GS81302S18/36AGD-400/375/333/300/250
Background
Separate I/O SRAMs, like SigmaQuad SRAMs, are attractive in applications where alternating reads and writes are needed. On the
other hand, Common I/O SRAMs like the SigmaCIO family are popular in applications where bursts of read or write traffic are
needed. The SigmaSIO SRAM is a hybrid of these two devices. Like the SigmaQuad family devices, the SigmaSIO features a
separate I/O data path, offering the user independent Data In and Data Out pins. However, the SigmaSIO devices offer a control
protocol like that offered on the SigmaCIO devices. Therefore, while SigmaQuad SRAMs allow a user to operate both data ports at
the same time, they force alternating loads of read and write addresses. SigmaSIO SRAMs allow continuous loads of read or write
addresses like SigmaCIO SRAMs, but in a separate I/O configuration.
Like a SigmaQuad SRAM, a SigmaSIO DDR-II SRAM can execute an alternating sequence of reads and writes. However, doing
so results in the Data In port and the Data Out port stalling with nothing to do on alternate transfers. A SigmaQuad device would
keep both ports running at capacity full time. On the other hand, the SigmaSIO device can accept a continuous stream of read
commands and read data or a continuous stream of write commands and write data. The SigmaQuad device, by contrast, restricts
the user from loading a continuous stream of read or write addresses. The advantage of the SigmaSIO device is that it allows twice
the random address bandwidth for either reads or writes than could be acheived with a SigmaQuad version of the device.
SigmaDDR (CIO) SRAMs offer this same advantage, but do not have the separate Data In and Data Out pins offered on the
SigmaSIO SRAMs. Therefore, SigmaSIO devices are useful in psuedo dual port SRAM applications where communication of
burst traffic between two electrically independent busses is desired.
Each of the three SigmaQuad Family SRAMs—SigmaQuad, SigmaDDR, and SigmaSIO—supports similar address rates because
random address rate is determined by the internal performance of the RAM. In addition, all three SigmaQuad Family SRAMs are
based on the same internal circuits. Differences between the truth tables of the different devices proceed from differences in how
the RAM’s interface is contrived to interact with the rest of the system. Each mode of operation has its own advantages and
disadvantages. The user should consider the nature of the work to be done by the RAM to evaluate which version is best suited to
the application at hand.
Burst of 2 SigmaSIO DDR-II SRAM DDR Read
The status of the Address Input, R/W, and LD pins are sampled at each rising edge of K. LD high causes chip disable. A high on
the R/W pin begins a read cycle. The two resulting data output transfers begin after the next rising edge of the K clock. Data is
clocked out by the next rising edge of the C if it is active. Otherwise, data is clocked out at the next rising edge of K. The next data
chunk is clocked out on the rising edge of C, if active. Otherwise, data is clocked out on the rising edge of K.
Burst of 2 SigmaSIO DDR-II SRAM DDR Write
The status of the Address Input, R/W, and LD pins are sampled at each rising edge of K. LD high causes chip disable. A low on the
R/W pin, begins a write cycle. Data is clocked in by the next rising edge of K and then the rising edge of K.
Rev: 1.01a 3/2019
5/31
© 2017, GSI Technology
Specifications cited are subject to change without notice. For latest documentation see http://www.gsitechnology.com.
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