Camera module reference design with MIPI CSI-2 video interface, FPD-Link III and POC

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Overview

MIPI CSI-3SM is a camera subsystem interface that can be used to integrate digital still cameras, high-resolution and high-frame-rate sensors, teleconferencing and camcorder functionalities on a UniPro network.

MIPI CSI-3 is developed by the MIPI Camera Working Group.

The specification is available as v1.1 and defines the MIPI camera application layer for the MIPI UniPro v1.6 multi-layer network stack whose physical layer is, in turn, defined by the MIPI M-PHY v3.0 specification. CSI-3 and UniPro collectively support high image data transfer rates, fast bidirectional in-band camera control, device discovery capabilities, data delivery with acknowledgement, and management of multiple data streams using virtual channels.

The MIPI M-PHY physical layer supports bidirectional transfers using a pair of unidirectional serial links, and depending on speed, provides up to six predefined and high-speed serial transfer rates (or fixed gears) in either the forward or reverse direction. Each two-wire lane within each link in either direction also has its own embedded clock implemented using 8B10B line coding.

A basic CSI-3 v1.1 link configuration using four forward lanes and one reverse lane (10 total wires) can support up to 14.88 Gbps (usable bit rate, including 8B10B and UniPro overhead) in the forward direction and typically supports 1 Mbps or more in the reverse direction. The UniPro stack itself uses some link bandwidth (primarily in the reverse direction) for the purpose of guaranteeing reliable packet delivery to the receiver. Cameras implementing a minimal MIPI CSI-3 configuration consisting of one forward and one reverse lane (four total wires) can transmit 12 BPP 4K video at about 40 FPS.

Introduction:

Camera serial interfaces interconnect the camera in a device to the application processor or image signal processor. MIPI Alliance offers camera and imaging interfaces, as well as a standardized camera command set. Each can be used to bring high-resolution imaging, rich color and advanced video capabilities to smartphones, tablets, automobiles, video game devices, camera drones, wearables and other products.

MIPI Camera Serial Interface 2 (MIPI CSI-2) operates on the MIPI C-PHY and/or MIPI D-PHY physical layers. MIPI CSI-2 is widely adopted across mobile and mobile-influenced industries.

MIPI Camera Command Set (MIPI CCS) defines a standard set of functionalities for implementing and controlling image sensors.

MIPI Camera Serial Interface 3 (MIPI CSI-3) operates on the MIPI UniPro transport layer using the MIPI M-PHY physical layer.

All of these specifications have been developed by the MIPI Camera Working Group.

MIPI for Vision
Today's SoCs with MIPI/CSI-2 inputs generally offer hardware-accelerated image pre-processing operations via an Image Signal Processor (ISP). The ISP takes over operations such as de-mosaicking or color correction and, on some platforms, even demanding tasks such as H.264/H.265 coding or distortion correction. The ISPs usually only process data that is delivered via the MIPI/CSI-2 inputs. This excludes, therefore, the processing of data from GigE-Vision or USB3-Vision devices via ISP. Optimal use of the SoC's hardware resources (including ISP), however, requires the MIPI/CSI-2 interface. The SoC performs almost all image pre-processing tasks (i.e. operations that were often calculated directly in the camera in the case of industrial cameras) allowing for the use of compact and cost-effective camera designs. Another driver for MIPI/CSI-2 is currently the automotive industry's use of intelligent driver assistance systems. Today, hardly a vehicle rolls off the assembly line without camera modules or displays. In addition to digital rear-view mirrors, surround view, distance control or collision avoidance, MIPI Alliance protocols are also used for such components as infotainment systems.

Methodology

MIPI CSI-2SM is the most widely used camera interface in the mobile industry. It has achieved widespread adoption for its ease of use and ability to support a broad range of high-performance applications, including 1080p, 4K, 8K and beyond video, and high-resolution photography.

Designers should feel comfortable using MIPI CSI-2 for any single- or multi-camera implementation in mobile devices. The interface can also be used to interconnect cameras in head-mounted virtual reality devices; automotive smart-car applications for infotainment, safety, or gesture-based controls; camera drones; IoT appliances; wearables; and 3D facial recognition security or surveillance systems.

The latest release, MIPI CSI-2 v2.1 can be implemented on either of two physical layers from MIPI Alliance: MIPI C-PHYSM v1.2 and MIPI D-PHYSM v2.1. It is backward compatible with all previous MIPI CSI-2 specifications. Performance is lane-scalable, delivering, for example, up to 24 Gbps using a three-lane (nine-wire) MIPI C-PHY v1.2 interface, or 18 Gbps using four-lane (ten-wire) MIPI D-PHY v2.1 interface under MIPI CSI-2 v2.1.

Performance highlights:

· RAW-16 and RAW-20 color depth optimizes intra-scene High Dynamic Range (HDR) and Signal to Noise Ratio (SNR) to bring “advanced vision” capabilities to autonomous vehicles and systems

· Option to use up to 32 virtual channels accommodates the proliferation of image sensors with multiple data types and supports multi-exposure and multi-range sensor fusion for Advanced Driver Assistance Systems (ADAS) applications such as enhanced collision avoidance

· Latency Reduction and Transport Efficiency (LRTE) provides image sensor aggregation without adding to system cost; facilitates real-time perception, processing and decision-making; and optimizes transport to reduce the number of wires, toggle rate and power consumption

· Differential Pulse Code Modulation (DPCM) 12-10-12 compression reduces bandwidth while delivering superior SNR images devoid of compression artifacts for mission-critical vision applications

· Scrambling to reduce Power Spectral Density (PSD) emissions, minimize radio interference and allow further reach for longer channels

MIPI CSI-2 v2.1 adds the capability of the Camera Command Interface (CCI) to work with the MIPI I3C v1.0 sensor interface. It also offers enhanced performance when used with I2C. The updates provide dramatic improvements in CCI data speeds to support advanced imaging performance requirements for auto focus and optical image stabilization (OIS), among other applications.

CCI is a bidirectional, two-wire interface that host processors may use to configure and control cameras before, during or after image streaming using the high-speed MIPI D-PHY or MIPI C-PHY interfaces. Previously, CCI operated at 400 Kbps on I2C. With the new MIPI CSI-2 v2.1 release, CCI implementations can use I2C Fast Mode+ (FM+), which supports up to 1 Mbps. When used with MIPI I3C v1.0 Single Data Rate (SDR) mode, the interface delivers data at 12.5 Mbps. It delivers 25 Mbps when used with MIPI I3C v1.0 High Data Rate (HDR) Double Data Rate (DDR) mode.

MIPI CSI-2 v2.1 also includes technical adjustments and clarifications, requested by members, to optimize interoperability and minimize risks when using the specification for product development.
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