SN74ACT3632
512 × 36 × 2
CLOCKED BIDIRECTIONAL FIRST-IN, FIRST-OUT MEMORY
SCAS224D – JUNE 1992 – REVISED APRIL 1998
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
almost-empty flags (AEA, AEB)
The AE flag of a FIFO is synchronized to the port clock that reads data from its array . The almost-empty state
is defined by the contents of register X1 for AEB
and register X2 for AEA. These registers are loaded with preset
values during a FIFO reset or programmed from port A (see
almost-empty flag and almost-full flag offset
programming
). An AE flag is low when its FIFO contains X or fewer words and is high when its FIFO contains
(X + 1) or more words. A data word present in the FIFO output register has been read from memory.
Two low-to-high transitions of the AE
flag synchronizing clock are required after a FIFO write for its AE flag to
reflect the new level of fill. Therefore, the AE
flag of a FIFO containing (X + 1) or more words remains low if two
cycles of its synchronizing clock have not elapsed since the write that filled the memory to the (X + 1) level. An
AE
flag is set high by the second low-to-high transition of its synchronizing clock after the FIFO write that fills
memory to the (X + 1) level. A low-to-high transition of an AE
flag synchronizing clock begins the first
synchronization cycle if it occurs at time t
sk2
, or greater, after the write that fills the FIFO to (X + 1) words.
Otherwise, the subsequent synchronizing clock cycle can be the first synchronization cycle
(see Figures 11 and 12).
almost-full flags (AFA, AFB)
The AF flag of a FIFO is synchronized to the port clock that writes data to its array . The almost-full state is defined
by the contents of register Y1 for AFA
and register Y2 for AFB. These registers are loaded with preset values
during a FIFO reset or programmed from port A (see
almost-empty flag and almost-full flag offset programming
).
An AF
flag is low when its FIFO contains (512 – Y) or more words and is high when its FIFO contains
[512 – (Y + 1)] or fewer words. A data word is present in the FIFO output register has been read from memory .
Two low-to-high transitions of the AF
flag synchronizing clock are required after a FIFO read for its AF flag to
reflect the new level of fill. Therefore, the AF
flag of a FIFO containing [512 – (Y + 1)] or fewer words remains
low if two cycles of its synchronizing clock have not elapsed since the read that reduced the number of words
in memory to [512 – (Y + 1)]. An AF
flag is set high by the second low-to-high transition of its synchronizing clock
after the FIFO read that reduces the number of words in memory to [512 – (Y + 1)]. A low-to-high transition of
an AF
flag synchronizing clock begins the first synchronization cycle if it occurs at time t
sk2
, or greater, after the
read that reduces the number of words in memory to [512 – (Y + 1)]. Otherwise, the subsequent synchronizing
clock cycle can be the first synchronization cycle (see Figures 13 and 14).
mailbox registers
Each FIFO has a 36-bit bypass register to pass command and control information between port A and port B
without putting it in queue. 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 and 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 and 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 the mail flag is low.
When data outputs of a port are active, the data on the bus comes from the FIFO output register when the port
mailbox select input is low and from the mail register when the port-mailbox select input is high. 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 and 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 and 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.