Chapter 4 Software Development
page 4-4
standard device requests, and standard device
descriptors
Chapter 10 USB Host: Hardware
and Software
Functions and operation of host hardware and
software
Chapter 11 Hub Specification Hub port operation, requests, and descriptors
Chapter 4 provides a firm grounding in the USB core specifications. All developers of USB
equipment must study Chapter 5 very carefully. Hardware developers must read Chapter 7; firmware
developers, Chapters 8 and 9. Firmware developers must pay particular attention to the timing
specifications in Chapter 7.
What follows are key points from the core specifications. For complete details, refer to the
specifications available from the USB Implementers Forum web site.
4-1-4. Data Flow Types
The USB specifications define four data flow types with the following characteristics. Flexibly
combining these four data flow types provides solutions to the communications needs of a wide
variety of applications.
Table 4.1.2. USB Data Flow Types
Control Communication of commands and responses for device configuration and pipe
control
Bulk Transfer of relatively large, bursty data volumes with wide dynamic latitude in
transmission constraints
Interrupt Transfer of small data volumes within time limits based on human-perceptible echo
or feedback response characteristics
Isochronous Transfer using prenegotiated USB bandwidth with a prenegotiated delivery latency--
audio data, for example--with no procedure for retransmitting data
4-1-5. Bus Transactions
Data transfers consist of bus transactions, exchanges of basic packets between the host and a specific
device. The example below shows two such transactions.
The first data request (IN) from the host arrives when the device has no data for delivery, so the
latter returns a NAK handshake, completing the transaction. The NAK indicates two things: that the
device has no data ready and that the host should resend the request later.
The second data request (IN), in contrast, causes the device to deliver the data (DATA0) that it has
ready. The host acknowledges successful receipt of this data with an ACK, completing the
transaction.
Transactions require cirtain bit time order response, so are implemented in hardware.
Host Device
(1) Data request (IN)
→
←
(2) No data (NAK)
(3) Data request (IN)
→
←
(4) Data transfer (DATA0)
(5) Successful receipt acknowledgment (ACK)
→