When we perform output transfers to the endpoint for flash in addition to the 1 millisecond periodic outputs, we start seeing intermittent errors on the 1 ms. So long as we use only one of the endpoints to transfer the 1 millisecond repetitive data, we see no problem. The interrupt service routine examines the descriptors to see which ones have completed, and sets operating system even flags associated with the completed transfers. The software mostly polls the EHCI, and the only interrupt is the Interrupt On Completion of transfer descriptors. The USB host controller is an Intel EHCI device built into the chip-set. The other two endpoints transfer 512 bytes or less of output or input data randomly to communicate with flash memory. Two of the endpoints transfer about 1072 bytes of output data, and 245 bytes of input data continually at a 1 millisecond rate. The USB device is on-board with the CPU chip-set (Intel SCH US15W) We transfer data using four endpoints. Our devices are limited to one USB port with one USB High Speed device (the CY7C68000). We have an embedded software design used for process control and have written a simple embedded USB driver. We are using the Cypress Semiconductor CY7C68001 EZ-USB SX2 High Speed USB Interface Device.
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