FROM EDN EUROPE: Go serial for simpler mobile-phone-display interconnect
By Graham Prophet -- EDN, March 4, 2004
Another high-speed serial link? Don't panic. This one targets a specific circuit function and has a tightly defined area of application—in its first implementation, at least. National Semiconductor has designed MPL (Mobile Pixel Link) to interconnect the system board and the display module in mobile phones and handheld devices, especially those with a clamshell design. The problem, which has a number of solutions vying for attention in the marketplace, is that routing the signals to drive high-resolution colour displays and feeding data from an onboard camera stretches parallel buses to the limit. You will also face problems of power to drive all of the lines, EMC generated by the high-speed edges on those lines, and physical restrictions due to the need to route the bus around the hinge line of a clamshell phone; the demands on a flexible ribbon interconnect are severe. As cameras on phones extend into the megapixel range, and colour displays of quarter-VGA resolution are called on to show reasonable-frame-rate video, the problems become more intractable.
National Semiconductor is, therefore, proposing MPL as an "open" offering, in the hope that it will become a de-facto standard. MPL uses a low-current-mode system in which signalling occurs by a shift in current level; the current is unidirectional with no reversals, leading to low radiated emissions. You will be able to achieve a comparable data rate with around 5% of the power of a full LVDS (low-voltage- differential-signal) link, and the company says that power will also be lower than RSDS (reduced-swing differential signalling), or 8-bit parallel buses running at 2.8 or 1.8V. (The comparison is for an MPL operating from 3V.)
As an interim "fix" for existing and in-progress designs, National Semiconductor is introducing the LM25601 camera and 2502 display interface chips, which implement just the physical layer of the link. You can reduce the link to a handful of wires in each case; the chips come as serialiser/deserialiser pairs and will replace the physical-layer links with 8- and 16-bit-wide interfaces, respectively. The company refers to this introduction as Level 0 of its concept. It will later introduce Level 1, which will include an extended media-access definition and a command-set data structure for tighter integration into portable designs.
MPL will include guidance on connectors or on physical media, which can be either a pc board or a flexible interconnect. If the proposal gains traction as a standard, the company envisages a Level 2 that might include features such as a graphics-API (application-programming-interface) definition.
National Semiconductor, www.national.com.
