Tag Archives: FeRAM

Non-volatile memory for low power designs

I came across a mention in a newsletter recently of Texas Instruments’s line of FRAM based MSP430 microcontrollers and thought a short post on FRAM would be of interest.

If you aren’t familiar with FRAM (also called FeRAM), think of it as essentially a non-volatile SRAM. FRAM was commercially developed primarily by Ramtron International starting in the mid 1980s. The ‘F’ in FRAM stands for ferroelectric but the underlying technology isn’t based on magnetics as might be assumed. The ferroelectric film used in constructing an FRAM is often described as creating a capacitor that can be charged or discharged to store a ‘1’ or a ‘0’. These are near perfect capacitors that can retain a charge for over 150 years. Reading a FRAM location causes the charge to degrade so read operations must be followed by a re-write of the data to that location. This is very similar to the way a DRAM operates and the re-write cycle is transparent to the user (except that read operations count toward the device’s endurance cycle count) and refresh cycles are not required like they are for DRAM.

FRAM is interesting for low power designs that require non-volatile memory. FRAM devices typically have standby currents comparable to or slightly less than EEPROM and NOR Flash devices. For write cycles, the FRAM has significantly lower currents and significantly faster write cycles. For applications that primarily read from non-volatile memory FRAM provides a modest power savings. For applications that frequently write to non-volatile memory FRAM can provide a huge power savings and does not require erase operations like Flash devices do.  Unlike EEPROM and Flash devices where the number of write cycles can limit or prevent their use in some applications, FRAM devices are typically spec’d for 10 trillion write cycles or more. The main drawbacks to FRAM are higher cost and relatively low density compared to EEPROM and Flash but with 8Mb parts on the market, for most low power embedded applications that really should not be an issue.

The table below presents some performance specs for typical FRAM, EEPROM and NOR Flash devices along with a few characteristics of the technologies. The devices used in the comparison all have SPI interfaces and capacities in the lower end of the capacity range for each part type.

FRAM1

EEPROM2

Flash3

Standby Current

6uA @ 3.3V

5uA @ 5.5V

1uA @ 2.5V

15uA @ 3.3V

Slow Read Current

200uA @ 1Mhz, 3.3V

2.5mA @ 5Mhz, 2.5V

Not specified

Fast Read Current

3mA @ 20Mhz, 3.3V

6mA @ 10Mhz, 5.5V

10mA @ 20Mhz, 3.3V

Write Current

Same as read

3mA @ 5.5V

30mA @ 3.3V

Erase time

N/A

N/A

25mS (sector or block)

Byte write time

< SPI transfer time

5mS

20uS

Endurance

> 1 trillion read/writes

> 1,000,000 writes

> 100,000 writes

Data retention (years)

150

200

100

Density range

4Kb – 8Mb

64b – 8Gb

256Kb – 2Gb

1 = FRAM specs from Cypress FM25L16B, 2Kx8 organization, 2.7V to 3.6V operating voltage

2 = EEPROM specs from Microchip 25LC160, 2Kx8 organization, 2.5V to 5.5V operating voltage

3 = Flash specs from SST (Microchip) SST25VF512, 64Kx8 organization, 2.7V to 3.6V operating voltage

FRAMs are available with SPI, I2C and parallel interfaces. There is a considerable amount of more detailed information on the Internet about FRAMs. The primary suppliers are Cypress Semiconductor (acquired Ramtron in 2012), Fujitsu Semiconductor (RAMtron’s primary foundry) and Rohm Semiconductor.

Back to what originally caught my eye regarding FRAM, Texas Instruments has integrated FRAM into their MSP430 micros. The MSP430FR series of micros has from 4KB to 128KB of on-chip FRAM along with a smaller amount of SRAM (presumably for stack and other frequently used variables). In contrast to some micros that lose some or all of their RAM contents in a deep sleep mode, with some consideration over whether variables are stored in FRAM or SRAM these devices could be completely powered down while retaining important state information and data.  With operating currents as low as 100uA/Mhz the MSP430 micros are strong players in the ultra low power micro market but the addition of internal FRAM provides them with a distinct advantage in certain types of application.

 

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