Oki JOB60851 user manual Bus Transactions, USB Data Flow Types

Chapter 4 Software Development

		standard device requests, and standard device
		descriptors
Chapter 10	USB Host: Hardware	Functions and operation of host hardware and
	and Software	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

(1)Data request (IN)

(3)Data request (IN)

(5)Successful receipt acknowledgment (ACK)

→

←

→

←

→

Device

(2) No data (NAK)

(4) Data transfer (DATA0)

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