SN74ALVC7814

64

× 18

LOW-POWER FIRST-IN, FIRST-OUT MEMORY

SCAS592A – OCTOBER 1997 – REVISED APRIL 1998

5

POST OFFICE BOX 655303

DATA-WORD NUMBERS FOR FLAG TRANSITIONS

DEVICE

TRANSITION WORD

DEVICE

A

B

C

D

E

F

G

H

I

SN74ALVC7814

W32

W(64 – Y)

W64

W33

W34

W(64 – X)

W(65 – X)

W64

W64

Figure 1. Write, Read, and Flag Timing Reference (Continued)

offset values for AF/AE

The AF/AE flag has two programmable limits: the almost-empty offset value (X) and the almost-full offset

value (Y). They can be programmed after the FIFO is reset and before the first word is written to memory. The

AF/AE flag is high when the FIFO contains X or fewer words or (512 – Y) or more words.

To program the offset values, PEN can be brought low after reset. On the following low-to-high transition of

LDCK, the binary value on D0–D7 is stored as the almost-empty offset value (X) and the almost-full offset

value (Y). Holding PEN low for another low-to-high transition of LDCK reprograms Y to the binary value on

D0–D7 at the time of the second LDCK low-to-high transition. Writes to the FIFO memory are disabled while

the offsets are programmed. A maximum value of 32 can be programmed for either X or Y (see Figure 2). To

LDCK

RESET

X and Y

Y

PEN

D0–D17

EMPTY

Don’t Care

Don’t Care

Figure 2. Programming X and Y Separately