Abstract: Since the first NV SRAM modules were developed nearly 20 years ago, new generations have been produced to keep pace with various applications and the growing demand for new packaging technologies. This article takes a look back at the first NV SRAM devices, describes the improvements made that led to the current generation of parts, draws a comparison with existing technologies, and concludes with an overview of the key application areas.
Past and Present
The first-generation NV SRAM modules were offered in a high-profile DIP package (Figure 1). The package height is due to the fact that both the battery and the RAM chip are mounted one on top of the other in the DIP. The advantage of the DIP package is that by plugging it into a DIP socket, it can be replaced, stored, or transferred from one board to another. Although these advantages are still relevant today, an expansion to surface-mount technology and the migration from 5V to 3.3V was necessary.
Figure 1. First-generation NV SRAM in DIP module.
Second-generation NV SRAM modules were offered as a two-piece device—a PowerCap module (PCM) consisting of the base containing the SRAM, which was soldered directly to the board, and the PowerCap, a Lithium energy source attached to the base through snap-in contacts (Figure 2). These devices have two key advantages when compared to the DIP modules: they are surface-mountable and the pin configuration is standardized. In other words, the package and the pin count remain the same regardless of the NV SRAM size. As a result, system storage requirements could now grow without the designer having to worry about making changes to his PCB layout. The battery can also be easily replaced.
Figure 2. Second-generation NV SRAM in PowerCap module.
The third and newest generation NV SRAM modules are based on addressing the limitations in the previous generations while adding more functionality. These new NV SRAMs are single-piece BGA modules containing a rechargeable lithium battery (Figure 3). Like the PCM approach, these modules are offered in the same package size and pinout for all SRAM sizes. The module is surface mountable, and only one piece is used. The design is also more rugged, withstanding a higher mechanical shock level than its predecessors. Because the battery is rechargeable, the concept of data-retention time takes on another meaning. A more appropriate specification is equivalent service lifetime, which can be up to 200 years! The module is capable of withstanding a reflow temperature of +230°C, and a lead-free version will be available which can withstand a temperature of +260°C.
Figure 3. Third-generation NV SRAM in single-piece module.
Comparison of NV SRAM with Other Technologies
SRAMs provide a fast and reliable means to access and store data. They are made nonvolatile by the addition of an energy source, either the system supply or an attached energy source, such as a battery. Table 1 gives an overview of the advantages and disadvantages of selected nonvolatile memory storage technologies.
Table 1. Comparison of Nonvolatile Memories
Feature
Flash
SRAM
OUM
EEPROM
FRAM
MRAM
Write speed
slow
fast
fast
slow
fast
fast
Read speed
medium
fast
fast
medium
fast
fast
Read endurance
infinite
infinite
infinite
infinite
1,00E+12
infinite
Write endurance
1,00E+06
infinite
1,00E+12
1,00E+6
1,00E+12
1,00E+12
Power consumption
high
low
medium
high
low
high
Lifetime
low
high
high
low
medium
high
Density
high
medium
high
low
medium
high
Read
non-destructive
non-destructive
non-destructive
non-destructive
destructive
non-destructive
Data retention at +140°C
good
good
good
medium
good
poor
The battery used in NV SRAM plays a crucial role in data retention. Poor contacts to the battery can significantly impact the lifetime of the battery. Other factors such as shock, humidity, and the inclusion of a battery switchover circuit can reduce the battery's service life and impact the overall reliability of the system. An apparent consequence of these problems occurs when the end-customer is required to change the battery in significantly shorter time intervals, such as every year as opposed to every five to ten years.
Applications
NV SRAM is typically employed in systems where maintenance must be kept to a minimum, many read/write operations take place, and speed is crucial in saving data. One example can be found in numerically controlled milling machines. Suppose that the machine was in the middle of performing a milling operation when suddenly the main power drops out. For safety reasons, the drill bit needs to be withdrawn and parked in a safe position. As soon as the power comes up, the machine must remember the point where it left off. This is a task that can easily be accomplished with the help of an NV SRAM module. An alternative way to do this is to use a wide-input-range, step-up DC-DC converter powered by a bank of capacitors or a supercap. The hold-up time is chosen to be equal or greater than the time required to store program information in an EEPROM. SRAM is still required because it serves as a data cache until the data is transferred to EEPROM. In such cases, the cost of the extra components usually exceeds the cost of an NV SRAM module and reliability is compromised.
Data logging in tamper-proof applications is gaining importance, especially in POS terminals. Smart terminals are now able to approve payment transactions without having to obtain approval from a remote server—a time-consuming task. Because secure data resides in the terminal, a single-piece BGA module lends itself perfectly here. Secure microcontrollers have the ability to store encryption keys in a "secure zone" within the IC. However, extra steps are still required to prevent the encrypted data, which is stored in SRAM outside the microcontroller, from being read and decoded. The level of protection currently offered today is not enough to keep out today's sophisticated hackers. To address this concern, Maxim/Dallas Semiconductor offers custom-made BGA modules containing a secure micro, SRAM, tamper-detection circuitry, and timing and control functions.
Another data-logging application is found in motor-vehicle crash boxes. Data at the time of a crash needs to be quickly and reliably stored in memory. The single-piece BGA is once again an ideal candidate especially because of its ruggedness. Data tampering is an issue, but can be practically addressed by simply storing encrypted data in the NV SRAM. The effort involved in deciphering the encrypted data is not worth a hacker's time as much as a payment transaction terminal, for example.
Gambling machines require reliable data-logging in order to validate customer claims. In this case, the two-piece PCM approach is not effective because the gambling machine operator can open the machine, remove the battery, and say that the data got lost accidentally. An EEPROM could be written to every time the machine was opened, but again, this involves additional cost and complexity. A BGA module, from which the battery cannot be removed, easily solves this problem.
High-voltage fault protection equipment needs to constantly monitor the status of the power grid and be ready to store a significant amount of data in the event of a fault condition. The equipment has to be in service for at least ten years without requiring the involvement of a field-service technician. The use of a BGA module is ideal in this case because the fault-protection unit is usually powered by the power grid itself, and the battery inside the BGA module does not wear out. As a result, significantly longer service intervals can be expected. Software designers working on newer generations of a fault-protection unit can benefit from an increase in SRAM memory size by simply going to a higher density NV SRAM module without incurring changes to the existing board layout. This is possible because the pin assignment for all NV SRAM modules is set up so that as SRAM density increases, more "no connection" (NC) pins become converted to the required address lines.
Conclusion
NV SRAM modules not only provide fast and reliable data storage, but also offer significant advantages based on its packaging technology. These devices are ideal for applications requiring secure data and a bare minimum of field service.
A similar article was published in German in the October 2005 issue of Elektronik Informationen.
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