Differential serial interface solution for high-speed image data transmission in mobile phones

The Mobile Shrink DataLink (MSDL) differential serial interface solution independently developed by ROHM can greatly save the number of lines in the increasingly complex clamshell connections of foldable mobile phones. As LCD panels continue to move towards high precision, more lines are needed to transmit mobile phone image data, and high-speed data transmission will also generate electromagnetic interference. MSDL can not only solve the above-mentioned similar problems, but also solve many issues encountered in mobile phone research and development, so it will play a huge role in the application of foldable mobile phones.

Figure 1: MSDL has a unique low-amplitude transmission mode.

Problems faced by foldable phones

In recent years, mobile phones are moving from standard configurations of megapixel cameras to configurations of higher-definition cameras. As a result, the amount of data processed inside mobile phones is gradually increasing. Among them, the amount of image data is rapidly expanding due to the continuous high-quality and high-precision LCD panels and built-in cameras.

In foldable phones, the transmission lines of these image data are generally connected between the application processor, LCD panel and camera through the clamshell connection. In this case, the following two problems will be faced:

1. Users require mobile phone manufacturers to design mobile phones that are more novel and beautiful, so new models of rotatable foldable phones and dual-axis clamshells are on the scene, and their shell structures are becoming increasingly complex. On the other hand, since a digital camera is configured on one side of the LCD panel, more lines are required to be arranged. More lines make product structure design more difficult and will also seriously affect product reliability, so solving these problems has become an urgent issue.

2. Since mobile phones need to transmit data at high speed, the data lines of video signals at the clamshell connection will generate EMI. In order to reduce the impact of electromagnetic wave interference, it is necessary to strengthen the attenuation countermeasures of RF (wireless) characteristics. The EMI emitted by the data line at the flip cover joint will increase as the data transmission rate increases. Usually the flip cover joint is located near the antenna, so in order to reduce the impact on the wireless characteristics, it is necessary to strengthen the countermeasures such as shielding or inserting filters.

Figure 2: Current mode transmission adopted by MSDL can reduce EMI.

Technology development of MSDL

Currently, as the interface of LCD panel modules of PCs and LCD TVs, the differential interface of low voltage differential signaling (LVDS) mode has become a global standard specification. ROHM has first-class technology and management in this field due to its advantages in high speed, low EMI, and low power consumption. By applying these technologies and mobile phone dedicated system LSI, ROHM successfully developed MSDL, a new differential serial port that can solve the difficulties encountered in the development of folding mobile phones. Using one such IC on the LCD screen circuit board and the host circuit board of the folding mobile phone (the IC converts the parallel data transmitted from the application processor to the LCD panel into MSDL format), various functions can be realized.

MSDL adopts current mode differential transmission technology, and the differential amplitude of the line part is transmitted with a small amplitude of tens of millivolts (Figure 1). The EMI characteristics are outstanding, and the noise level can be reduced by about 10dB compared with the previous CMOS transmission (Figure 2).

By reducing this EMI noise, two effects can be achieved: it will not have an adverse effect on RF (wireless) characteristics, and the design period can be shortened. Reducing EMI noise can also shorten the design period because it can reduce the process of solving the noise problem. Usually, in order to control EMI noise, mobile phone manufacturers will install anti-EMI components such as filters or shielding sheets near the circuit of the flip cover connection to eliminate noise. However, since it is impossible to accurately estimate the impact of EMI noise in the actual use of the mobile phone at the beginning of the design, in many cases, it is necessary to repeatedly adjust the filter characteristics or replace the filter installation point, which often leads to the design cycle exceeding expectations and increases the design and development cost. [page]

The receiver BU7285GU using this MSDL communication method can greatly save line layout by transmitting image data through MSDL. For example, in the 18-bit color, QVGA size LCD panel interface, 22 LCD image dedicated lines were previously required, including the control signal line, but if MSDL is used, the lines can be reduced to 7 (Figure 3). The following are the main features of the BU7285GU series products:

1. The differential serial port MSDL greatly reduces the number of lines between the LCD panel and the CPU;

for example, the previous LCD interface: data + control signal lines = 20, and the interface control lines are 2. If a differential interface is used, in fact, only 6 lines + 1 control signal line are needed, a total of 7 lines.

Figure 3: System example based on BU7285GU.

2. Supports 18-bit color, QVGA-sized LCD panels (60fps transmission is possible);

3. Reduces EMI noise generated by the line to about 10dB through the CMOS interface;

4. Adopts the thin and light package that is most suitable for mobile phones-VBGA063T050;

5. LCD I/F: supports RGB stream;

data, HS, VS, DCLK

Vdd (I/O) = 1.7? 3.3V (built-in level conversion)

6. Built-in PLL: Generates serial port transmission clock.

Future topics

Following the 18-bit BU7285GU product that supports QVGA, ROHM is stepping up the development of other LCD data transmission series products, and will further develop interfaces that support higher-definition LCD panels, camera modules, and devices that support higher-speed data transmission. In the future, the newly developed mobile phone-specific LSI and LCD driver interface will be configured in mobile phones to meet customer needs for high-speed transmission, reduced line count, low EMI, and low power consumption.