SN74ACT3641

1024

× 36

CLOCKED FIRST-IN, FIRST-OUT MEMORY

SCAS338C – JANUARY 1994 – REVISED OCTOBER 1997

6

POST OFFICE BOX 655303

Terminal Functions (Continued)

TERMINAL

NAME

I/O

DESCRIPTION

W/RA

I

Port-A write/read select. A high on W/RA selects a write operation and a low selects a read operation on port A for a

low-to-high transition of CLKA. The A0 – A35 outputs are in the high-impedance state when W/RA is high.

W/RB

I

Port-B write/read select. A low on W/RB selects a write operation and a high selects a read operation on port B for a

low-to-high transition of CLKB. The B0 – B35 outputs are in the high-impedance state when W/RB is low.

detailed description

reset

The SN74ACT3641 is reset by taking the reset (RST) input low for at least four port-A clock (CLKA) and four

port-B clock (CLKB) low-to-high transitions. RST can switch asynchronously to the clocks. A reset initializes the

memory read and write pointers and forces the IR flag low, the OR flag low, the AE flag low, and the AF flag high.

Resetting the device also forces the mailbox flags (MBF1, MBF2) high. After a FIFO is reset, its flag is set high

after at least two clock cycles to begin normal operation. A FIFO must be reset after power up before data is

written to its memory.

almost-empty flag and almost-full flag offset programming

Two registers in the SN74ACT3641 are used to hold the offset values for the AE and AF flags. The AE flag offset

register is labeled X, and the AF flag offset register is labeled Y. The offset registers can be loaded with a value

in three ways: one of two preset values are loaded into the offset registers, parallel load from port A, or serial

load. The offset register programming mode is chosen by the flag select (FS1, FS0) inputs during a low-to-high

transition on RST (see Table 1).

Table 1. Flag Programming

FS1

FS0

RST

X AND Y REGISTERS†

H

H

↑

Serial load

H

L

↑

64

L

H

↑

8

L

L

↑

Parallel load from port A

† X register holds the offset for AE; Y register holds the

offset for AF.

preset values

If a preset value of 8 or 64 is chosen by FS1 and FS0 at the time of a RST low-to-high transition according to

Table 1, the preset value is automatically loaded into the X and Y registers. No other device initialization is

necessary to begin normal operation, and the IR flag is set high after two low-to-high transitions on CLKA.

parallel load from port A

To program the X and Y registers from port A, the device is reset with FS0 and FS1 low during the low-to-high

transition of RST. After this reset is complete, IR is set high after two low-to-high transitions on CLKA. The first

two writes to the FIFO do not store data in its memory but load the offset registers in the order Y, X. Each offset

register of the SN74ACT3641 uses port-A inputs (A9 – A0). Data input A9 is used as the most-significant bit of

the binary number. Each register value can be programmed from 1 to 1020. After both offset registers are

programmed from port A, subsequent FIFO writes store data in the SRAM.