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Number of USB devices connected to the USB host | Cypress Semiconductor

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Number of USB devices connected to the USB host

Last Updated: September 15, 2011

Is it possible to have numerous devices connected to the host at a single time (>64) or so. What is the maximum and how is this limitation imposed.


Technically, each root hub can support 127 device on a five tier level (as per the spec).  The limitation is due to the  number of different addresses that
can be assigned  to the device.  The address field is a 7 bit field and hence can have 127 individual addresses. See section of the USB spec for more details on this.

In reality, the host  need to keep up with the devices.  The host will only configure a device (the device will enumerate) if the bandwidth that it needs is available, and the power requirements are within the  spec limitation.  Please see chapter 7 and 11 of the USB spec for this.    Of course if any of this is violated  the device will fail to  enumerate.  In reality rarely is the case when 127 devices are attached to the same Hub port.

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