Altera Arria V GZ Avalon-ST - Manual (Page 72)

Signal Direction Description
tx_st_ready
Output Indicates that the Transaction Layer is ready to accept data for
transmission. The core deasserts this signal to throttle the data
stream.
tx_st_ready
may be asserted during reset. The Applica‐
tion Layer should wait at least 2 clock cycles after the reset is
released before issuing packets on the Avalon-ST TX interface.
The
reset_status
signal can also be used to monitor when the
IP core has come out of reset.
If
tx_st_ready
is asserted by the Transaction Layer on cycle
, then
+ readyLatency>
is a ready cycle, during which
the Application Layer may assert
valid
and transfer data.
When
tx_st_ready
,
tx_st_valid
and
tx_st_data
are
registered (the typical case), Altera recommends a
readyLa-
tency
of 2 cycles to facilitate timing closure; however, a
readyLatency
of 1 cycle is possible. If no other delays are added
to the read-valid latency, the resulting delay corresponds to a
readyLatency
of 2.
tx_st_valid
Input Clocks
tx_st_data
to the core when
tx_st_ready
is also
asserted. Between
tx_st_sop
and
tx_st_eop
,
tx_st_valid
must
not be deasserted in the middle of a TLP except in response to
tx_st_ready
deassertion. When
tx_st_ready
deasserts, this
signal must deassert within 1 or 2 clock cycles. When
tx_st_
ready
reasserts, and
tx_st_data
is in mid-TLP, this signal must
reassert within 2 cycles. The figure entitled64-Bit Transaction
Layer Backpressures the Application Layer illustrates the timing of
this signal.
For 256-bit data, when you turn on Enable multiple packets per
cycle, the bit 0 applies to the entire bus
tx_st_data[255:0]
. Bit
1 is not used.
To facilitate timing closure, Altera recommends that you register
both the
tx_st_ready
and
tx_st_valid
signals. If no other
delays are added to the ready-valid latency, the resulting delay
corresponds to a
readyLatency
of 2.
tx_st_empty[1:0] Input Indicates the number of qwords that are empty during cycles that
contain the end of a packet. When asserted, the empty dwords
are in the high-order bits. Valid only when
tx_st_eop
is asserted.
Not used when
tx_st_data
is 64 bits. For 128-bit data, only bit 0
applies and indicates whether the upper qword contains data. For
256-bit data, both bits are used to indicate the number of upper
5-20 Avalon-ST TX Interface
UG-01127_avst
2014.12.15
Altera Corporation Interfaces and Signal Descriptions
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