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Getting Started with Serial SRAM
Introduction:
Microchip’s Serial SRAMs offer an easy and inexpensive way to add RAM to an application. With small, 8-pin packages and the SPI interface (which only requires 4 I/Os) these devices give designers added flexibility with regard to their RAM needs. The Serial NVSRAM devices offer the industry’s least expensive non-volatile SRAM or FRAM solution. These low power NVSRAM devices offer unlimited, instantaneous writes to the memory array while automatically backing up data in the event of a power shutdown using an external battery.
RAM Expansion Options:
There have traditionally been two ways to increase an application’s RAM:
1. Use a larger microcontroller. This option is less attractive if you must buy a larger, more expensive micro just to get more RAM.
2. Use external parallel RAM. But, parallel RAM uses large packages and typically requires at least 16-20 I/Os.
Serial SRAM now offers the flexibility to add RAM to a design without the disadvantages of a large micro or parallel RAM and uses the simple 4-pin SPI interface. These devices also offer improved performance with the SDI and SQI interface that offers up to a 4x improvement in data rates.
Product Overview:
Benefits over other memory technologies:
Serial SRAM offers customers the least Expensive RAM expansion solution. With its 5V capability, easy to use SPI interface and tiny 8 pin package, Serial SRAM devices are ideal for applications that need additional RAM or applications that can offload repetitive, data intensive functions such as data buffering, data logging, graphics, math, audio and video to an off-chip memory. Serial SRAMs are available from 64Kbit up to 1Mbit in density.
The Serial NVSRAM is significantly lower cost than any other parallel, serial NVSRAM or FRAM. The serial NVSRAM uses an external battery for non-volatile storage. This is beneficial for applications such as meters, black boxes and other data recorders, which require unlimited endurance or instantaneous writes along with non-volatile storage. Serial NVSRAMs are available up from 512Kbit 1Mbit in density.
Serial SRAM and NVSRAM offer a lot of benefits – size, voltage, speed, power and costs.
Serial SRAM Bus Modes – SPI, SDI and SQI:
Serial SRAM devices are designed to work on the standard SPI interface at 20MHz. On the 512Kb, 1Mb devices – the SO, SI (pin 2, pin 5) along with SIO2, SIO3 (pin 4, pin 7) are designed to work as bi-directional I/O pins.
In SQI mode, all 4 pins (pins 2, 4, 5, 7) are configured to read or write at any given time effectively ensuring 4 bits are transferred across the bus in a single clock cycle – this, in effect, multiplies the data rate by 4. So for example, a task that takes 40 clock cycles to complete in standard SPI mode can now be completed in 40/4 = 10 clock cycles. This feature is extremely useful when large amounts of data need to be transferred quickly.
In SDI Mode, only 2 of the pins are configured as bi-directional I/O pins (pin 2, pin 5), allowing you to get a maximum of 2x data rates. Our standard 512Kb and 1Mb SRAM devices can be used in SDI and SQI modes.
The same device in the same footprint can be used for SDI and SQI.
The graph below shows the time required to write 128Kbytes of memory at 20MHz in SPI, SDI and SQI modes on the 23LC1024 1Mbit SPI SRAM. As can be seen the SQI is 75% faster than standard SPI.
Web Seminars:
Serial SRAM Overview:
Recommended Usage for Serial SRAM:
For more information:
We have several on-line resources available:
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