Datasheet

SN74V263, SN74V273, SN74V283, SN74V293
8192 × 18, 16384 × 18, 32768 × 18, 65536 × 18
3.3-V CMOS FIRST-IN, FIRST-OUT MEMORIES
SCAS669D – JUNE 2001 – REVISED FEBRUARY 2003
26
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
retransmit operation
The retransmit operation allows data that has already been read to be accessed again. There are two modes
of retransmit operation: normal latency and zero latency. There are two stages to retransmit. The first stage is
a setup procedure that resets the read pointer to the first location of memory. The second stage is the actual
retransmit, which consists of reading out the memory contents, starting at the beginning of memory.
Retransmit setup is initiated by holding RT
low during a rising RCLK edge. REN and WEN must be high before
RCLK goes high while RT
is low. When zero latency is utilized, REN need not be high before bringing RT low.
At least two words, but no more than D – 2 words, should have been written into the FIFO and read from the
FIFO between reset (master or partial) and the time of retransmit setup. If ×18 input or ×8 output bus width is
selected, D = 8192 for the SN74V263, D = 16384 for the SN74V273, D = 32768 for the SN74V283, and
D = 65536 for the SN74V293. If both ×9 input and ×9 output bus widths are selected, D = 16384 for the
SN74V263, D = 32768 for the SN74V273, D = 65536 for the SN74V283, and D = 131072 for the SN74V293.
In FWFT mode, if ×18 input or ×18 output bus width is selected, D = 8193 for the SN74V263, D = 16385 for the
SN74V273, D = 32769 for the SN74V283, and D = 65537 for the SN74V293. If both ×9 input and ×9 output bus
widths are selected, D = 16385 for the SN74V263, D = 32769 for the SN74V273, D = 65537 for the SN74V283,
and D = 131073 for the SN74V293.
In normal retransmit mode, if FWFT mode is selected, the FIFO marks the beginning of the retransmit setup
by setting OR
high. During this period, the internal read pointer is set to the first location of the RAM array.
When OR
goes low, retransmit setup is complete. At the same time, contents of the first location appear on the
outputs. Since FWFT mode is selected, the first word appears on the outputs; no low on REN
is necessary.
Reading all subsequent words requires a low on REN
to enable the rising edge of RCLK.
See Figure 12 for timing information.
If standard mode is selected, the FIFO marks the beginning of the retransmit setup by setting EF
low. The
change in level is noticeable only if EF
was high before setup. During this period, the internal read pointer is
initialized to the first location of the RAM array.
When EF
goes high, retransmit setup is complete and read operations can begin, starting with the first location
in memory. Since standard mode is selected, every word read, including the first word following retransmit
setup, requires a low on REN
to enable the rising edge of RCLK.
See Figure 11 for timing information.
For either FWFT mode or standard mode, updating of the PAE
, HF, and PAF flags begins with the rising edge
of RCLK that the RT
is set up on. PAE is synchronized to RCLK, thus, on the second rising edge of RCLK after
RT
is set up, the PAE flag is updated. HF is asynchronous, thus, the rising edge of RCLK that RT is set up on
updates HF
. PAF is synchronized to WCLK, thus, the second rising edge of WCLK that occurs t
sk
after the rising
edge of RCLK that RT
is set up on updates PAF. RT is synchronized to RCLK.
The retransmit function has the option of two modes of operation, either normal latency or zero latency.
Figures 11 and 12 show to normal latency. Figures 13 and 14 show the zero-latency retransmit operation. Zero
latency means, basically, that the first data word to be retransmitted is placed in the output register with respect
to the RCLK pulse that initiated the retransmit.