1.10 The USB Core
D
A
T
A
1
D
A
T
A
0
C
R
C
1
C
R
C
1
A
D
D
R
E
N
D
P
C
R
C
5
A
D
D
R
E
N
D
P
C
R
C
5
O
U
T
A
C
K
O
U
T
A
C
K
Payload
Data
Payload
Data
6
6
Token Packet
Data Packet
Data Packet
H/S Pkt
Token Packet
H/S Pkt
Payload
Data
Serial
D+
Interface
Engine
(SIE)
Payload
Data
D-
A
C
K
USB
Tranceiver
Figure 1-8. What the SIE Does
Every USB device has a Serial Interface Engine (SIE). The SIE connects to the USB data
lines D+ and D-, and delivers bytes to and from the USB device. Figure 1-8 illustrates a
USB bulk transfer, with time moving from left to right. The SIE decodes the packet PIDs,
performs error checking on the data using the transmitted CRC bits, and delivers payload
data to the USB device. If the SIE encounters an error in the data, it automatically indi-
cates no response instead of supplying a handshake PID. This instructs the host to re-
transmit the data at a later time.
Bulk transfers such as the one illustrated in Figure 1-8 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 peripheral
device has successfully transferred the data, it commands the SIE to send an ACK hand-
shake packet, indicating success.
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 the two-wire USB. Because the USB uses a
self-clocking data format (NRZI), the SIE also inserts bits at appropriate places in the bit
stream to guarantee a certain number of transitions in the serial data. This is called “bit
stuffing,” and is transparently handled by the SIE.
Page 1-10
Chapter 1. Introducing EZ-USB
EZ-USB TRM v1.9
CYPRESS [ CYPRESS ]
