SN74ACT3651

2048

× 36

CLOCKED FIRST-IN, FIRST-OUT MEMORY

SCAS439D – JUNE 1994 – REVISED FEBRUARY 1999

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 SN74ACT3651 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. The reset input can switch asynchronously to the clocks. A reset

initializes the memory-read and-write pointers and forces the IR flag low, the OR flag high, 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,

IR 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 SN74ACT3651 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 is 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 an 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, the IR flag 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 SN74ACT3651 uses port-A inputs (A10 – A0). The highest number input is used as

the most-significant bit of the binary number in each case. Each register value can be programmed from 1 to

2044. After both offset registers are programmed from port A, subsequent FIFO writes store data in the SRAM.