The next-generation Secure Digital (SD) memory card standard, SD Specification Version 4.00 (provisional name), is slated for completion in spring 2010, and may well feature a new dedicated pin for high-speed data transfer (Table 1). The plan is to use low-voltage differential signaling (LVDS) to boost the max data transfer rate on the read read/write bus (the "SD bus") from its current 104Mbyte/s to 300Mbyte/s.
The serial transfer
technology will also be made available for use in SD Input/Output
(SDIO), with the objective of making it possible to use the SD bus as
the internal bus for embedded systems. According to a source involved
with the SD Association of the US, "Specifically, the target is the
mobile phone internal bus."
As the resolution of handset internal cameras continues to rise, along with other evolutionary changes, they are handling increasingly large data flows, and the data rate of the internal bus is rising to keep pace. Many products on the market now use parallel transfer schemes, but some people in the industry feel that the data rate ceiling is approaching. There is also increasing call for a unified internal bus technology. The idea of adopting the SD bus as the internal bus in 4.00-compliant cards does indeed look like a good one.
Another serial transfer technology for the mobile phone internal bus is the mobile industry processor interface (MIPI), supported by Nokia Corp of Finland and other firms. There are plans to boost MIPI's data transfer rate as well, from its current level of about 1Gbps.
MIPI is not yet widely used in the market, however, and the high-speed spec standardization process is well behind schedule. Several sources in the industry suggest the SD bus has a real chance of being adopted as the handset internal bus instead of MIPI.
The next-gen standard will have a new pin added for serial transfer, but the external card dimensions will remain the same. Both measures are to ensure backward compatibility.
One possible cause for concern in the new specification is susceptibility to crosstalk due to the need for two types of signal lines, with different data rates, transmission protocols, etc. The latest Universal Serial Bus (USB) specification, USB 3.0, also adopted a new pin specifically for high-speed datacom, but crosstalk issues contributed to the delayed finalization of the compliance test spec. It is possible that the next-gen, high-speed specification for SD memory cards will run into similar problems.
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