Pocket PC using XScale SDIO Performance
By Chris De Herrera, Copyright 2002
 Version 1.00  Revised 8/27/2002

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Recently I have been reviewing the options for using SDIO (Secure Digital Input/Output) peripherals in Pocket PC 2002 devices. My interest in SDIO is piqued because vendors like Socket Communications are planning to offer a SDIO Bluetooth card. Also Sycard has created a SDIO WLAN, 802.11b wireless LAN card as well

Basically after reviewing the SDIO specifications, there are some significant differences in the options that vendors can choose to implement. The SDIO specifications allow for either 1 bit (SD Narrow) or 4 bit (SD Wide) bus and a clock rate of 20 mhz or 25 mhz. The combination of the bus size and the clock rate allow us to calculate the fastest that data can be transferred over the SDIO bus.

Intel’s implementation in their PXA250 chip http://www.intel.com/design/pca/applicationsprocessors/applnots/278533-001.pdf is to use a 1 bit bus and a 20 mhz bus speed. So the fastest data rate we will see is approximately 2.5 megabytes per second excluding any delays to process commands or read or write data on the SDIO card. The MultiMedia Card (MMC) uses the 1 bit bus and 20 mhz bus speed which the Pocket PCs support today.

If a chipset supported the 4 bit bus and 25 mhz bus speed, it would be using the fastest options for SDIO. The throughput in this configuration would be 12.5 megabytes. Right now none of the current crop of Pocket PCs uses this implementation.

Since Intel implemented the slowest version of SDIO, any peripheral that can move data at over 2.5 megabytes per second cannot be used at their fullest speed. Since 802.11b uses 11 megabits and has a theoretical throughput of 1.375 megabytes per second, the slower version of SDIO should work at approximately the full capabilities of the 802.11b protocol standard and the SDIO’s functional speed subject to the ability of the Pocket PC to process the data and send the appropriate commands to the SDIO card in a timely fashion.

Obviously speed is an important factor in using these wireless peripherals. I believe that vendors should work together like BSquare proposes in their SDIO Now program http://www.bsquare.com/device/products/devtools/sdionow/ to support common standards and implement the highest speeds that the hardware can support.