How Will Timekeeping Design for IoT Devices Change: Why FeRAM Is Gaining Attention

Why do batteries remain even though SRAM has been eliminated?

Replacing SRAM with FeRAM (FRAM, ferroelectric memory) eliminates the need for backup circuits for data retention. However, backup circuits cannot be completely eliminated because of the RTC (Real-Time Clock IC). This article examines the issue from the perspective of rethinking timekeeping design itself, given the advancement of IoT.

A Clock That’s Always Running – What’s Its Power Source?

Clocks have traditionally been designed with the assumption that they must always maintain time. This assumption itself may limit design flexibility. That’s where a Real-Time Clock (RTC), also known as a clock IC, comes in.

It works in conjunction with a 32.768kHz crystal oscillator to keep time constantly. Incidentally, dividing this frequency 15 times yields one second. Wristwatches and other clocks around us use crystal oscillators of the same frequency. And the power source for these clocks is a battery.

Similarly, many electronic devices use lithium batteries as their power source. Their primary purpose is data retention for SRAM, and the RTC has traditionally shared the same backup battery used for SRAM retention.

In actual terms, the RTC’s power consumption is about 10% of that of SRAM, so it hasn’t been a particularly noticeable component.

The Impact of FeRAM Introduction

Introducing FeRAM eliminates the need for SRAM. As a result, the load on the backup power supply is concentrated on the RTC. In terms of power consumption, it’s possible to extend the product lifespan to about 10 times that of conventional systems.

However, even with minimal current consumption, the backup circuit cannot be eliminated as long as the time is being kept. And this backup circuit is surprisingly troublesome, causing headaches for designers. Ideally, we would like to consider eliminating the backup circuit at the same time as introducing FeRAM.

Methods for Eliminating the Backup Circuit

So, is this backup circuit truly necessary? The design options can be broadly categorized into two:

• Skip backup entirely (synchronize the time only when needed)
• Eliminate the RTC itself

Both require a fundamental redesign of the system, but IoT adoption is a major factor in supporting this decision.

[Diagram 1: Comparison of Simplification of Backup Circuits]

Is a Backup Really Necessary?

Let’s reconsider why a backup is necessary in the first place. The components being backed up are SRAM and RTC. While SRAM holds data, replacing it with FeRAM eliminates the need for a backup.

The RTC requires constant clock operation to obtain time information. Therefore, strictly speaking, it’s an operation rather than a backup, but we’ll consider its advantages and disadvantages.

Advantages of Maintaining an RTC

What are the advantages of continuously backing up the RTC?

First, it allows for the constant acquisition of time information guaranteed by a crystal oscillator. Many electronic devices frequently utilize time information. It allows for the acquisition of time information that is self-contained within the product, completely independent of the external environment.

Furthermore, using the CPU’s interrupt function enables automatic system startup. It also becomes easier to use it as an interval timer. For standalone devices that use time information, an RTC is necessary.

How Does Eliminating the RTC Change the Design?

There is only one disadvantage: you lose the ability to obtain time information in real time. Now let’s consider the advantages.

First, the backup circuit — lithium battery, diodes, and so on — can be eliminated entirely. In designs where the lithium battery was made replaceable, the battery holder that enabled that also becomes unnecessary. Furthermore, if the RTC (Real-Time Clock) can be eliminated, the crystal oscillator and its passive-component oscillator circuit also become unnecessary.

This also reduces manufacturing costs by eliminating the backup circuit and reduces the printed circuit board area required for implementation. Together, these changes can reduce both BOM cost and PCB complexity. This is especially effective when the lithium battery is directly soldered to the printed circuit board.

Where Should System Review Begin?

For IoT devices connected to a network, time information can be obtained from NTP (Network Time Protocol), etc., at startup. Review the system and consider eliminating the RTC and backup circuit.

For standalone devices not connected to a network, the review proceeds in two directions. One is to reconsider whether time information itself is truly necessary. If data retention is reliable, the system design may work even without time information from the RTC.

The other is functional expansion through network connectivity. For example, there are home appliances with time display functions, such as rice cookers and microwave ovens. If these can obtain time information from the network at startup through IoT integration, a design without an RTC is possible. In this case, since time information can be obtained using network time synchronization such as NTP, it leads to a review of the RTC and backup circuit.

Many electronic devices are becoming IoT-enabled. The switch to FeRAM could be a catalyst for IoT implementation.

What Changes When the RTC Is Eliminated?

The elimination of the RTC due to the adoption of FeRAM also means the elimination of lithium batteries. Here, we will consider the impact of eliminating these components.

Impact on Product Lifespan

The RTC starts operating when the product is assembled at the factory. In many cases, time setting is done during the manufacturing process. Product lifespan is often determined by battery life. Therefore, the actual product lifespan must be considered from the point of manufacture at the factory.

For electronic devices that are transported and stored in warehouses for extended periods after manufacturing, the question of how to handle product warranties for that time becomes an issue. Because users consider the time from the start of use, a discrepancy between reality and the warranty occurs. If lithium batteries can be eliminated, product lifespan can be significantly extended.

[Diagram 2: Conceptual Changes in Product Lifespan]

Operation During Transportation and Storage

Often overlooked, the RTC (Real-Time Clock) continues to operate during transportation and storage. Engineers should be aware that the 32.768kHz clock is constantly running. Without an RTC, no circuits would operate unless external power is supplied.

Crystal Oscillator Accuracy

The temperature characteristics of the RTC’s crystal oscillator are also important. While highly accurate at room temperature, accuracy deteriorates at low or high temperatures. Depending on the temperature environment during transportation and storage, the time can drift by several minutes per month. Network time information is highly accurate, and correction information can be obtained even if there is a drift.

Maintenance-Free Added Value

In electronic devices, lithium batteries significantly impact product lifespan. Eliminating lithium batteries results in maintenance-free operation. This is a new added value for both users and designers.

The introduction of FeRAM goes beyond simply replacing memory. It provides an opportunity to re-examine the design itself for continuously maintaining time.

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