in-pixel histogram circuit operative to:
generate a histogram for each pixel or groups of pixels whereby photon detection counts are maintained for a plurality of discrete time bins over an exposure period with each time bin corresponding to a particular delay interval equivalent to distance; and
simultaneously generate an intensity measurement for each pixel.
2 . The time of flight depth measuring system according to claim 1 , wherein said array is organized as macropixels, each macropixel comprising a plurality of pixel photon detectors.
3 . The time of flight depth measuring system according to claim 1 , wherein said accumulator is N-bits wide and comprises at least two separate LFSR circuits, each LFSR circuit being less than N-bits wide.
4 . The time of flight depth measuring system according to claim 1 , wherein said accumulator comprises a memory storage slave portion for storing LFSR data and a shared master portion for reading, incrementing, and writing the LFSR data stored in said slave portion.
5 . The time of flight depth measuring system according to claim 1 , further comprising one or more shared decoder circuits operative to convert an output of each divided LFSR circuit to binary form, each decoder circuit selected from a group consisting of a look up table (LUT) and combinatorial logic gates.
6 . The time of flight depth measuring system according to claim 1 , wherein pixels in said array operate independently and concurrently from each other, allowing the system to operate in global shutter mode within the exposure period.
7 . The time of flight depth measuring system according to claim 1 , wherein pixels in said array as well as said in-pixel histogram circuits operate off a single global clock signal that is utilized for real time photon synchronization, memory indexing, and operation of said system.
8 . A method of time of flight distance measuring, comprising:
detecting photons, using an array of pixel photon detectors, for detecting photons reflected off one or more targets in response to a series of laser events;
in-pixel histogram circuit circuits disposed under said plurality of pixel photon detectors, each in-pixel histogram circuit incorporating an accumulator per discrete time bin, each accumulator including a linear feedback shift register (LFSR) circuit, wherein each LFSR circuit is divided into multiple LFSR circuits coupled by one or more carry bits; generating a histogram for each pixel or groups of pixels whereby photon detection counts are maintained for a plurality of discrete time bins over an exposure period with each time bin corresponding to a particular delay interval equivalent to distance; and simultaneously generating an intensity measurement for each individual pixel.
9 . The method according to claim 8 , wherein said array is organized as macropixels, each macropixel comprising a plurality of pixel photon detectors.
10 . The method according to claim 8 , wherein said accumulator is N-bits wide and comprises at least two separate LFSR circuits, each LFSR circuit being less than N-bits wide.
11 . The method according to claim 8 , wherein said accumulator is divided into a memory storage array slave portion for storing LFSR data and a shared master portion for reading, incrementing, and writing the LFSR data stored in said slave portion.
12 . The method according to claim 8 , wherein an output of each divided LFSR circuit is converted to binary form using a one or more shared decoder circuits.
13 . The method according to claim 8 , wherein pixels in said array operate independently of each other whereby said histograms and intensity data are generated in real time for all pixels simultaneously in parallel and in a single shot within the exposure period.
14 . The method according to claim 8 , wherein pixels in said array as well as said in-pixel histogram circuits operate off a single clock signal that is utilized for real time photon synchronization, memory indexing, and operation of said system.
15 . A time of flight distance measuring system, comprising:
an array of laser emitters operative to generate and emit pulses of laser light as a series of laser events that illuminate an area;
an array of pixel photon detectors for detecting photons reflected off one or more targets in response to the series of laser events;
in-pixel histogram circuits disposed under said plurality of pixel photon detectors, each in-pixel histogram circuit incorporating an accumulator per discrete time bin for counting photons, each accumulator including a linear feedback shift register (LFSR) circuit, wherein each LFSR circuit is divided into multiple LFSR circuits coupled by one or more carry bits, said in-pixel histogram circuit operative to: generate a histogram for each macropixel whereby photon detection counts are maintained for a plurality of discrete time bins over an exposure period with each time bin corresponding to a particular delay interval equivalent to distance; and
simultaneously generate an intensity measurement for each individual pixel.
16 . The time of flight depth measuring system according to claim 15 , wherein said accumulator comprises N-bits and comprises a circuit selected from a group consisting of two groups of N/2-bit LFSR circuits, three groups of N/3-bit LFSR circuits, and four groups of N/4-bit LFSR circuits, where N is a positive integer greater than or equal to four.
17 . The time of flight depth measuring system according to claim 15 , wherein pixels in said array operate independently and concurrently from each other, allowing the system to operate in global shutter mode within the exposure period.
18 . The time of flight depth measuring system according to claim 5 , wherein said one or more shared decoder circuits are shared across a plurality of in-pixel histogram circuits.
19 . The method according to claim 12 , wherein said one or more shared decoder circuits are shared across a plurality of in-pixel histogram circuits.