10
IDT72255LA/72265LA CMOS SuperSync FIFO™
8,192 x 18 and 16,384 x 18
COMMERCIAL AND INDUSTRIAL
TEMPERATURE RANGES
SERIAL PROGRAMMING MODE
If Serial Programming mode has been selected, as described above, then
programming of PAE and PAF values can be achieved by using a combi-
nation of the LD, SEN, WCLK and SI input pins. Programming
PAE and
PAF proceeds as follows: when LD and SEN are set LOW, data on the
SI input are written, one bit for each WCLK rising edge, starting with the
Empty Offset LSB and ending with the Full Offset MSB. A total of 26 bits for
the IDT72255LA and 28 bits for the IDT72265LA. See Figure 13, Serial
Loading of Programmable Flag Registers, for the timing diagram for this
mode.
Using the serial method, individual registers cannot be programmed se-
lectively. PAE and PAF can show a valid status only after the complete
set of bits (for all offset registers) has been entered. The registers can be
reprogrammed as long as the complete set of new offset bits is entered.
When LD is LOW and SEN is HIGH, no serial write to the registers can
occur.
Write operations to the FIFO are allowed before and during the serial
programming sequence. In this case, the programming of all offset bits does
not have to occur at once. A select number of bits can be written to the SI
input and then, by bringing LD and SEN HIGH, data can be written to
FIFO memory via Dn by toggling WEN. When WEN is brought HIGH with
LD and SEN restored to a LOW, the next offset bit in sequence is written to
the registers via SI. If an interruption of serial programming is desired, it is
sufficient either to set LD LOW and deactivate SEN or to set SEN LOW
and deactivate LD. Once LD and SEN are both restored to a LOW level,
serial offset programming continues.
From the time serial programming has begun, neither partial flag will be
valid until the full set of bits required to fill all the offset registers has been
written. Measuring from the rising WCLK edge that achieves the above
criteria; PAF will be valid after two more rising WCLK edges plus t
PAF
,
PAE will be valid after the next two rising RCLK edges plus t
PAE
plus
t
SKEW2
.
It is not possible to read the flag offset values in a serial mode.
PARALLEL MODE
If Parallel Programming mode has been selected, as described above,
then programming of PAE and PAF values can be achieved by using a
combination of the LD, WCLK , WEN and Dn input pins. ProgrammingPAE
and PAF proceeds as follows: when LD and WEN are set LOW, data on
the inputs Dn are written into the Empty Offset Register on the first LOW-to-
HIGH transition of WCLK. Upon the second LOW-to-HIGH transition of WCLK,
data are written into the Full Offset Register. The third transition of WCLK
writes, once again, to the Empty Offset Register. See Figure 14, Parallel
Loading of Programmable Flag Registers, for the timing diagram for this
mode.
The act of writing offsets in parallel employs a dedicated write offset
register pointer. The act of reading offsets employs a dedicated read offset
register pointer. The two pointers operate independently; however, a read
and a write should not be performed simultaneously to the offset registers.
A Master Reset initializes both pointers to the Empty Offset (LSB) register.
A Partial Reset has no effect on the position of these pointers.
Write operations to the FIFO are allowed before and during the parallel
programming sequence. In this case, the programming of all offset
registers does not have to occur at one time. One, two or more offset reg-
isters can be written and then by bringing LD HIGH, write operations can
be redirected to the FIFO memory. When LD is set LOW again, and WEN
is LOW, the next offset register in sequence is written to. As an
alternative to holding WEN LOW and toggling LD, parallel programming
can also be interrupted by setting LD LOW and toggling WEN.
Note that the status of a partial flag (PAE or PAF) output is invalid dur-
ing the programming process. From the time parallel programming has
begun, a partial flag output will not be valid until the appropriate offset word
has been written to the register(s) pertaining to that flag. Measuring from
the rising WCLK edge that achieves the above criteria; PAF will be valid
after two more rising WCLK edges plus t
PAF
, PAE will be valid after the
next two rising RCLK edges plus t
PAE
plus t
SKEW2
.
The act of reading the offset registers employs a dedicated read offset
register pointer. The contents of the offset registers can be read on the Q
0
-
Qn pins when LD is set LOW and REN is set LOW. Data are read via Qn
from the Empty Offset Register on the first LOW-to-HIGH transition of RCLK.
Upon the second LOW-to-HIGH transition of RCLK, data are read from the
Full Offset Register. The third transition of RCLK reads, once again, from
the Empty Offset Register. See Figure 15, Parallel Read of Programmable
Flag Registers, for the timing diagram for this mode.
It is permissible to interrupt the offset register read sequence with reads
or writes to the FIFO. The interruption is accomplished by deasserting REN,
LD, or both together. When REN and LD are restored to a LOW level,
reading of the offset registers continues where it left off. It should be noted,
and care should be taken from the fact that when a parallel read of the flag
offsets is performed, the data word that was present on the output lines Qn
will be overwritten.
Parallel reading of the offset registers is always permitted regardless of
which timing mode (IDT Standard or FWFT modes) has been selected.
RETRANSMIT OPERATION
The Retransmit operation allows data that has already been read to be
accessed again. There are two stages: first, a setup procedure that resets
the read pointer to the first location of memory, then 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 bringing RT LOW. At least
one word, but no more than D - 2 words should have been written into the
FIFO between Reset (Master or Partial) and the time of Retransmit setup.
D = 8,192 for the IDT72255LA and D = 16,384 for the IDT72265LA. In
FWFT mode, D = 8,193 for the IDT72255LA and D = 16,385 for the
IDT72265LA.
If IDT Standard mode is selected, the FIFO will mark the beginning of
the Retransmit setup by setting EF LOW. The change in level will only be
noticeable 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 opera-
tions may begin starting with the first location in memory. Since IDT Stan-
dard 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, Retransmit Timing (IDT Standard Mode), for the
relevant timing diagram.
If FWFT mode is selected, the FIFO will mark 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,
the 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
