SN74ACT7808

2048

× 9 STROBED FIRST-IN, FIRST-OUT MEMORY

SCAS205D – FEBRUARY 1991 – REVISED APRIL 1998

6

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. If the

offsets are not programmed, the default values of X = Y = 256 are used. The AF/AE flag is high when the FIFO

contains X or fewer words or (2048 – Y) or more words.

To program the offset values, program enable (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–D8 and DP9 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–D8 and DP9 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 1023 can be

programmed for either X or Y (see Figure 1). To use the default values of X = Y = 256, PEN must be held high.

RESET

X and Y

Y

PEN

D0–D8

LDCK

Don’t Care

EMPTY

X and Y MSB

Y MSB

Don’t Care

DP9

Don’t Care

Figure 1. Programming X and Y Separately