Unveiling the Camera Technology of Xiaomi MIX4: Understanding MIPI D-PHY Interface

The recently released Xiaomi MIX4 features significant innovations in both the front and rear cameras, boasting a 6.67-inch full-screen display that completely hides the front camera beneath the screen. The rear camera is equipped with a 100-megapixel professional main camera. Today, let’s discuss the technical information hidden behind the product: MIPI unveiled, let’s talk through waveforms.

Whether it’s the screen, camera, or RF, there is a crucial set of high-speed interfaces in smartphones, namely MIPI. Today’s protagonist is the MIPI interface, which will be discussed in terms of actual waveforms.

MIPI stands for Mobile Industry Processor Interface. It is an open standard initiated by the MIPI Alliance for mobile application processors. The purpose is to standardize internal interfaces of the smartphone, such as camera and display interfaces, thereby reducing the complexity of smartphone design and increasing design flexibility.

The MIPI protocol includes CSI, DSI, etc. CSI stands for Camera Serial Interface, used in camera working groups; DSI stands for Display Serial Interface, used in display screen working groups. Additionally, there are SLIMbus for low-speed multipoint connections and SPMI for power management working groups.

CSI and DSI are protocol layers, and their physical layers can support D-PHY and C-PHY. D-PHY uses DDR (double data rate) data transmission, transmitting data on both the rising and falling edges of the clock, with a dedicated clock channel; C-PHY does not require a dedicated clock channel, as its clock information is embedded in the data itself. We will focus on the waveform characteristics of D-PHY.

D-PHY has a dedicated clock channel for transmitting clock information, as well as data channels, with both clock and data transmitted in differential pairs, providing fast speed and high stability.

D-PHY has two main operating modes: LP (low power) and HS (high speed). In LP mode, the speed is slow, and the voltage amplitude is high; in HS mode, the amplitude is low, and the speed is fast.

The following image shows the actual waveform results collected from the camera. The yellow and blue represent data P and data N, while the red and green represent clock P and clock N. Ignoring the impact of the bandwidth of the collecting device, the image below shows that one frame of data takes about 33ms, corresponding to a frame rate of 30Hz.

Upon magnification, we can see the specific data of one row in a frame. If the measured clock frequency is k MHz, then the D-PHY rate is 2*k Mbps/lane.

The following is a magnified waveform of a row of data. In LP mode, the signal is single-ended at 1.2V; in HS mode, the signal oscillates on a 200mV baseline, with P and N providing the differential result.

This concludes the introduction to MIPI D-PHY.