SN74ACT7804

512

× 18 STROBED FIRST-IN, FIRST-OUT MEMORY

SCAS204C – APRIL 1992 – REVISED APRIL 1998

4

POST OFFICE BOX 655303

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 only when LDCK is low. 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 1). To use the default values of X = Y = 64, PEN must be held high.

RESET

X and Y

Y

PEN

D0–D7

EMPTY

Don’t Care

LDCK

Don’t Care

Figure 1. Programming X and Y Separately