SRAM Histogram-on-Pixel DTOF for Small-Pitch SPAD Arrays
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Solution Overview
Problem
Traditional time of flight (ToF) depth sensors face challenges in achieving high resolution and power efficiency, particularly when the SPAD pitch is small, leading to degraded performance in terms of signal-to-noise ratio (SNR) and spatial resolution due to the inability to fit counters for histogram bins under the SPAD pixel array.
Innovation Solution
The implementation of a static random access memory (SRAM) based accumulation and storage approach using high-density SRAM cells under the SPAD pixel array in a histogram on pixel Direct Time of Flight (DTOF) depth sensor, which updates SRAM values only when photon-detection events occur, enabling high-quality histogram generation and efficient power usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by stationary object
If traditional TDC-based depth sensors are used, then depth measurement functionality is achieved, but power consumption is high and frame rates are limited
Solution Approach 1:
The patent implements event-driven operation where the SRAM-based histogram counter only updates when photon-detection events occur, rather than continuously operating like traditional TDC-based sensors. This periodic/event-triggered action significantly reduces power consumption while maintaining high frame rate capability, as the system transitions to a low-power state between events.
Solution Approach 2:
The SRAM-based histogram counter is integrated directly under the SPAD pixel array, enabling the sensor to perform histogram accumulation and depth calculation locally without requiring external TDC circuits. This self-contained architecture reduces overall system power consumption while improving processing speed and frame rate.
2Measurement precision
If counters for histogram bins are fitted under SPAD pixel array with small pitch, then spatial resolution is improved, but device complexity and area constraints make it infeasible
Solution Approach 1:
The patent changes the fundamental parameter of how histogram data is stored and processed by using high-density SRAM cells instead of traditional counter circuits. This parameter change allows for compact integration under small-pitch SPAD arrays, achieving high spatial resolution without the complexity of fitting traditional histogram counters in limited space.
Solution Approach 2:
The patent uses SRAM cells to store histogram bin data, effectively creating a memory-based copy of the photon detection events organized by time bins. This copying approach in SRAM memory structure provides a compact and scalable solution that fits under small-pitch SPAD arrays while maintaining high spatial and temporal resolution.
3Area of stationary object
If high-density SRAM cells are used under SPAD pixel array, then compact area and improved spatial resolution are achieved, but manufacturing complexity increases
Solution Approach 1:
The SRAM-based histogram counter is designed to be compatible with standard CMOS fabrication processes, allowing it to be manufactured using the same universal manufacturing infrastructure as the SPAD pixel array itself. This multi-functionality approach enables both the photodetector and memory components to be produced in the same process node, reducing manufacturing complexity despite the high density requirements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution provides significant power savings and high frame rates compared to traditional TDC-based sensors, while offering improved spatial resolution and sensitivity, enabling accurate detection of small SPAD pitch and fast-moving objects in low-light environments.
Implementation Method 1
The SPAD pixel array can be used in a sensitive photodetector whose high gain arises from avalanching within a photodiode
Implementation Method 2
DToF depth sensors can measure a time period from the irradiation time moment of the laser event, such as a laser diode (LD) pulse, to the time moment corresponding to the output pulse
Implementation Method 3
the implementation of a static random access memory (SRAM) based accumulation and storage approach using high-density SRAM cells under the SPAD pixel array
Implementation Method 4
DToF depth sensors can measure a time period from the irradiation time moment of the laser event, such as a laser diode (LD) pulse
Data Source
AI summary
In one embodiment, a system may comprise a plurality of photon sensors for detecting photons, a plurality of event registers for storing photon-detection events detected by the plurality of photon sensors during an exposure window after a laser event, and an SRAM disposed under the plurality of photon sensors. The SRAM may comprise a plurality of memory cells associated with each photon sensor of the plurality of photon sensors to store a histogram of photon-detection events. Each memory cell may store photon-detection events detected during a predetermined time period after the laser event. The SRAM may comprise an in-memory incrementor to update the plurality of memory cells based on the photon-detection events. The in-memory incrementor may read an event count stored in a selected one of the plurality of memory cells, increment the event count, and write the incremented event count back to the selected memory cell.