Re: [usb] usb clarifications

["Vikas T Rao " <vikasraot@myw.ltindia.com>]

** PRIVATE **
** Proprietary **

ravi,

if the data payload of an isochronous device(which is configured) is 1023 bytes, then the host cannot configure one more iso device with 1023 bytes payload. if the other device has less payload(<= 256 bytes), then only it can be configured by the host. like this if we connect as many isochronous devices as possible, the payload of the new device should be less such that the combined bandwidth of all devices(including interrupt transfer devices) should not exceed 90%(approx.) of the frame time. the remaining time frame will be used for control/bulk transfers(if possible).

let's take an example where all the 5 iso devices can be configured. let their payload be 1023, 256, 64, 16, 4 bytes. then the total data bytes transferred is 1363 bytes. still a little amount of time will be left for other transfers. the above situation is where there are only iso devices. if the 1st configured device has many more iso/interrupt endpoints, then there will be a prob in configuring other devices with a greater payload than mentioned above.

coming to ur question,

isochronous endpoint need not always have 1023 bytes payload. it's the max value only. it can have lesser payloads also mentioned in their descriptors(remember that the payload value in descriptor is not alterable during the device operation). but if one device of 1023 bytes payload is configured, the other device with 1023 bytes can't be configured. that's the limitation of USB1.1 spec which has been rectified in USB2.0. the payload of iso endpoints remain constant and they don't change to suit the bandwdith requirements. i hope only control endpoints have the capability to transfer lesser payloads than mentioned in their descriptors.

so, 5 iso devices(with 1023 bytes payload) will not work together at all in USB1.1 environment. though their(iso devices) buffer sizes allow them to transfer lesser payloads, they have to transfer the same payload as mentioned in their descriptor.

i hope u have understood.

can u tell me where r u working ravi.

...vikas.

>>> ravikumar_vlsi@yahoo.co.in 06/17/02 02:33PM >>>
hi folks,
It is mentioned in usb 1.1 spec 

'the data payload during isochoronous transfer is 1023
bytes during each frame '

if i connect nearly 5 isochronous device to undertake
transaction .
how the bandwidth wil be allocated ?

a)for each device 1023/5(Five isochronous usb devices)
 = 204 bytes wil be alocated in each frame

or 

b)each device  will take unique frame to tranfer its
data content , therefore consuming 5 frames .

pls clarify 
Regards
Ravikumar


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