What are the considerations to determine whether I should use an nvSRAM or an F-RAM™?
Interface standards, density, power consumption, throughput, and endurance are the major considerations while choosing between Cypress’s nonvolatile devices: nvSRAM or F-RAM.
Cypress’s nonvolatile products are available with parallel, SPI, I2C, and ONFI interfaces.
Parallel: Some controllers have parallel interfaces (asynchronous memory interfaces) to use peripheral devices Cypress’s parallel nvSRAMs and F-RAMs can be used with controllers that have an asynchronous SRAM interface.
Table 1. Parallel nvSRAMs and F-RAMs
Item nvSRAM (Parallel) F-RAM (Parallel) Density range 64 Kbit to 16 Mbit 64 Kbit to 4 Mbit Speed range 20 ns to 55 ns 55 ns to 70 ns Operating current (typ) 70 mA 12 mA Standby current (max) 5 mA 150 µA
SPI: SPI is a 4-wire communication interface. Most of the controllers will have this interface. SPI nvSRAMs and F-RAMs are recommended to use with such controllers.
Table 2. SPI nvSRAMs and F-RAMs
Item nvSRAM (SPI) F-RAM (SPI) Density range 64 Kbit to 1 Mbit 4 Kbit to 2 Mbit Speed range Up to 40 MHz Up to 40 MHz Operating current (typ) 10 mA 3 mA Standby current (max) 5 mA 150 µA
I2C: I2C is a 2-wire communication interface. Most of the controllers will have this interface. I2C nvSRAMs and F-RAMs are recommended for use with such controllers.
Table 3. I2C nvSRAMs and F-RAMs
Item nvSRAM (I2C) F-RAM (I2C) Density range 64 Kbit to 1 Mbit 4 Kbit to 1 Mbit Speed range Up to 3.4 MHz Up to 3.4 MHz Operating current (typ) 1 mA 1 mA Standby current (max) 250 µA 150 µA
ONFI 1.0 (Open NAND Flash Interface): Some RAID controllers have ONFI-compliant controllers. Cypress’s ONFI nvSRAMs can be interfaced with such controllers.
The power consumption of F-RAMs is much less than that of nvSRAMs. Tables 1, 2, and 3 provide a comparison of operating current and standby current between nvSRAMs and F-RAMs.
The throughput comparison of Cypress parallel and serial nvSRAM is given in the below table,
Table 4. Throughput Comparison Between Parallel and Serial nvSRAMs (1 Mbit)
nvSRAM (Parallel) nvSRAM (I2C) nvSRAM (SPI) 400 Mbits/s 3.4 Mbits/s 40 Mbits/s
Table 5. Throughput Comparison Between Parallel and Serial F-RAMs (1 Mbit)
F-RAM (Parallel) F-RAM (I2C) F-RAM (SPI) 88 Mbits/s 3.4 Mbits/s 40 Mbits/s
As seen from this table, parallel nvSRAMs have the highest throughput compared to serial nvSRAMs and F-RAMs.
Both nvSRAMs and F-RAMs provide similar performances in terms of endurance. All writes to nvSRAMs are carried out through the SRAM memory and do not use up any endurance cycles of nvSRAMs. Endurance of F-RAMs is 100 trillion (1014) accesses (read or write operations). Even though F-RAMs have finite endurance, it will take a long time to reach the end of life of the part in terms of endurance. For example, an F-RAM operating at 40 MHz with 73,520 endurance cycles per second will take 43.1 years to use all the endurance cycles.
Parallel nvSRAMs are recommended for use in applications that require a high throughput. On the other hand, serial F-RAMs are recommended for use in applications that have low power requirements. Tradeoff among throughput, power consumption, and number of devices connected to a single controller will determine the interface type.
NOTE: nvSRAMs need an external capacitor to perform nonvolatile store and recall operations.