Pin-Compatible Replacement Possible? Improving the Quality of Existing Products Through FeRAM Implementation

Many hardware designers may initially view FeRAM (FRAM, ferroelectric memory) as an ideal memory technology.

Memory requires both capacity and access speed. And, as the name suggests, it needs to retain information. While it naturally retains information during operation, there’s a lot of information that needs to be retained even when the power is off.

SRAM is used in areas requiring high-speed access. However, since its contents are erased upon power loss, backup batteries are used in applications where data retention is necessary.

However, replacing SRAM with FeRAM eliminates the need for backups, as data retention is possible even without power. Furthermore, FeRAM boasts faster rewrite speeds and higher rewrite endurance compared to flash memory and EEPROM. It truly deserves to be called the ideal memory.

While you might want to implement it immediately, you might think that redesigning the printed circuit board would be costly and time-consuming. In fact, depending on your current EEPROM or SRAM, it may be possible to replace it with FeRAM without changing the printed circuit board. The key to this is “pin compatibility.”

What is Pin Compatibility?

Pin compatibility refers to the matching of pin configurations, package shapes, and signal specifications between different memory products. With pin compatibility, replacement can be completed simply by reusing the printed circuit board footprint and swapping components. No hardware design changes are required.

However, even within the category of pin compatibility, the difficulty of replacement differs significantly between 8-pin serial products and 40-pin or more parallel products.

8-Pin Serial Products Are Relatively Simple

For 8-pin SOP products with SPI or I2C connections, checking pin compatibility is relatively easy. EEPROM and FeRAM often use the same 8-pin SOP package (150mil or 208mil width), making them compatible with products from multiple manufacturers such as Microchip, STMicroelectronics, and Infineon.

The primary considerations are supply-voltage compatibility and package width (150mil or 208mil). If those match, replacement is usually possible without modification.

The Roles of the Pins in an 8-Pin SOP Package

The following are the pin names for many memory modules, including RAMXEED’s 2Mbit SPI product MB85RS2MTA.

Pin number	Pin name	Function
1	/CS	Chip select
2	S.O.	Serial data output
3	/WP	Write protect
4	VSS	Ground
5	S.I.	Serial data input
6	SCLK	Serial clock
7	/HOLD	Hold
8	VDD	Power supply voltage

After checking the pin functionality and confirming there are no issues with the electrical specifications, replacement is possible. If the package dimensions are the same, the printed circuit board footprint can be used as is. Since FeRAM is backward compatible with EEPROM, try replacing only the memory in a system currently using EEPROM and verify its operation.

[Diagram 1: 8-pin SOP package (150mil) dimensions] Quoted from MB85RS2MTA datasheet

Parallel Bus Components: What Hardware Engineers Need to Know

When it comes to multi-pin parallel bus connections, the situation changes. This is where hardware designers truly need to verify the following points.

RAMXEED’s parallel FeRAM comes in packages such as TSOP-44 pin and BGA-48 pin. Even for the same 8Mbit product, different packages result in completely different printed circuit board footprints.

Package Selection

The TSOP-44 pin (e.g., MB85R8M2TAFN) has a package width of 10.16mm x length of 18.41mm and a gull-wing lead shape.

On the other hand, the BGA-48 pin (e.g., MB85R8M2TABGL) has a package width of 8.00mm x length of 6.00mm, resulting in a significantly smaller footprint.

For replacing existing printed circuit boards, TSOP is practical, but BGA has advantages when miniaturization is required. However, BGA does not allow for visual solder joint verification. X-ray inspection may be required on the manufacturing line, and prototyping for replacement verification can be challenging.

For hardware engineers, the TSOP package may be easier to handle.

[Diagram 2: Comparison of TSOP and BGA Packages] Quoted from MB85R8M2TA datasheet

Left: TSOP-44 pin, lead pitch 0.8mm / width 10.16mm x length 18.41mm / height 1.2mm or less / gull-wing

Right: BGA-48 pin, ball pitch 0.75mm / width 8.00mm x length 6.00mm / height 1.00mm or less / solder ball

Package Thickness

Mounting height is another important parameter that is often overlooked.  For TSOP products, it should be 1.2mm or less; for BGA products, 1.00mm or less.

This number is crucial in designs with tight clearances within the enclosure. Furthermore, the exact height must be registered in the pick-and-place machine during manufacturing.

Always compare the dimensions with existing SRAM.

Pin Pitch and Printed Circuit Board Footprint

The lead pitch of TSOP products is 0.8mm. If the original SRAM has the same 0.8mm pitch, the printed circuit board is likely to be usable as is. It’s also necessary to check the gull-wing shaped contact points on the board and the thickness of the leads. These checks will help reduce mounting defects.

Always compare the outline dimensions in the datasheet before and after replacement.

Signals to Pay Attention to Regarding Pin Configuration

The pin configuration of parallel FeRAM is almost identical to that of standard asynchronous SRAM. The address bus (A0-A18), data bus (I/O0-I/O15), and pins /CE, /OE, /WE, /LB, and /UB are arranged in that order. Most pins match those of existing SRAM.

However, the /ZZ (sleep mode) pin requires special attention.

This pin is a unique feature of FeRAM; setting it low puts the device into sleep mode, reducing the quiescent current to the order of microamperes. Existing SRAM may lack this pin, or it may be assigned to another function (such as the SRAM’s Snooze function or CE2). Furthermore, some FeRAM part numbers may not have a /ZZ pin at all.

When replacing, you should check how this pin is handled on the current printed circuit board. If it’s unconnected or pulled up, there’s no problem, but if it’s connected to another signal, it will affect operation. Even if the pinout is almost identical, this single pin can cause the device to malfunction.

When designing a printed circuit board (PCB) with future FeRAM replacement in mind, it’s a good idea to prepare the footprints for jumpers and resistors in advance. For pins whose function is uncertain during the PCB design phase, such as the /ZZ pin, this approach can help avoid costly redesigns in the future.

S2 is the signal line before replacement, and a pull-down resistor is used to prevent it from becoming a noise source after replacement. The design allows switching the jumper to the J1 side when FeRAM is used. Connecting to Vcc with a pull-up resistor keeps the /ZZ pin high. S1 is intended for the sleep mode control signal, but if this function is not used, only the pull-up resistor R1 needs to be connected. Note that high resistance values are generally used for pull-up and pull-down resistors to maintain the signal state while suppressing unnecessary current consumption.

[Diagram 3: Example of a control signal switching circuit for memory replacement]

Capacity and Package Selection: How to Choose

RAMXEED’s parallel FeRAM offers a wide range of capacities from 256Kbit to 8Mbit. While choosing the same capacity as the original is the basic approach, selecting a higher capacity allows for future functional expansion.

Regarding packaging, for existing printed circuit board replacements, TSOP packaging makes physical verification easy. We recommend confirming supply stability early, especially with mass production in mind.

Hardware Replacement Alone Improves Product Quality

Replacing with FeRAM is more than just a component change.

While EEPROM has a rewrite endurance of 1 million cycles, FeRAM boasts 10 to 100 trillion cycles. This eliminates the need for lithium batteries for SRAM backup, extending product lifespan.

Replacing with pin-compatible FeRAM is a practical way to minimize development effort while increasing product reliability, provided you consider package dimensions, pin pitch, mounting height, and /ZZ pin handling. Please consider this option when making minor changes or component changes to existing products.

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