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Table 1: PID Types
1) SYNC Pattern
The synchronization pattern is a 0x80 byte sent before
each packet. The Start-of-Packet SOP delimiter is
part of the SYNC field.
2) Packet Identifier Field
The PID indicates the type of packet, its format and
type of error detection. It is always the second byte
sent and has its own error detection field. After the 4bit
packet type field follows the one’s complement of
it to ensure the correct decoding of the entire packet.
3) Address and Endpoint Fields
The address field or ADDR, it has 7 bits to specify
the address of the function that can be either the
source or destination of the packet.
The endpoint field or ENDP has four bits that, except
for the control endpoint zero, are functionspecific.
4) Frame Number Field
The frame interval used for full-speed devices is
1.00 ± 0.0005 ms and the host needs to send a packet
every time a new frame begins. The frame number
field is an 11-bit field that is incremented by the host
each start of frame.
5) Data Field
The data field may range from zero to 1,024 bytes
and must always be an integral number of bytes.
6) Cyclic Redundancy Check Fields
The CRCs provide protection for all single- and
double-bit errors on all non-PID fields. They are generated
with a specific polynomial before bit stuffing.
There are 2 different CRCs used in USB: the CRC-5
52
PID Type
Token
Data
Handshake
PID
Name
PID
bits
Fields
OUT 0001 ADDR+ENDP
IN 1001 ADDR+ENDP
SOF 0101 Frame Number
DATA0 0011 Data
DATA1 1011 Data
ACK 0010 ---
NACK 1010 ---
VHDL BASED CIRCUIT DESCRIPTION TO TRANSMIT,
RECEIVE AND ANALYSE USB DATA PACKETS
Table 2: CRC‘s
Generator
Polynomial
Token Packets
G(X) = X 5 + X 2 + 1
Binary Bit
Representation
00101B
Binary Bit Residual 01100B
Generator
Polynomial
Data Packets
G(X) = X 16 + X 15 + X 2 + 1
Binary Bit
Representation
1000000000000101B
Binary Bit Residual 1000000000001101B
Figure 2: General Solution
used for the token packets and the CRC-16 used only
for data packets; both are described in Table 2.
A. General Solution
III. DESIGN AND SIMULATION
The USB transceiver is divided into two main
blocks, one for receiving data and another one to
transmit it. Each of these blocks work independently
from one another, both connected to the microcontroller
by several control lines. A special block was created
to generate different frequency clocks to use as
reference and to keep track of the SOFs (see figure 2).
Every block in the structure was simulated before the
implementation.
B. Transmitter
The transmitter is in itself divided into 4 different
structures: the interface with the microcontroller µC,
the block for data packets, the block for token and
handshake packets, and finally the block in charge of
giving the proper format to the transmitted string.
Each of these blocks are shown in Figure 3; the dotted