1.4
Tokens and PIDs
In this manual, you will read statements like, “When the host sends an IN token...” or “The
device responds with an ACK.” What do these terms mean? A USB transaction consists
of data packets identified by special codes called Packet IDs or PIDs. A PID signifies
what kind of packet is being transmitted. There are four PID types, as shown in Table 1-1.
Table 1-1. USB PIDs
PID Type
PID Name
Token Data IN, OUT, SOF, SETUP, DATA0, DATA1
Handshake ACK, NAK, STALL
Special
PRE
D
A
T
A
0
C
R
C
1
D
A
T
A
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
4
Data Packet
5
H/S Pkt
6
Token Packet
1
Data Packet
2
H/S Pkt
3
Figure 1-3. USB Packets
Figure 1-3 illustrates a USB transfer. Packet j is an OUT token, indicated by the OUT
PID. The OUT token signifies that data from the host is about to be transmitted over the
bus. Packet !k contains data, as indicated by the DATA1 PID. Packet l is a handshake
packet, sent by the device using the ACK (acknowledge) PID to signify to the host that the
device received the data error-free.
Continuing with Figure 1-3, a second transaction begins with another OUT token m, fol-
lowed by more data n, this time using the DATA0 PID. Finally, the device again indicates
success by transmitting the ACK PID in a handshake packet o.
Why two DATA PIDs, DATA0 and DATA1? It’s because the USB architects took error
correction very seriously. As mentioned previously, the ACK handshake is a signal to the
host that the peripheral received data without error (the CRC portion of the packet is used
to detect errors). But what if a handshake packet itself is garbled in transmission? To
detect this, each side, host and device maintains a data toggle bit, which is toggled
between data packet transfers. The state of this internal toggle bit is compared with the
Page 1-4
Chapter 1. Introducing EZ-USB
EZ-USB TRM v1.9
CYPRESS [ CYPRESS ]
