Uncrewed Systems Technology 049 - April/May 2023

44 Focus | Image sensors While it might be possible to use a different, leading-edge wafer technology in a stacked wafer, it is a much more difficult process for building the stack, and risks raising issues with reliability. Instead, the MIPI interface is used to connect to the NPU for processing. Interconnection For the connection to local processing over a distance of 10 cm, MIPI works well, and it is an industry standard. Connections beyond 10 cm though require a serialiser/deserialiser (SerDes) interface, and there are two specifications for this that are not compatible. It means that the SerDes interface for the vehicle’s ECU has to be used by the sensor, reducing the ability of system designers to use sensors from different suppliers; also, a camera might need to be redesigned to work with a different ECU. Industry is currently pushing to create a single standard for SerDes connections, but that will limit the integration of SerDes into the sensor, requiring additional SerDes chips. MIPI The latest version of theMIPI camera system interface (CSI), MIPI CSI- 2 v4.0.1, was agreed in November 2022 and uses several of theMIPI physical layer (PHY) standards. This protocol specification provides selectable data rates from 1.25Gbit/s up to 5.8Gbit/s, with different clock speeds to avoid EMI problems. The v4.0 specification adds an advanced ‘always-on’ imaging solution that operates over as few as two wires to lower the cost and complexity, reducing the size and weight of UAV camera systems. It also adds multi-pixel compression for the latest generation of advanced image sensors and RAW28 colour depth to provide higher image quality and signal-to-noise ratio. The MIPI CSI protocol is typically implemented for shorter-reach applications on either a MIPI C-PHY or MIPI D-PHY interface – v4.0 is the first to support transmission of CSI-2 image frames over the low-cost, low- pin-count MIPI I3C/I3C Basic two-wire interface. However, a key advantage of CSI-2 v4.0 is that it is backward-compatible with all previous versions of the MIPI specification. CSI-2 v4.0 includes an Always-On Sentinel Conduit (AOSC) that enables the development of always-on machine vision systems, where the image sensor and video signal processor (VSP) can continuously monitor their surrounding environments and then wake their more powerful host CPUs only when significant events happen. Typically, the VSP will be either a separate device or integrated with the host CPU within a larger SoC. AOSC enables image frames to be economically streamed from an image sensor to a VSP over a low-power MIPI I3C bus in a highly efficient manner, with scaling options to add extra I3C lanes and bandwidth as defined by the I3C specification. In addition, Multi-Pixel Compression (MPC) provides optimised pixel compression for the latest generation of CMOS image sensors with multi-pixel colour filter arrays. It helps alleviate the high bandwidth demands of the sensors by compressing both multi-pixel and standard images more efficiently and with potentially higher quality than current methods. Reference code has also been made available to help integrators April/May 2023 | Uncrewed Systems Technology An end-to-end SerDes implementation (Courtesy of MIPI) A SerDes board for the ASA specification (Courtesy of VSI)