Книга: Advanced PIC Microcontroller Projects in C

8.3.1 Packets

8.3.1 Packets

Data is transmitted on a USB bus in packets. A packet starts with a sync pattern to allow the receiver clock to synchronize with the data. The data bytes of the packet follow, ending with an end of packet signal.

A packet identifier (PID) byte immediately follows the sync field of every USB packet. A PID itself is 4 bits long, and the 4 bits are repeated in a complemented form. There are seventeen different PID values, as shown in Table 8.4. These include one reserved value and one that is used twice, with two different meanings.

Table 8.4: PID values

There are four packet formats, based on which PID is at the start of the packet: token packets, data packets, handshake packets, and special packets.

Figure 8.4 shows the format of a token packet, which is used for OUT, IN, SOF (start of frame), and SETUP. The packet contains a 7-bit address, a 4-bit ENDP (endpoint number), a 5-bit CRC checksum, and an EOP (end of packet).

Figure 8.4: Token packet

A data packet is used for DATA0, DATA1, DATA2, and MDATA data transactions. The packet format is shown in Figure 8.5 and consists of the PID, 0–1024 bytes of data, a 2-byte CRC checksum, and an EOP.

Figure 8.5: Data packet

Figure 8.6 shows the format of a handshake packet, which is used for ACK, NAK, STALL, and NYET. ACK is used when a receiver acknowledges that it has received an error-free data packet. NAK is used when the receiving device cannot accept the packet. STALL indicates when the endpoint is halted, and NYET is used when there is no response from the receiver.

Figure 8.6: Handshake packet

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