Tip of the Week: Un-parallelize the video interface--Go serial

Ever-increasing bandwidth
Displays are increasing in both physical size and resolution for portable applications. Smaller bezel sizes allow for a larger active area to fit into current application form factors. Resolutions are also increasing, providing a finer viewing experience. Frame rates are also on the rise to 50-60 frames per second (fps) to provide smooth video, and 18-bit and even 24-bit color depth is replacing the older RGB565 (16-bit) color depth. These trends require more bandwidth. For example an 18-bit RGB HVGA display (320 x 480), at 60 fps and with 10 percent blanking requires a Pixel Clock (PCLK) of 10 MHz, and a resulting pixel bandwidth of 180 Mbps.

The parallel paradox
The solution in the past to upgrade a parallel bus from RGB565 to full 24-bit RGB would be to simply add more lines. Thus 24 RGB signals, up to 3 video control signals (HS, VS and DE) and a PCLK are required for a total of 28 signals. With these changes were implemented, the problems began.

The first challenge is physical space, more signals means more interconnect which is physically larger and not aligned with target small portable applications. Second is cost, a wider flex costs more since it requires more connector pins, and also pins on the host and target devices--all of which are cost drivers. Battery life is always of utmost importance. More signals is a multiplier in terms of power dissipation, again not the trend desired for portable applications. The next key attribute, is EMI. Noise generated from the streaming video interface generally increases as the number of signals increase.

To support the increasing bandwidth requirements, faster transition times are required. This is problematic, as the faster edges will generate high frequency noise components. The larger number of signals will also increase emissions. In portable applications, multiple radios are now commonplace. These range from WiFi, to Bluetooth, to cellular (GSM with multiple bands for example). Noise in the radio bands tends to degrade the radio receiver's sensitivity which is totally undesired. A common solution for lowering EMI is to filter the edge rates with additional capacitive loading. While effective for controlling EMI, it drives up power dissipation greatly which in turn reduces battery operating time.

Go serial!
Serial Interfaces address the demanding needs of the portable application on all fronts.

A serialized bus by nature is less signal lines. Instead of 28 signals, the video interface can be reduced to only a few signals or even all the way to a single signal. This reduces the interconnect's physical size, connector pin counts, IC pins, and generally lowers system cost. Of course the smaller interface is easy to route about the system from host chip (GUI) to display. This can be especially important in applications that flip, twist, or have other mechanical challenges such as being ultra thin.

Next, the physical layer is optimized for the targeted application, and is not simply a common place LVCMOS. Instead it is a specialty physical layer optimized for the application at hand. Different options are available such as LVDS-like to single ended current-mode physical layers. What is unique is their optimization for short-haul embedded applications that allows for smaller swings to be used. With small swings, power dissipation required to charge up and down the load capacitance is greatly reduced.

At higher switching rates, required by higher resolution displays/higher frame rates/and full color, the current-mode transmission tends to consume less power than a comparable LVCMOS interface with heavy capacitive (required for EMI filtering) loads. The features that provide low power also provide low radiated EMI. With the controlled current switching, current signaling transmission, and resulting small voltage swing, EMI generation off the interface (even at the faster rates) is extremely low.

Application requirements are fully addressed with serial video interfaces. The smaller physical buses require less space. Also the smaller busses tend to allow for more elaborate physical / mechanical interconnects. With the low swings, power dissipation is minimized, allowing battery charge to be used by other functions or to run longer. Small signals also tend to radiate less, with lower noise, less shielding is required and impact to the radio's receiver (de-sense) is minimized. To break the parallel paradox, go serial!

See Related information at: National Semi raises video experience bar in latest handheld multimedia devices

About the Author
John Goldie is a Display Interface Applications Engineer at National Semiconductor. He holds a BSEE degree from San Francisco State University. He can be reached at: john.goldie@nsm.com