CY7C4281
CY7C4291
Document #: 38-06007 Rev. *C Page 3 of 16
Functional Description (continued)
The CY7C4281/91 provides four status pins: Empty, Full,
Programmable Almost Empty, and Programmable Almost
Full. The Almost Empty/Almost Full flags are programmable to
single-word granularity. The programmable flags default to
Empty+7 and Full-7.
The flags are synchronous, i.e., they change state relative to
either the read clock (RCLK) or the write clock (WCLK). When
entering or exiting the Empty and Almost Empty states, the
flags are updated exclusively by the RCLK. The flags denoting
Almost Full and Full states are updated exclusively by WCLK.
The synchronous flag architecture guarantees that the flags
maintain their status for at least one cycle.
All configurations are fabricated using an advanced 0.5µ
CMOS technology. Input ESD protection is greater than
2001V, and latch-up is prevented by the use of guard rings.
Architecture
The CY7C4281/91 consists of an array of 64K to 128K words
of nine bits each (implemented by a dual-port array of SRAM
cells), a read pointer, a write pointer, control signals (RCLK,
WCLK, REN1
, REN2, WEN1, WEN2, RS), and flags (EF,
PAE
, PAF, FF).
Resetting the FIFO
Upon power-up, the FIFO must be reset with a Reset (RS)
cycle. This causes the FIFO to enter the Empty condition
signified by EF being LOW. All data outputs (Q
0–8
) go LOW
t
RSF
after the rising edge of RS. In order for the FIFO to reset
to its default state, the user must not read or write while RS
is
LOW. All flags are guaranteed to be valid t
RSF
after RS is taken
LOW.
FIFO Operation
When the WEN1 signal is active LOW, WEN2 is active HIGH,
and FF
is active HIGH, data present on the D
0–8
pins is written
into the FIFO on each rising edge of the WCLK signal.
Similarly, when the REN1
and REN2 signals are active LOW
and EF
is active HIGH, data in the FIFO memory will be
presented on the Q
0–8
outputs. New data will be presented on
each rising edge of RCLK while REN1
and REN2 are active.
REN1
and REN2 must set up t
ENS
before RCLK for it to be
a valid read function. WEN1
and WEN2 must occur t
ENS
before WCLK for it to be a valid write function.
An output enable (OE
) pin is provided to three-state the Q
0–8
outputs when OE is asserted. When OE is enabled (LOW),
data in the output register will be available to the Q
0–8
outputs
after t
OE
. If devices are cascaded, the OE function will only
output data on the FIFO that is read enabled.
The FIFO contains overflow circuitry to disallow additional
writes when the FIFO is full, and underflow circuitry to disallow
additional reads when the FIFO is empty. An empty FIFO
maintains the data of the last valid read on its Q
0–8
outputs
even after additional reads occur.
Write Enable 1 (WEN1
) — If the FIFO is configured for
programmable flags, Write Enable 1 (WEN1
) is the only write
enable control pin. In this configuration, when Write Enable 1
(WEN1
) is LOW, data can be loaded into the input register and
RAM array on the LOW-to-HIGH transition of every write clock
(WCLK). Data is stored is the RAM array sequentially and
independently of any on-going read operation.
Write Enable 2/Load (WEN2/LD
) — This is a dual-purpose
pin. The FIFO is configured at Reset to have programmable
flags or to have two write enables, which allows for depth
expansion. If Write Enable 2/Load (WEN2/LD
) is set active
HIGH at Reset (RS
= LOW), this pin operates as a second
write enable pin.
If the FIFO is configured to have two write enables, when Write
Enable (WEN1
) is LOW and Write Enable 2/Load (WEN2/LD)
is HIGH, data can be loaded into the input register and RAM
array on the LOW-to-HIGH transition of every write clock
(WCLK). Data is stored in the RAM array sequentially and
independently of any on-going read operation.
Programming
When WEN2/LD is held LOW during Reset, this pin is the load
(LD
) enable for flag offset programming. In this configuration,
WEN2/LD
can be used to access the four nine-bit offset
registers contained in the CY7C4281/4291 for writing or
reading data to these registers.
When the device is configured for programmable flags and
both WEN2/LD
and WEN1 are LOW, the first LOW-to-HIGH
transition of WCLK writes data from the data inputs to the
empty offset least significant bit (LSB) register. The second,
third, and fourth LOW-to-HIGH transitions of WCLK store data
in the empty offset most significant bit (MSB) register, full
offset LSB register, and full offset MSB register, respectively,
when WEN2/LD
and WEN1 are LOW. The fifth LOW-to-HIGH
transition of WCLK while WEN2/LD
and WEN1 are LOW
writes data to the empty LSB register again. Figure 1 shows
the registers sizes and default values for the various device
types.
Figure 1. Offset Register Location and Default Values
64K × 9
8
0
8
0
8
0
Empty Offset (LSB) Reg.
Default Value = 007h
Full Offset (LSB) Reg
Default Value = 007h
(MSB)
7
7
7
8
0
8
0
8
0
8
0
Empty Offset (LSB) Reg.
Default Value = 007h
Full Offset (LSB) Reg
Default Value = 007h
(MSB)
(MSB)
7
7
128K× 9
8
0
(MSB)
7
Default Value = 000h
Default Value = 000h
Default Value= 000h
Default Value = 000h
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