SN74ACT7807

2048

× 9

CLOCKED FIRST-IN, FIRST-OUT MEMORY

SCAS200D – JANUARY 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.

Program enable (PEN) should be held high throughout the reset cycle. PEN can be brought low only when IR

is high and WRTCLK is low. On the following low-to-high transition of WRTCLK, the binary value on D0–D8 and

WRTEN1/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 WRTCLK reprograms Y to the binary value on D0–D8 and

WRTEN1/DP9 at the time of the second WRTCLK low-to-high transition. While the offsets are programmed,

data is not written to the FIFO memory, regardless of the state of the write enables (WRTEN1/DP9, WRTEN2).

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.

X and Y

MSB

34

RESET

PEN

D0–D8

WRTCLK

WRTEN1/DP9

WRTEN2

IR

X and Y

Y

YMSB

Word 1

Figure 1. Programming X and Y Separately