UPSD3422 datasheet(146/293 Pages) STMICROELECTRONICS

146 / 293 page

USB interface

uPSD34xx

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The data packet contains a DATA1 or DATA0 PID. In a USB system, the host or device that is

sending data is responsible for toggling the data PID between DATA0 and DATA1. The

receiving device keeps track of the Toggle Bit and compares it with the data PID that is

received. This provides a means for the receiving host or device to detect a corrupted

handshake packet. The Payload Data is the data that the host is sending to the device and

the CRC16 is used for error checking.

For an OUT transaction, the host sends the token and data packets. The receiving device

sends a handshake packet to notify the host whether it was able to accept the packet or not.

There are three handshake PIDs as follows:

●

ACK: this PID indicates that the device received the data successfully.

●

NAK: this handshake indicates that the device was not able to receive the data (it is

busy). A NAK does not mean there was an error, since errors are indicated by a “no

handshake” packet. When the host receives a NAK PID or does not receive a

handshake packet at all, the host retries sending the data at a later time.

●

STALL: this handshake indicates that something is wrong. For example, the host has

sent a device request that is not understood, the host is trying to access a resource that

is not available, or something is wrong with the device.

Figure 50.

USB Packets in a USB Transfer Example

25.1.4

Data transfers with the host

The host issues OUT tokens followed by Data Tokens to send data to a device. The device

responds with an appropriate handshake packet (ACK/NAK), indicating whether it was able

to receive the data. If the device does not receive the data packet OK (because there is

some error), it does not respond with a handshake packet. In the case of a NAK or no

response, the host retries sending the data at a later time.

USB devices are not able to send data to a host whenever they have it ready. When a device

has data ready, it loads data into its endpoint buffer, making it ready for a transfer. The data

will remain in the buffer until the host issues an IN token to that device’s endpoint, at which

time the data will be sent. If the host receives the data OK, it follows with an ACK handshake

(a host never NAKs). If the host did not receive the data OK, there is no handshake packet.

In this case, the device should reload its endpoint buffer as appropriate and the host will

retry again later to retrieve the data.

Token Packet

Data Packet

Handshake Packet

OUT

ADDR

ENDP

CRC5

CRC16

ACK

Data1

Payload

Data

Token Packet

Data Packet

Handshake Packet

OUT

ADDR

ENDP

CRC5

CRC16

ACK

Data0

Payload

Data

AI10489

UPSD3422 Datasheet(PDF) 146 Page - STMicroelectronics

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