Altera HyperTransport MegaCore Function - Manual (Page 48)

3–22 Chapter 3: Specifications
HyperTransport MegaCore Function Specification
HyperTransport MegaCore Function User Guide © November 2009 Altera Corporation
Preliminary
Tx Command/Data Buffer Interfaces Operation
The Tx command/data buffers are temporary storage for all packets to be transmitted
to the link. The Tx command/data buffer interfaces are slave sink Atlantic interfaces.
Each packet stored in the buffer contains a 32- or 64-bit command followed by its
associated data, if there is any. The
Sop_i signal indicates the start of packet, which
must always be an HT command packet. Transmitted packets that do not have data
are always one DWORD, and have
Eop_i and Sop_i asserted.
HT command packets can be either 32 or 64 bits. 32-bit commands should be stored in
the least significant bytes
Dat_i[31:0] . The buffer automatically decodes the
command, determines if it is 32 or 64 bits, and determines whether or not there is data
associated with it. The buffer ignores
Mty for the command word. It is illegal to have a
non-zero value for the Mty signals at the command word if the command has data.
The Eop signal indicates the end of the packet. When Eop is valid, the Mty signals are
valid and indicate which bytes are invalid. Only the most significant four bytes can be
invalid.
The Mty signals have only two valid values for the HyperTransport MegaCore
function:
■ 100 indicates that Dat_i[63:32] is invalid
■ 000 indicates that all bytes are valid
After a packet is written to the buffer, the local-side application cannot retrieve that
packet or prevent its transmission. A HT link warm or cold reset resets all buffer
status bits and effectively destroys all data in the buffers. After the first word of a
packet is written to the buffer, the entire packet must follow before a new packet is
started. The transmitter circuitry begins transmission of a packet only after the entire
packet is written into the buffer. This implementation allows the local application to
use as much time as it needs to complete the packet.
Tx Command/Data Buffer Interface Signals
Table 3–7 describes the Tx buffer interface signals. Each of these signals has one of the
following three types:
■ Tx response interface (HT MegaCore function signal names are prefixed with TxR)
■ Tx posted interface (HT MegaCore function signal names are prefixed with TxP)
■ Tx non-posted interface (HT MegaCore function signal names are prefixed with
TxNp)
Because each channel uses similar interface signals, they are only described once. Any
channels that have specific differences are noted in
Table 3–7 .