SN74ALVC7804

512

× 18

FIRST-IN, FIRST-OUT MEMORY

SCAS432 – JANUARY 1995

5

POST OFFICE BOX 655303

DATA WORD NUMBERS FOR FLAG TRANSITIONS

DEVICE

TRANSITION WORD

DEVICE

A

B

C

D

E

F

G

H

I

SN74ALVC7804

W256

W(512 – Y)

W512

W257

W258

W(512 – X)

W(513 – X)

W511

W512

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 less words or (512 minus 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 255 can be programmed for either X or Y (see Figure 2). To use the

LDCK

RESET

X and Y

Y

PEN

D0 – D17

EMPTY

Don’t Care

Don’t Care

Figure 2. Programming X and Y Separately