Whether the flag offset registers are programmed as
described in
Table 1
or the default values are used, the
programmable almost-empty flag (PAE) and programmable
almost-full flag (PAF) states are determined by their corre-
sponding offset registers and the difference between the Read
and Write pointers.
The number formed by the empty offset least significant bit
register and empty offset most significant register is referred
to as
n
and determines the operation of PAE. PAE is synchro-
nized 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 synchro-
nized 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 CY7C4421. (64 鈥?m), CY7C4201
Table 2. Status Flags
Number of Words in FIFO
CY7C4421
0
1 to n
[2]
(n + 1) to 32
33 to (64 鈥?(m + 1))
(64 鈥?m)
64
[3]
(256 鈥?m), CY7C4211 (512 鈥?m), CY7C4221 (1K 鈥?m),
CY7C4231 (2K 鈥?m), CY7C4241 (4K 鈥?m), and CY7C4251
(8K 鈥?m). PAF is set HIGH by the LOW-to-HIGH transition of
WCLK when the number of available memory locations is
greater than m.
Table 1. Writing the Offset Registers
LD
0
WEN
0
WCLK
[1]
Selection
Empty Offset (LSB)
Empty Offset (MSB)
Full Offset (LSB)
Full Offset (MSB)
No Operation
Write Into FIFO
No Operation
0
1
1
1
0
1
CY7C4201
0
1 to n
[2]
(n + 1) to 128
129 to (256 鈥?(m + 1))
(256 鈥?m)
256
[3]
CY7C4211
0
1 to n
[2]
(n + 1) to 256
257 to (512 鈥?(m + 1))
(512 鈥?m)
512
[3]
FF
H
H
H
H
H
L
PAF
H
H
H
H
L
L
PAE
L
L
H
H
H
H
EF
L
H
H
H
H
H
to 63
to 255
to 511
Number of Words in FIFO
CY7C4221
0
1 to n
[2]
(n + 1) to 512
(1024 鈥?m)
[3]
to 1023
1024
0
1 to n
[2]
(n + 1) to 1024
(2048 鈥?m)
[3]
to 2047
2048
CY7C4231
0
1 to n
[2]
(n + 1) to 2048
(4096 鈥?m)
[3]
to 4095
4096
CY7C4241
0
1 to n
[2]
(n + 1) to 4096
(8192 鈥?m)
[3]
to 8191
8192
CY7C4251
FF
H
H
H
H
H
L
PAF
H
H
H
H
L
L
PAE
L
L
H
H
H
H
EF
L
H
H
H
H
H
513 to (1024 鈥?(m + 1)) 1025 to (2048 鈥?(m + 1)) 2049 to (4096 鈥?(m + 1)) 4097 to (8192 鈥?(m + 1))
Notes:
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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
