UPSD3422 datasheet(145/293 Pages) STMICROELECTRONICS

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uPSD34xx

USB interface

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25.1.2

Endpoints

Each USB device contains a collection of independent endpoints, with an endpoint being the

destination of a communication flow between client software and the device. By design,

each USB device’s endpoints are given specific unique identifiers called endpoint numbers.

In addition, each endpoint has an associated direction for the data flow, either in (from

device to host) or out (from host to device). At the time a device is connected to the USB, it

is assigned a unique address. The combination of the device address, endpoint number,

and direction allows each endpoint to be uniquely referenced.

Each endpoint has some associated characteristics for the communication flow with the

client software running on the host. Those characteristics include:

●

Endpoint number;

●

Frequency and latency requirements;

●

Bandwidth requirements;

●

Maximum packet size capability;

●

Error handling requirements;

●

Data transfer direction; and

●

Transfer type.

All USB devices are required to implement a default control method that uses both the input

and output endpoints with Endpoint zero. The USB System Software uses this default

control method to initialize and generically manipulate the logical device as the Default

Control Pipe. Endpoint zero is always accessible and provides access to the device’s

configuration and status information as well as some basic control access.

Additional (non-zero) endpoints provide the communication flow required for the

functionality of the device. The non-zero endpoints are available for use only after the device

is configured per the normal device configuration process (see Chapter 9 of the USB

specification, http://www.usb.org).

25.1.3

Packets

USB transactions consist of data packets that contain special codes called Packet IDs

(PIDs). A PID signifies the kind of packet that is being transmitted. While there are more

types of PIDs in a USB system, the uPSD34xx responds to the three types shown in

Table 67

Table 67.

Types of packet IDs

Figure 50 shows an example of packets sent during a USB transfer. The first packet is a

Token Packet with an OUT PID. The OUT PID indicates that the host is going to send data to

the addressed device’s endpoint. The ADDR field contains the address of the device and the

ENDP field contains the endpoint within the addressed device. The CRC5 is a Cyclic

Redundancy Check for error checking.

PID Type

PID Name

Token

IN, OUT, SETUP

Data

DATA0, DATA1

Handshake

ACK, NAK, STALL

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