Altera HyperTransport MegaCore Function - Manual (Page 29)

Chapter 3: Specifications 3–3
HyperTransport MegaCore Function Specification
© November 2009 Altera Corporation HyperTransport MegaCore Function User Guide
Preliminary
HT Flow Control
All commands and data are separated into one of three separate virtual channels,
which provide higher performance and allow certain types of requests to pass others
to avoid deadlocks. However, in some cases, requests in one channel cannot pass
those in other channels to preserve ordering as required by systems. The three virtual
channels are:
■ Non-Posted Requests—Requests that require a response (all read requests and
optionally write requests)
■ Posted Requests—Requests that do not require a response (typically write requests)
■ Responses—Responses to non-posted requests (read responses or target done
responses to non-posted writes)
The HT flow control mechanism is a credit-based scheme maintained per virtual
channel for an individual link. A transmitter consumes a buffer credit each time it
transmits a packet and cannot transmit a packet to the receiver unless a buffer credit is
available. The receiver provides buffer credits for each available buffer at link
initialization, and it provides an additional buffer credit each time a buffer is freed
thereafter.
Buffer credits are transmitted in the opposite direction of the data flow as part of NOP
packets.
HyperTransport MegaCore Function Specification
This section describes the functionality and features of the 8-bit end-chain
HyperTransport MegaCore function.
Figure 3–3 shows the block diagram of the
HyperTransport MegaCore function. The HyperTransport MegaCore function is
partitioned into three layers:
■ Physical Interface
■ Synchronization and Alignment
■ Protocol Interface