CY7C4261
CY7C4271
Document #: 38-06015 Rev. *B
Page 4 of 18
The number formed by the empty offset least significant bit
register and empty offset most significant bit register is
referred to as n and determines the operation of PAE. PAF is
synchronized to the LOW-to-HIGH transition of RCLK by one
flip-flop and is LOW when the FIFO contains n or fewer unread
words. PAE is set HIGH by the LOW-to-HIGH transition of
RCLK when the FIFO contains (n+1) or greater unread words.
The number formed by the full offset least significant bit
register and full offset most significant bit register is referred to
as m and determines the operation of PAF. PAE is synchronized
to the LOW-to-HIGH transition of WCLK by one flip-flop and is
set LOW when the number of unread words in the FIFO is
greater than or equal to CY7C4261 (16K-m) and CY7C4271
(32K-m). PAF is set HIGH by the LOW-to-HIGH transition of
WCLK when the number of available memory locations is
greater than m.
Width-Expansion Configuration
Word width may be increased simply by connecting the corre-
sponding input controls signals of multiple devices. A
composite flag should be created for each of the end-point
status flags (EF and FF). The partial status flags (PAE and PAF)
can be detected from any one device. Figure 2 demonstrates
a 18-bit word width by using two CY7C4261/71s. Any word
width can be attained by adding additional CY7C4261/71s.
When the CY7C4261/71 is in a Width-Expansion Configu-
ration, the Read Enable (REN2) control input can be grounded
(see Figure 2). In this configuration, the Write Enable 2/Load
(WEN2/LD) pin is set to LOW at Reset so that the pin operates
as a control to load and read the programmable flag offsets.
Flag Operation
The CY7C4261/71 devices provide four flag pins to indicate
the condition of the FIFO contents. Empty, Full, PAE, and PAF
are synchronous.
Full Flag
The Full Flag (FF) will go LOW when the device is full. Write
operations are inhibited whenever FF is LOW regardless of the
state of WEN1 and WEN2/LD. FF is synchronized to WCLK, i.e.,
it is exclusively updated by each rising edge of WCLK.
Empty Flag
The Empty Flag (EF) will go LOW when the device is empty.
Read operations are inhibited whenever EF is LOW,
regardless of the state of REN1 and REN2. EF is synchronized
to RCLK, i.e., it is exclusively updated by each rising edge of
RCLK.
Note:
1.
The same selection sequence applies to reading from the registers. REN1 and REN2 are enabled and a read is performed on the LOW-to-HIGH transition of RCLK.
2.
n = Empty Offset (n = 7 default value).
3.
m = Full Offset (m = 7 default value).
Table 1. Writing the Offset Registers
LD
WEN
WCLK[1]
Selection
00
0
1
No Operation
1
0
Write Into FIFO
1
1
No Operation
Empty Offset (LSB)
Empty Offset (MSB)
Full Offset (LSB)
Full Offset (MSB)
Table 2. Status Flags
Number of Words in FIFO
FF
PAF
PAE
EF
CY7C4261
CY7C4271
00
H
H
L
L
1 to n[2]
1 to n[2]
HH
L
H
(n+1) to (16384
− (m+1))
(n+1) to (32768
− (m+1))
H
H
H
H
(16384
− m)[3] to 16383
(32768
− m)[3] to 32767
H
L
H
H
16384
32768
L
L
H
H
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