Electronic Design

Short-Run Serial Interface May Become Standard

The new Mobile Pixel Link (MPL) physical-layer (PHY) chips from National Semiconductor are cheap and simple serial interfaces for transmitting data between chips and other subsystems in multimedia cell phones.

These phones now use 8- and even 16-bit buses on flexible printed circuits to carry the video data from the processor to the screen or CCD sensor to the processor. Flexible buses are expensive and bulky, especially in clam-shell phones, whose cables must pass through the hinge. Such interfaces and cabling are also sources of ugly electromagnetic-interference (EMI) problems.

Today, virtually all digital interfaces are going serial. While parallel data buses are fast, they run out of steam above several hundred megabits/s and distances over a few inches. To push past this limitation, most computer and networking equipment has adopted serial data interfaces that can easily achieve multiple gigabit data rates at extended distances.

This trend is now moving into cell phones and other multimedia devices with high-resolution color LCD screens and/or digital-camera sensors. They cram in unbelievable amounts of circuitry that must run at extreme speeds to satisfy the display and the camera sensor.

The MPL chips use a switching current source that takes the incoming serial video data and switches it between the low-current levels. The current-source transmitter drives a 50-Ω impedance line made with a pair of conductors on flexible pc-board material. This is certainly narrower, simpler, and cheaper than the traditional multiple conductor bus. A separate clock line makes for a fast synchronous link.

At the receiving end, an op amp converts the current pulses back into the serial data voltage. With binary current levels of 150 and 450 µA over a distance of less than 20 cm, a data rate of 160 Mbits/s is easily achievable. Moreover, this current-mode approach drops the EMI level so much that it's no longer a problem.

National Semiconductor has even dubbed it the MPL Level 0 Whisperbus. Power consumption is extremely low when stacked up against comparable devices. A level 1 version is expected, with higher currents accompanied by a command/data format that should reach serial rates as high as 1 Gbit/s.

Two National chips support this promising and potential technology. The LM2501 is a camera-to-baseband interface, and the LM2502 is a baseband-to-LCD screen interface (see the figure). Samples are available now. Production will occur late in the second quarter. While MPL requires the addition of these chips to the handset, the cost is more than offset by the reduction in flexible pc-board buses and their attendant cost and physical hassle.

National hopes the MPL will become a standard for multimedia data transfer in smart phones, digital cameras, and PDAs. It will offer MPL as an open standard, meaning royalty-free with no licensing hassles. While similar interfaces are being designed and considered, the MPL offers some benefits worth considering.

Engineering samples for the LM2501 and LM2502 are available now. Call for detailed volume pricing.

National Semiconductor Corp.

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