CY7C1243KV18/CY7C1245KV18
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).
on the rising edge of the positive input clock only (K). Each port
select input can deselect the specified port. Deselecting a port
does not affect the other port. All pending transactions (read and
write) are completed before the device is deselected.
When deselected, the write port ignores all inputs after the
pending write operations have been completed.
Programmable Impedance
An external resistor, RQ, must be connected between the ZQ pin
on the SRAM and VSS to allow the SRAM to adjust its output
driver impedance. The value of RQ must be 5 × the value of the
intended line impedance driven by the SRAM, the allowable
range of RQ to guarantee impedance matching with a tolerance
of ±15% is between 175 and 350 , with VDDQ = 1.5 V. The
output impedance is adjusted every 1024 cycles upon power-up
to account for drifts in supply voltage and temperature.
Byte Write Operations
Byte write operations are supported by the CY7C1243KV18. A
write operation is initiated as described in Write Operations on
page 6. The bytes that are written are determined by BWS0 and
BWS1, 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.
Echo Clocks
Echo clocks are provided on the QDR II+ to simplify data capture
on high-speed systems. Two echo clocks are generated by the
QDR II+. CQ is referenced with respect to K and CQ is
referenced with respect to K. These are free-running clocks and
are synchronized to the input clock of the QDR II+. The timing
for the echo clocks is shown in the Switching Characteristics on
page 24.
Concurrent Transactions
The read and write ports on the CY7C1243KV18 operates
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.
Valid Data Indicator (QVLD)
QVLD is provided on the QDR II+ to simplify data capture on high
speed systems. The QVLD is generated by the QDR II+ device
along with data output. This signal is also edge-aligned with the
echo clock and follows the timing of any data pin. This signal is
asserted half a cycle before valid data arrives.
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 takes priority (as read operations can not be
initiated on consecutive cycles). If a write was initiated on the
previous cycle, the read port takes priority (as write operations
can not 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.
PLL
These chips use a PLL that is designed to function between
120 MHz and the specified maximum clock frequency. During
power up, when the DOFF is tied HIGH, the PLL is locked after
20 s of stable clock. The PLL can also be reset by slowing or
stopping the input clocks K and K for a minimum of 30 ns.
However, it is not necessary to reset the PLL to lock to the
desired frequency. The PLL automatically locks 20 s after a
stable clock is presented. The PLL may be disabled by applying
ground to the DOFF pin. When the PLL is turned off, the device
behaves in QDR I mode (with one cycle latency and a longer
access time). For information refer to the application note PLL
Considerations in QDRII/DDRII/QDRII+/DDRII+.
Depth Expansion
The CY7C1243KV18 has a port select input for each port. This
enables for easy depth expansion. Both port selects are sampled
Document Number: 001-57832 Rev. *J
Page 7 of 31
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