Datasheet

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Debugging Serial Buses in Embedded System Designs
Data Packets
Data packets contain a PID byte, data bytes and 16-bit CRC
as shown in Figure 20.
DATA0 and DATA1 packets have a 1-bit sequence number
that is used in stop and wait automatic repeat-request
handshake. DATA0 and DATA1 packets alternate in error
free transmission. Data packets are resent with the same
sequence number when a transmission error occurs.
An error free data transaction is when the host sends a DATA0
packet to the device, the device sends a handshake ACK
packet and then the host sends a DATA1 packet.
If the host does not receive a handshake ACK packet or
received a NAK from the device, it resends the DATA0 packet.
If the device sent an ACK packet and receives the data packet
with the same sequence number, the device acknowledges
the data packet but ignores the data as a duplicate.
Start of Frame
Start of Frame (SOF) packet, shown in Figure 21, is used to
synchronize isochronous and polled data flows. The 11-bit
frame number is incremented by one in each consecutive SOF.
Handshake Packets
Handshake packets such as data packet accepted (ACK) and
data packet not accepted (NAK) are composed of the Sync
byte, PID byte and EOP as seen in Figure 18.
Token Packets
Host sent token packets are composed of the SYNC, PID
followed by two bytes composed of an 11-bit address and
5-bit cyclic redundancy check (CRC) (Figure 19).
The OUT, IN and SETUP tokens 11-bit address is subdivided
into a 7-bit device address and a 4-bit endpoint identifier.
Address zero is special and is for a device that has not been
assigned an address at the beginning of the enumeration
process. Later in the enumeration process the host assigns a
nonzero address to the device.
All devices have an endpoint zero. Endpoint zero is used for
device control and status. Other device endpoints are for data
sources and/or sinks.
The host sends an OUT token to a device followed by a data
packet. The host sends an IN token to a device and expects
to receive a data packet or handshake packet such as NAK
from the device.
Figure 18. Handshake packet organization.
Figure 20. Data Packets with the PID of DATA0 or DATA1.
Figure 19. OUT, IN and SETUP token packet organization.
Figure 21. Start of frame packets.
Sync PID EOP
Sync PID Data
16-bit
CRC
EOP
Sync PID
11-bit
Address
5-bit
CRC
EOP
Sync PID
Frame
Number
5-bit
CRC
EOP