Synchronous & Asynchronous Nand Flash Memory
Solid state drives utilizing Nand flash memory, versus the traditional magnet hard disk drive, have long been overdue to take over desktop and laptop computing. The price for SSD have been high for years, and the capacities quite low in comparison to HDD. However, with the emerging ultrabook market segment as well as the latest offerings from Apple, SSD's are starting to become more mainstream. Prices are still a bit high for consumers and capacities a bit low for professionals, but one thing that no one has complained about is the vastly improved read and write speeds of flash memory solid state drives.
A relatively unknown technical term in reference to flash memory and read/write speeds is DDR toggle mode. Early flash memory up to USB2.0, encompassing SD/TF cards, MP3/MP4, and tablet PC flash memory, have all been asynchronous flash, meaning asynchronous read operation that improves the read performance of flash memory. On power up or reset, flash memory defaults to asynchronous read array mode to enable processors to immediately read from the flash memory, and the main advantage of asynchronous flash is a simpler system design with lower power consumption compared to other more performance based chips. Meanwhile, synchronous (DDR toggle mode) is a clock-synchronous read operation that improves the read performance of flash memory over that of single reads and page mode reads, yielding greater read and write speeds over asynchronous. Synchronous provides the highest flash read performance - data is output on clock edges at the clock frequency. Synchronous flash is best utilized in USB3.0 applications and SATA III SSD because of the transfer limitations of SATA II. In all applications, the synchronous speeds are most dependent on the flash controller used as well as the firmware and not all flash memory devices can take advantage of synchronous performance speeds.
With all this talk of synchronous vs asynchronous read and write speeds, what are some actual real world figures? The following is a pretty comprehensive list of popular flash ID's and their subsequent tested read/write speeds. Some feature direct comparisons between synchronous and asynchronous speeds of the same flash ID which is a clear indication of the performance differences. Also please note, synchronous flash doesn't make USB2.0 any faster because of the transfer limitations, as is the case with SATA II vs SATA III SSD synchronous performance. All testing was done in a controlled environment using the industry standard Crystal Disk Mark 3.0 with Asus motherboard 2GB RAM and Intel CPU.
Under the "Synch." column, checked boxes denote synchronous flash while blank boxes denote asynchronous flash.