How do USB 2.0 devices attain high throuput?

The actual 'service' frequency can be much higher. The 125 us you are referring to is the micro frame size. Some endpoints, like interrupts, are only polled once per micro frame, but bulk data can be transferred in a burst. Typically, when the device responds with a full sized data packet (512) bytes, the host will immediately ask for the next one. Of course, this also depends on the driver, which should try to read large blocks at once. If the device responds with a smaller packet, this is usually interpreted as the end of the transfer, and the host could stop polling until the driver has set up the next request.

When sending data to a device, if the device responds with NYET, some hosts will also introduce a delay before each PING. If you want high throughput, it's important to avoid the NYET responses as much as possible. How you do that, depends on the device.

Using 64KB bulk requests, I can read more than 300 Mbit/sec from my USB

Thank you very much. .I will revisit the spec. Good info!

The 125uS timebase allows you to do 13 of 512 byte bulk transfers in each frame. This allows you to transfer 5324800 bytes per seconds which is approximately

2. Actually getting that data rate depends on the endpoint not issuing any NAKs during the microframe. If the host controller sees a NAK it has to stop sending data.

If you need timing guarentees then you'd need to do Isochronous transfers and the limit is 2 of 3072 byte transfers ie about 46 Mbytes/s base 2

In practice actually achieving either of these maxima usually requires that your device is the only thing on the bus except for the host controller.

Download and look at chapter 5, particularly 5.4, 5.6 and 5.8 of usb_20.pdf in the zipfile from

Personally I prefer IEEE1394 which looks inferior if you look at marketing blurb but is better in practice.

-p

good stuff .. Thank you Paul.