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Flexible MIPI Switching in Multi-Camera Smartphones and More

By Kevin Wu, Product Marketing Manager, Diodes Incorporated

The latest generations of smartphones are utilizing twin- and triple-camera designs to meet the high expectations of today’s users, ranging from capturing flattering selfies to semi-pro shooting in support of digital businesses. Wide-angle, telephoto, and normal modes are needed, and the performance of both front and rear cameras are important buying criteria. Multiple cameras are also key to the 3D photography capabilities of some phones, with certain models increasing the number of cameras per handset.

Each camera needs a high-performance connection to the application processor. If you are designing a smartphone, or embedding one or more cameras in equipment, such as drones, IoT devices, or automotive applications, an MIPI® interface is the most likely choice.


MIPI – An Interface Moving with the Times

According to the MIPI Alliance, the MIPI Camera Serial Interface 2 (MIPI CSI-2) is the most widely used camera interface in the mobile industry. MIPI has brought to the world a family of interfaces that provides much-needed standardization for connecting cameras and embedded displays. Moreover, the MIPI Camera Working Group continues to develop updates and new specifications as market demands and technological capabilities keep moving forward.

The working group aims to deliver enhancements to CSI-2 every two years. In 2019, CSI-2 v3.0 introduced features to simplify the connection between a sensor module and the application processor, and to enhance machine vision and AI inferencing. The next update is expected to introduce further machine-related features, as well as enhanced security and functional safety.

The CSI-2 interface can use either the D-PHY or later C-PHY physical layer. Whereas D-PHY is source-synchronous and uses a one clock lane with various numbers of data lanes, C-PHY embeds the clock with the data to eliminate the separate clock lane. With fewer wires needed to implement the connection, C-PHY increases data throughput per area. Thanks to the embedded clock, each C-PHY lane is also independent, which allows greater flexibility for designers when implementing C-PHY links.

On the other hand, the D-PHY interface uses conventional two-level logic signals, which allows relatively simple circuitry to implement selectable high-speed (HS) differential and low-power (LP) single-ended modes. C-PHY uses a three-level signaling scheme with encoding that adds complexity to the interface circuitry, including the HS and LP transmitters and receivers. However, with more bits per symbol due to the trio signaling scheme, C-PHY ensures a lower line-transition rate relative to the data rate. This permits lower power consumption in higher data-rate applications. C-PHY also allows support for new features, such as fast Bus Turnaround (BTA), Latency Reduction Transport Efficiency (LRTE), and Alternate Low-Power (ALP) mode as a longer-range alternative to the single-ended LP mode.


Designers’ Demands on MIPI Switches

Both specifications are active. They can coexist and can even be combined. Design engineers may need to connect either a C-PHY or a D-PHY module to the application processor at the heart of their system. Diodes Incorporated’s PI3WVR628 two-data-lane MIPI switch supports both C-PHY and D-PHY, offering a flexible solution for multiplexing CSI camera modules or Display Serial Interface (DSI) modules.

With six channels in a tiny 1.7mm x 2.4mm LGA outline, the PI3WVR628 permits a space-efficient solution. This is important because, although multi-camera smartphones are growing in popularity and the number of built-in cameras is likely to increase, very limited PCB space is available to house the extra functionality. In addition, at only 0.5mm tall, the PI3WVR628 is well suited for use in today’s ultra-low-profile handset designs.

Drawing a quiescent current of 11μA (typical) and falling to a maximum of 1μA when in a high-impedance mode, the PI3WVR628 also helps minimize power consumption to prolong battery life, operating from a supply voltage as low as 1.5V. The device can support data rates of up to 3.5Gsps for C-PHY-compliant modules, and 4.5Gbps for D-PHY-compliant modules, and is thus ready to meet popular camera specifications.

With its small size, low power, and support for both the MIPI D-PHY and C-PHY physical layers, the PI3WVR628 is ideal for a wide range of applications seeking to integrate multiple camera modules.

MIPI® is a registered trademark owned by MIPI Alliance.