- Limitation of speed with HW design, Fig.1
With the HW design, Fig.1, there is a bottleneck of speed when it connects with PC. To get maximum speed, the transfers, (1) and (2) should be done at the same time. But usual device driver for mass storage isn't designed considering it. For example, with Intel PXA27X BSP for Microsoft Windows Mobile, the transfer is done one after the other. Cause the FIFO size for USB endpoints is very small, typically 512Byets, comparing with the size of HDD, larger than 128KB, the speed is reduced even by one-fifth of its maximum.
The Fig.2 may be alternative HW design. With direct connection between HDD controller and USB controller, a fastest Ultra DMA could be used. Typical currently used USB controller supports direct connection by ATAPI standard interface, like Ultra DMA.
In case of connection with PC, we can get the maximum speed without any CPU load, (1). When it is need for CPU to access the HDD, (2), DMA can be used.
