Sharp Corp has prototyped an optical interconnection system intended mainly for mobile phones. The system was developed with a focus on reducing the power consumption and mounting area.
The power consumed by the entire system to transmit data at 1.25 Gbps was reduced to about 108mW.
The prototype system is composed of an optical connector incorporating optical elements for transmission and reception, and the "main chip," an IC for driving light-emitting elements and processing electrical signals. The area of the IC was reduced by about 30% compared with existing ICs.
Sharp has been working on the research and development of optical transmission systems so as to reduce the EMI (electromagnetic interference) generated from data transmission channels. Data transmission speeds inside mobile phones are increasing in line with their improved functionality and the increased resolution of their LCD panels. With the increase in data transmission speed, the EMI generated from transmission channels became a major issue, according to the company.
The reduction in both power consumption and mounting area was realized by the common use of VCOs (voltage controlled oscillators) by both the transmitting and receiving sides. In addition, digital CDR (clock data recovery) was adopted because it is more suitable for the common use of VCOs than analog CDR, according to Sharp.
However, "It is very difficult to employ digital CDR for a high-speed transmission of GHz-order," the company said. This is because of signals called the "Glitch."
In CDR, the data output from the light receiving element and the phases of clock signals, which are transmitted from the VCO and selected via a multiplexer for use in CDR, are monitored by a comparator to select optimal clock signals to switch to. In high-speed transmission, an instantaneous pulse signal called "glitch" could be generated when the clock signals are being switched, increasing the bit error rate of reproduced data.
As a countermeasure, the timing for switching clock signals was controlled to reduce the generation of glitches. For example, if the current clock signal (CLK(n)) is "low" and the next clock signal (CLK(n+1)) is "high," glitches are generated. Therefore, the timing was controlled so that switching occurs when CLK(n) and CLK(n+1) are both in "high" state to reduce the generation of glitches.
Sharp plans to further reduce the power consumption for adoption in mobile phones, aiming at 50mW.
The details were disclosed at IEEE Asian Solid-State Circuits Conference (A-SSCC) 2008, which took place from Nov 3 to 5, 2008, in Fukuoka Prefecture, Japan. (Lecture No.: Industry 2-3)
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