CY7C1541V18, CY7C1556V18
CY7C1543V18, CY7C1545V18
Document Number: 001-05389 Rev. *F
Page 8 of 28
Functional Overview
The CY7C1541V18, CY7C1556V18, CY7C1543V18, and
CY7C1545V18 are synchronous pipelined burst SRAMs
equipped with a read port and a write port. The read port is
dedicated to read operations and the write port is dedicated to
write operations. 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
CY7C1541V18, four 9-bit data transfers in the case of
CY7C1556V18, four 18-bit data transfers in the case of
CY7C1543V18, and four 36-bit data transfers in the case of
CY7C1545V18, in two clock cycles.
Accesses for both ports are initiated on the positive input clock
(K). All synchronous input and output timing are referenced from
the rising edge of the input clocks (K and K).
All synchronous data inputs (D
[x:0]
) 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) as well.
All synchronous control (RPS, WPS, NWS
[x:0]
,
BWS
[x:0]
) inputs
pass through input registers controlled by the rising edge of the
input clocks (K and K).
CY7C1543V18 is described in the following sections. The same
basic descriptions apply to CY7C1541V18, CY7C1556V18, and
CY7C1545V18.
Read Operations
The CY7C1543V18 is organized internally as four arrays of 1M
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
address presented to address inputs are stored in the read
address register. Following the next two K clock rise, the corre-
sponding 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 CY7C1543V18 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 for 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 cock (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 CY7C1543V18. 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 writes it
into the device. Deasserting the Byte Write Select input during
the data portion of a write enables the data stored in the device
for that byte to remain unaltered. This feature can be used to
simplify read, modify, or write operations to a byte write
operation.
Concurrent Transactions
The read and write ports on the CY7C1543V18 operate
completely independently of one another. As each port latches
the address inputs on different clock edges, the user 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 access 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 are deselected, the
read port takes priority. If a read was initiated on the previous
cycle, the write port assumes priority (as read operations cannot
be initiated on consecutive cycles). If a write was initiated on the
previous cycle, the read port assumes priority (as write opera-
tions cannot be initiated on consecutive cycles). Therefore,
asserting both port selects active from a deselected state results
in alternating read or write operations being initiated, with the first
access being a read.
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