SN74ACT3651

2048

× 36

CLOCKED FIRST-IN, FIRST-OUT MEMORY

SCAS439D – JUNE 1994 – REVISED FEBRUARY 1999

11

POST OFFICE BOX 655303

mailbox registers

Two 36-bit bypass registers pass command and control information between port A and port B. The

mailbox-select (MBA, MBB) inputs choose between a mail register and a FIFO for a port data-transfer operation.

A low-to-high transition on CLKA writes A0 – A35 data to the mail1 register when a port-A write is selected by

CSA, W/RA, and ENA with MBA high. A low-to-high transition on CLKB writes B0 – B35 data to the mail2 register

when a port-B write is selected by CSB, W/RB, and ENB with MBB high. Writing data to a mail register sets its

corresponding flag (MBF1 or MBF2) low. Attempted writes to a mail register are ignored while its mail flag is

low.

When the port-B data (B0 – B35) outputs are active, the data on the bus comes from the FIFO output register

when the MBB input is low and from the mail1 register when MBB is high. Mail2 data is always present on the

port-A data (A0 – A35) outputs when they are active. The mail1 register flag (MBF1) is set high by a low-to-high

transition on CLKB when a port-B read is selected by CSB, W/RB, and ENB with MBB high. The mail2 register

flag (MBF2) is set high by a low-to-high transition on CLKA when a port-A read is selected by CSA, W/RA, and

ENA with MBA high. The data in a mail register remains intact after it is read and changes only when new data

is written to the register.

tpd(R-F)

CLKA

CLKB

RST

0,1

th(FS)

tsu(FS)

th(RS)

tsu(RS)

FS1, FS0

IR

tpd(C-IR)

tpd(C-IR)

OR

tpd(C-OR)

tpd(R-F)

AE

AF

MBF1,

MBF2

tpd(R-F)

Figure 1. FIFO Reset Loading X and Y With a Preset Value of Eight