Packetized Transfers

Transactions are constructed out of combinations of various packet types and carry the commands, address, and data associated with each transaction. Packets are organized in multiples of 4-byte blocks. If the link uses data paths that are narrower than 32 bits, successive bit-times are added to complete the packet transfer on an aligned 4-byte boundary. The primary packet types include:

• Control Packets ” used to manage various HT features, initiate transactions, and respond to transactions

• Data packets ” that carry the payload associated with a control packet (maximum payload is 64 bytes).

As illustrated in Figure 2-13, the control (CTL) signal differentiates control packets from data packets on the bus.

Figure 2-13. Distinguishing Control from Data Packets

For every group of 8 bits (or less) within the CAD path , there is a CLK signal. These groups of signals are transmitted source synchronously with the associated CLK signal. Source synchronous clocking requires that CLK and its associated group of CAD signals must all be routed with equal length traces in order to minimize skew between the signals.

Control Packets

Control packets manage various HT features, initiate transactions, and respond to transactions as listed below:

• Information packets

• Request packets

• Response packets

Information packet (4 bytes)

Information packets are exchanged between the two devices on a link. They are used by the two devices to synchronize the link, convey a serious error condition using the Sync Flood mechanism, and to update flow control buffer availability dynamically (using tags in NOP packets). The information packets are:

• NOP

• Sync/Error

Request packet (4 or 8 bytes)

Request packets initiate HT transactions and special functions. The request packets include:

• Sized Write (Posted)

• Broadcast Message

• Sized Write ( non-posted )

• Sized Read

• Flush

• Fence

• Atomic Read-Modify-Write

Response packet (4 bytes)

Response packets are used in HT split-transactions to reply to a previous request. The response may be a Read Response with data, or simply a Target Done Response confirming a non-posted write has reached its destination.

Data Packets

Some Request/Response command packets have data associated with them. Data packet structure varies with the command which caused it:

• Sized Dword Read Response or Write data packets are 1-16 dwords (4-64 bytes)

• Sized Byte Read Response data packets are 1 dword (any byte combination valid)

• Sized Byte Write data packets are 0-32 bytes (any byte combination valid)

• Read-Modify-Write