Chapter 22: Overview of USB Host Software

Depth in topology — The round-trip time required to send packets to a device and receive the response depends on the number of cable segments the transmission must cross. The maximum round-trip delay is specified as 16 bit times. Software may either use this worst case value when calculating transaction time or determine where in the topology the target device resides and specify the delay based on the number of cable segments the transmission must cross (70ns one-way trip delay per cable segment).

Once the total bus time required to support the pipe has been calculated, if the pipe can be supported, the USBD establishes the pipe and adds the transaction to the frame schedule.

Bus Bandwidth Reclamation

Since the bandwidth calculations in many instances are based on worst case values (e.g., bit stuffing), there will normally be bus bandwidth remaining after all scheduled transactions have completed during a frame. Host controllers can be designed to reclaim this leftover bandwidth and perform additional transactions to utilize the bus more efficiently. Control and bulk transfers are candidates for reclamation. Since isochronous and interrupt transfers are scheduled, they will already have completed. How a host controller implements bus bandwidth reclamation is implementation dependent.

Data Transfer Management

Each pipe that is set up during configuration is defined for a specific endpoint within a functional interface within a device. A given interface, and therefore each endpoint within an interface, is managed by exactly one software client driver. A client must initialize a pipe before it can be used. During pipe initialization, the client specifies the policy for that pipe. The policy defines the exact amount of data to be transferred per IRP and the maximum service interval (if required). The client may also request notification of status information (e.g., completion status) from the USBD. The client must also specify the location of its memory buffer where data read from the endpoint is to be stored or where data to be written to the endpoint resides.

The USBD may need to break the IRP into chunks that can be supported by both the endpoint (buffer size) and the constraints associated with the USB protocol.

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