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6.42
IDT70V24S/L
High-Speed 4K x 16 Dual-Port Static RAM Industrial and Commercial Temperature Ranges
Figure 3. Busy and chip enable routing for both width and depth expansion with IDT70V24 SRAMs.
prevented to a port by tying the
BUSY
pin for that port LOW.
The busy outputs on the IDT 70V24 SRAMin master mode, are push-
pull type outputs and do not require pull up resistors to operate. If these
RAMs are being expanded in depth, then the
BUSY
indication for the
resulting array requires the use of an external AND gate.
-+&8/;&
,.91
When expanding an IDT70V24 SRAMarray in width while using busy
logic, one master part is used to decide which side of the SRAMarray will
receive a
BUSY
indication, and to output that indication. Any number of
slaves to be addressed in the same address range as the master, use the
BUSY
signal as a write inhibit signal. Thus on the IDT70V24 SRAMthe
BUSY
pin is an output if the part is used as a master (M/
S
pin = V
IH
), and
the
BUSY
pin is an input if the part used as a slave (M/
S
pin = V
IL
) as shown
in Figure 3.
If two or more master parts were used when expanding in width, a
split decision could result with one master indicating
BUSY
on one side
of the array and another master indicating
BUSY
on one other side of
the array. This would inhibit the write operations fromone port for part
of a word and inhibit the write operations fromthe other port for the
other part of the word.
The
BUSY
arbitration, on a master, is based on the chip enable and
address signals only. It ignores whether an access is a read or write.
In a master/slave array, both address and chip enable must be valid
long enough for a
BUSY
flag to be output fromthe master before the
actual write pulse can be initiated with either the R/
W
signal or the byte
enables. Failure to observe this timng can result in a glitched internal
write inhibit signal and corrupted data in the slave.
BUSY
@
.&
The IDT70V24 is an extremely fast Dual-Port 4K x 16 CMOS Static
RAMwith an additional 8 address locations dedicated to binary
semaphore flags. These flags allow either processor on the left or right
side of the Dual-Port SRAMto claima privilege over the other
processor for functions defined by the systemdesigner
’
s software. As
an example, the semaphore can be used by one processor to inhibit
the other fromaccessing a portion of the Dual-Port SRAMor any other
shared resource.
2911 drw 18
MASTER
Dual Port
SRAM
L
R
MASTER
Dual Port
SRAM
L
R
SLAVE
Dual Port
SRAM
L
R
SLAVE
Dual Port
SRAM
L
R
CE
L
R
D
The IDT70V24 provides two ports with separate control, address
and I/O pins that permt independent access to any location in memory.
The IDT70V24 has an automatic power down feature controlled by
CE
.
The
CE
controls on-chip power down circuitry that permts the respec-
tive port to go into a standby mode when not selected (
CE
HIGH).
When a port is enabled, access to the entire memory array is
permtted.
(
If the user chooses the interrupt function, a memory location (mail
box or message center) is assigned to each port. The left port interrupt
flag (
INT
L
) is asserted when the right port writes to memory location
FFE (HEX), where a write is defined as the
CE
=R/
W
=V
IL
per Truth
Table III. The left port clears the interrupt by accessing address
location FFE when
CE
R
=
OE
R
= V
IL
, R/
W
is a "don't care". Likewise,
the right port interrupt flag (
INT
R
) is asserted when the left port writes
to memory location FFF (HEX) and to clear the interrupt flag (
INT
R
),
the right port must read the memory location FFF. The message (16
bits) at FFE or FFF is user-defined, since it is an addressable SRAM
location. If the interrupt function is not used, address locations FFE and
FFF are not used as mail boxes, but as part of the randomaccess
memory. Refer to Truth Table IIII for the interrupt operation.
1@
Busy Logic provides a hardware indication that both ports of the
SRAMhave accessed the same location at the same time. It also
allows one of the two accesses to proceed and signals the other side
that the SRAMis
“
busy
”
. The
BUSY
pin can then be used to stall the access
until the operation on the other side is completed. If a write operation has
been attemp-ted fromthe side that receives a
BUSY
indication, the write
signal is gated internally to prevent the write fromproceeding.
The use of
BUSY
logic is not required or desirable for all applications.
In some cases it may be useful to logically OR the
BUSY
outputs together
and use any
BUSY
indication as an interrupt source to flag the event of
an illegal or illogical operation. If the write inhibit function of
BUSY
logic is
not desirable, the
BUSY
logic can be disabled by placing the part in slave
mode with the M/
S
pin. Once in slave mode the
BUSY
pin operates solely
as a write inhibit input pin. Normal operation can be programmed by tying
the
BUSY
pins HIGH. If desired, unintended write operations can be