IDT72281/72291
CMOS SuperSync FIFO™ 65,536 x 9 and 131,072 x 9
COMMERCIALANDINDUSTRIAL
TEMPERATURERANGES
ter. See Figure 15, Parallel Loading of Programmable Flag Registers for
the IDT72291, for the timing diagram for this mode.
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
combination 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 32 bits for
the IDT72281 and 34 bits for the IDT72291. 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.
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 registers 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 during
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 tPAF, PAE will be valid after the next two
rising RCLK edges plus tPAE plus tSKEW2.
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 Q0-
Qn pins when LD is set LOW and REN is set LOW. For the IDT72281, data
are read via Qn from the Empty Offset LSB Register on the first LOW-to-
HIGH transition of RCLK. Upon the second LOW-to-HIGH transition of RCLK,
data are read from the Empty Offset MSB Register. Upon the third LOW-to-
HIGH transition of RCLK, data are read from the Full Offset LSB Register.
Upon the fourth LOW-to-HIGH transition of RCLK, data are read from the
Full Offset MSB Register. The fifth transition of RCLK reads, once again,
from the Empty Offset LSB Register. See Figure 16, Parallel Read of Pro-
grammable Flag Registers for the IDT72281, for the timing diagram for this
mode.
For the IDT72291, data is read via Qn from the Empty Offset LSB Regis-
ter on the first LOW-to-HIGH transition of RCLK. Upon the second LOW-to-
HIGH transition of RCLK, data are read from the Empty Offset Mid-Byte
Register. Upon the third LOW-to-HIGH transition of RCLK, data are read
from the Empty Offset MSB Register. Upon the fourth LOW-to-HIGH transi-
tion of RCLK, data are read from the Full Offset LSB Register. Upon the fifth
LOW-to-HIGH transition of RCLK, data are read from the Full Offset Mid-
Byte Register. Upon the sixth LOW-to-HIGH transition of RCLK, data are
read from the Full Offset MSB Register. The seventh transition of RCLK
reads, once again, from the Empty Offset LSB Register. See Figure 17,
Parallel Read of Programmable Flag Registers for the IDT72291, 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.
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 tPAF, PAE
will be valid after the next two rising RCLK edges plus tPAE plus tSKEW2.
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. For the IDT72281,
programming PAE and PAF proceeds as follows: when LD and WEN are
set LOW, data on the inputs Dn are written into the Empty Offset LSB
Register on the first LOW-to-HIGH transition of WCLK. Upon the second
LOW-to-HIGH transition of WCLK, data are written into the Empty Offset
MSB Register. Upon the third LOW-to-HIGH transition of WCLK, data are
written into the Full Offset LSB Register. Upon the fourth LOW-to-HIGH
transition of WCLK, data are written into the Full Offset MSB Register. The
fifth transition of WCLK writes, once again, to the Empty Offset LSB Register.
See Figure 14, Parallel Loading of Programmable Flag Registers for the
IDT72281, for the timing diagram for this mode.
For the IDT72291, programming PAE and PAF proceeds as follows:
when LD and WEN are set LOW, data on the inputs Dn are written into the
Empty Offset LSB Register on the first LOW-to-HIGH transition of WCLK.
Upon the second LOW-to-HIGH transition of WCLK, data are written into the
Empty Offset Mid-Byte Register. Upon the third LOW-to-HIGH transition of
WCLK, data are written into the Empty Offset MSB Register. Upon the fourth
LOW-to-HIGH transition of WCLK, data are written into the Full Offset LSB
Register. Upon the fifth LOW-to-HIGH transition of WCLK, data are written
into the Full Offset Mid-Byte Register. Upon the sixth LOW-to-HIGH transi-
tion of WCLK, data are written into the Full Offset MSB Register. The sev-
enth transition of WCLK writes, once again, into the Empty Offset LSB Regis-
Parallel reading of the offset registers is always permitted regardless of
which timing mode (IDT Standard or FWFT modes) has been selected.
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