The Packet-Based Protocol

HyperTransport employs a packet-based protocol in which all information ”address, commands, and data ” travel in packets which are multiples of four bytes each. Packets are used in link management (e.g. flow control and error reporting) and as building blocks in constructing more complex transactions such as read and write data transfers.

It should be noted that, while packet descriptions in this chapter are in terms of bytes, the link's bidirectional interface width (2, 4, 8, 16, or 32 bits) ultimately determines the amount of packet information sent during each bit time on HyperTransport links. There are two bit times per clock period.

Before looking at packet function and use, the following sections describe the mechanics of packet delivery over 2,4,8,16, and 32 bit scalable link interfaces.

8 Bit Interfaces

For 8-bit interfaces, one byte of packet information may be sent in each bit time. For example, a 4-byte request packet would be sent by the transmitter during four adjacent bit times, least significant byte first as shown in Figure 4-1 on page 61. Total time to complete a four-byte packet is two clock periods.

Figure 4-1. Four Byte Packet On An 8-Bit Interface

Interfaces Narrower Than 8 Bits

For link interfaces which are narrower than 8 bits, the first byte of packet information is shifted out over multiple bit times, least significant bits first. Referring to Figure 4-2 on page 62, a 2-bit interface would require four bit times to transmit each byte of information. After the first byte is sent, subsequent bytes in the packet are shifted out in the same manner. Total time to complete four byte packet: eight clock periods.

Figure 4-2. Four Byte Packet On A 2-Bit Interface

Interfaces Wider Than 8 Bits

For 16 or 32 bit interfaces, packet delivery is accelerated by sending multiple bytes of packet information in parallel with each other.

16 Bit Interfaces

On 16-bit interfaces, two bytes of information may be sent in each bit time. Referring to Figure 4-3 on page 63, note that even numbered bytes travel on the lower portion of the 16 bit interface, odd numbered bytes on the upper portion.

Figure 4-3. Four Byte Packet On A 16-Bit Interface

32 Bit Interfaces

Similarly, four bytes of information may be sent in each bit time on a 32-bit interface. This is shown in Figure 4-4 on page 64. Note that Byte 0 travels on the low portion of the interface (CAD0-7), Byte 1 on the second byte lane (CAD8-15), etc.

Figure 4-4. Four Byte Packet On A 32-Bit Interface

A reminder: Because all HyperTransport packets are multiples of 4 bytes, bits of packet information always divide evenly into the available bus width. There never is a need to "pad" unused bit lanes .