6.42
IDT71V321/71V421S/L
High Speed 3.3V 2K x 8 Dual-Port Static RAM with Interrupts Industrial and Commercial Temperature Ranges
13
being expanded in depth, then the BUSY indication for the resulting array
requires the use of an external AND gate.
Width Expansion with Busy Logic
Master/Slave Arrays
When expanding an SRAM array in width while using BUSY logic, one
master part is used to decide which side of the SRAM array will receive
a BUSY 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 IDT71V321/IDT71V421 SRAMs the BUSY pin is an output
if the part is Master (IDT71V321), and the BUSY pin is an input if the part
is a Slave (IDT71V421) as shown in Figure 3.
Functional Description
The IDT7V1321/IDT71V421 provides two ports with separate control,
address and I/O pins that permit independent access for reads or writes
to any location in memory. The IDT71V321/IDT71V421 has an automatic
power down feature controlled by CE. The CE controls on-chip power
down circuitry that permits the respective port to go into a standby mode
when not selected (CE = VIH). When a port is enabled, access to the entire
memory array is permitted.
Interrupts
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
(INTL) is asserted when the right port writes to memory location 7FE
(HEX), where a write is defined as the CER = R/WR = VIL per Truth Table
II. The left port clears the interrupt by accessing address location 7FE when
CEL = OEL = VIL, R/W is a "don't care". Likewise, the right port interrupt
flag (INTR) is asserted when the left port writes to memory location 7FF
(HEX) and to clear the interrupt flag (INTR), the right port must access the
memory location 7FF. The message (8 bits) at 7FE or 7FF is user-defined,
since it is an addressable SRAM location. If the interrupt function is not used,
address locations 7FE and 7FF are not used as mail boxes, but as part
of the random access memory. Refer to Truth Table II for the interrupt
operation.
Busy Logic
Busy Logic provides a hardware indication that both ports of the RAM
have 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 RAM is “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 attempted from
the side that receives a busy indication, the write signal is gated internally
to prevent the write from proceeding.
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.
The BUSY outputs on the IDT71V321 RAM master are totem-pole type
outputs and do not require pull-up resistors to operate. If these RAMs are
3026 drw 16
MASTER
Dual Port
RAM
BUSYLBUSYR
CE
MASTER
Dual Port
RAM
BUSYLBUSYR
CE
SLAVE
Dual Port
RAM
BUSYLBUSYR
CE
SLAVE
Dual Port
RAM
BUSYLBUSYR
CE
BUSYL
BUSYR
DECODER
Figure 3. Busy and chip enable routing for both width and depth
expansion with IDT71V321 (Master) and (Slave) IDT71V421 RAMs.
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 from one port for part of a word and
inhibit the write operations from the 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 from the master before the actual write
pulse can be initiated with either the R/W signal or the byte enables. Failure
to observe this timing can result in a glitched internal write inhibit signal and
corrupted data in the slave.