Oki JOB60851 user manual Packets, Endpoints, lists the USB packet types

Chapter 4 Software Development

Figure 4.1.2. Two USB Transactions

All bus transactions begin with a token packet from the host. Devices never initiate data transfers on their own.

The host controller is in charge of scheduling all traffic on the bus. It schedules the appropriate transactions for the four data flow types at 1-ms intervals, which the USB documentation calls frames.

Token packet types include data request (IN), data transfer notification (OUT), and command transfer notification (SETUP).

4-1-6. Packets

A packet is a continuous bit stream starting with the synchronization pattern and flowing in one direction.

Table 4.1.3. lists the USB packet types.

PID Type	PID Name	Transmitter	Description
Token	OUT	Host	Data transfer notification
	IN	Host	Data request
	SOF	Host	Start of frame
	SETUP	Host	Data transfer notification for control pipe
Data	DATA0	Host/Device	Data packet PID even
	DATA1	Host/Device	Data packet PID odd
Handshake	ACK	Host/Device	Receiver accepts error-free packet
	NAK	Device	Transmitter cannot send data
			Receiver cannot accept data
	STALL	Device	Control pipe request not supported
			Endpoint halted
Special	PRE	Host	Preamble enabling downstream bus traffic to low-speed devices

4-1-7. Endpoints

The USB provides separate logical communication flows, called pipes, between the host and a USB function. The function end of a pipe is called the endpoint. This endpoint must provide buffer space (FIFO) capable of holding at least one data packet (a.k.a. the maximum payload size).

Endpoints and pipes are characterized, at creation, by the direction (from or to host) and by the data flow type (control, bulk, interrupt, or isochronous). There are provisions for up to 16 pipes in each direction, for a total of 32 endpoints per function. The token packet that the host transmits to initiate a transaction specifies the endpoint buffer with the endpoint address made up of the device address and the endpoint number.

USB device controller capacity is expressed by the number of endpoints it has and such endpoint specifications as supported data flow types and endpoint buffer sizes. Examining these endpoint specifications thus reveals whether the USB device controller is suitable for the intended application.

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