Gray-Coded TDC Latching for High-Resolution ToF Histograms
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Solution Overview
Problem
Time-of-flight (ToF) systems face challenges in achieving high resolution and power efficiency while minimizing routing congestion, particularly in generating accurate ToF histograms using conventional counter-based time digital converters (TDCs).
Innovation Solution
The implementation of a gray-coded time reference address that is periodically updated across a Single Photon Avalanche Diode (SPAD) array, where each SPAD is associated with a TDC, allowing for efficient latching and timestamp generation, thereby reducing power consumption and improving resolution without increasing congestion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional counter-based TDC is used with high clock frequency to achieve high time resolution, then measurement precision is improved, but power consumption increases
Solution Approach 1:
The patent changes the encoding parameter from binary to gray code for the time reference address. This parameter change allows the system to achieve the same time resolution while reducing the number of bit transitions during clock cycles, thereby reducing dynamic power consumption in the SPAD array and TDC circuits.
Solution Approach 2:
The patent implements periodic updating of the gray-coded time reference address across the SPAD array. By periodically refreshing the reference address in a gray-coded sequence, the system maintains accurate time measurement capability while minimizing unnecessary bit transitions between consecutive updates, thus reducing power consumption during the periodic operation cycles.
2Device complexity
If binary-coded time reference address is used in ToF systems, then routing is simplified, but incorrect code latching occurs during transitions
Solution Approach 1:
The patent changes the encoding scheme from binary to gray code for the time reference address. In gray code, only one bit changes between consecutive values, which eliminates the problem of multiple bits changing simultaneously that causes incorrect latching in binary encoding. This parameter change maintains routing simplicity while significantly improving code latching reliability.
3Measurement precision
If high clock frequency is used in counter-based TDC, then time resolution is improved, but routing congestion increases
Solution Approach 1:
The patent changes the address encoding from binary to gray code, which reduces the frequency of bit transitions during clock cycles. This parameter change allows the system to maintain high effective resolution by reducing the actual switching activity, thereby lowering routing congestion and signal interference even when operating at higher clock frequencies.
Data Source
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AI summary
In an embodiment, a method includes: providing a gray-coded time reference to a time-to-digital converter (TDC); receiving an event from an event signal; latching the gray-coded time reference into a memory upon reception of the event signal; and updating a time-of-flight (ToF) histogram based on the latched gray-coded time reference.