Cypress CY7C1245V18 - Manual (Page 8)

CY7C1241V18, CY7C1256V18
CY7C1243V18, CY7C1245V18
Document Number: 001-06365 Rev. *D Page 8 of 28
Functional Overview
The CY7C1241V18, CY7C1256V18, CY7C1243V18, and
CY7C1245V18 are synchronous pipelined Burst SRAMs
equipped with a read and a write port. The read port is dedicated
to read operations and the write port is dedicated to write opera-
tions. Data flows into the SRAM through the write port and out
through the read port. These devices multiplex the address
inputs to minimize the number of address pins required. By
having separate read and write ports, the QDR-II+ completely
eliminates the need to “turn around” the data bus and avoids any
possible data contention, thereby simplifying system design.
Each access consists of four 8-bit data transfers in the case of
CY7C1241V18, four 9-bit data transfers in the case of
CY7C1256V18, four 18-bit data transfers in the case of
CY7C1243V18, and four 36-bit data transfers in the case of
CY7C1245V18, in two clock cycles.
Accesses for both ports are initiated on the Positive Input Clock
(K). All synchronous input and output timing refer to the rising
edge of the input clocks (K/K).
All synchronous data inputs (D
[x:0]
) inputs pass through input
registers controlled by the input clocks (K and K). All
synchronous data outputs (Q
[x:0]
) outputs pass through output
registers controlled by the rising edge of the Input clocks (K and
K).
All synchronous control (RPS, WPS, BWS
[x:0]
) inputs pass
through input registers controlled by the rising edge of the input
clocks (K/K).
CY7C1243V18 is described in the following sections. The same
basic descriptions apply to CY7C1241V18, CY7C1256V18, and
CY7C1245V18.
Read Operations
The CY7C1243V18 is organized internally as 4 arrays of 512K x
18. Accesses are completed in a burst of four sequential 18-bit
data words. Read operations are initiated by asserting RPS
active at the rising edge of the Positive Input Clock (K). The
addresses presented to Address inputs are stored in the Read
address register. Following the next two K clock rising edges, the
corresponding lowest order 18-bit word of data is driven onto the
Q
[17:0]
using K as the output timing reference. On the subse-
quent rising edge of K the next 18-bit data word is driven onto
the Q
[17:0]
. This process continues until all four 18-bit data words
have been driven out onto Q
[17:0]
. The requested data is valid
0.45 ns from the rising edge of the input clock (K or K). To
maintain the internal logic, each read access must be allowed to
complete. Each read access consists of four 18-bit data words
and takes two clock cycles to complete. Therefore, read
accesses to the device cannot be initiated on two consecutive K
clock rises. The internal logic of the device ignores the second
read request. Read accesses can be initiated on every other K
clock rise. Doing so pipelines the data flow such that data is
transferred out of the device on every rising edge of the input
clocks (K and K).
When the read port is deselected, the CY7C1243V18 first
completes the pending Read transactions. Synchronous internal
circuitry automatically tri-states the outputs following the next
rising edge of the Positive Input Clock (K). This enables a
seamless transition between devices without the insertion of wait
states in a depth expanded memory.
Write Operations
Write operations are initiated by asserting WPS active at the
rising edge of the Positive Input Clock (K). On the following K
clock rise, the data presented to D
[17:0]
is latched and stored into
the lower 18-bit Write Data register, provided BWS
[1:0]
are both
asserted active. On the subsequent rising edge of the Negative
Input Clock (K), the information presented to D
[17:0]
is also stored
into the Write Data register, provided BWS
[1:0]
are both asserted
active. This process continues for one more cycle until four 18-bit
words (a total of 72 bits) of data are stored in the SRAM. The 72
bits of data are then written into the memory array at the specified
location. Therefore, write accesses to the device cannot be
initiated on two consecutive K clock rises. The internal logic of
the device ignores the second write request. Write accesses can
be initiated on every other rising edge of the Positive Input Clock
(K). Doing so pipelines the data flow such that 18 bits of data can
be transferred into the device on every rising edge of the input
clocks (K and K).
When deselected, the write port ignores all inputs after the
pending write operations have been completed.
Byte Write Operations
Byte Write operations are supported by the CY7C1243V18. A
Write operation is initiated as described in the
Write Operations
section. The bytes that are written are determined by BWS
0
and
BWS
1
, which are sampled with each set of 18-bit data words.
Asserting the appropriate Byte Write Select input during the data
portion of a write latches the data being presented and written
into the device. Deasserting the Byte Write Select input during
the data portion of a write allows the data stored in the device for
that byte to remain unaltered. This feature can be used to
simplify read/modify/write operations to a Byte Write operation.
Concurrent Transactions
The read and write ports on the CY7C1243V18 operate
completely independently of one another. Since each port
latches the address inputs on different clock edges, you can read
or write to any location, regardless of the transaction on the other
port. If the ports access the same location when a read follows a
write in successive clock cycles, the SRAM delivers the most
recent information associated with the specified address
location. This includes forwarding data from a write cycle that
was initiated on the previous K clock rise.
Read accesses and write access must be scheduled such that
one transaction is initiated on any clock cycle. If both ports are
selected on the same K clock rise, the arbitration depends on the
previous state of the SRAM. If both ports were deselected, the
read port takes priority. If a read was initiated on the previous
cycle, the write port assumes priority (because read operations
cannot be initiated on consecutive cycles). If a write was initiated
on the previous cycle, the Read port assumes priority (because
write operations cannot be initiated on consecutive cycles).
Therefore, asserting both port selects active from a deselected
state results in alternating read/write operations being initiated,
with the first access being a read.
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