Cypress Semiconductor FX2LP Technické informace stáhnout pdf (Strana 35)

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Chapter 1. Introducing EZ-USB FX2 Page 1-9

1.10 The Serial Interface Engine (SIE)

Figure 1-6. What the SIE Does

Every USB device has a Serial Interface Engine (SIE) which connects to the USB data lines (D+

and D-) and delivers data to and from the USB device. Figure 1-6 illustrates the SIE’s role: it

decodes the packet PIDs, performs error checking on the data using the transmitted CRC bits, and

delivers payload data to the USB device.

Bulk transfers are asynchronous, meaning that they include a flow control mechanism using ACK

and NAK handshake PIDs. The SIE indicates busy to the host by sending a NAK handshake

packet. When the USB device has successfully transferred the data, it commands the SIE to send

an ACK handshake packet, indicating success. If the SIE encounters an error in the data, it auto-

matically indicates no response instead of supplying a handshake PID. This instructs the host to

retransmit the data at a later time.

To send data to the host, the SIE accepts bytes and control signals from the USB device, formats it

for USB transfer, and sends it over D+ and D-. Because USB uses a self-clocking data format

(NRZI), the SIE also inserts bits at appropriate places in the bit stream to guarantee a certain num-

ber of transitions in the serial data. This is called “bit stuffing,” and is handled automatically by the

FX2’s SIE.

One of the most important features of the FX2 (and the other EZ-USB chips) family is that its con-

figuration is soft. Instead of requiring ROM or other fixed memory, it contains internal program/data

Serial

Interface

Engine

(SIE)

D+

D-

USB

Transceiver

Token Packet

Payload

Data

Data Packet

Token Packet

Payload

Data

Data Packet

H/S Pkt

Payload

Data

Payload

Data

H/S Pkt

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