Does Your System Need an RTC Module Circuit?

Anytime you need to keep track of time in your digital system, you need to convert clock pulses into a date and time. Running on a clock circuit isn’t enough, conversion takes some digital arithmetic and requires comparing a count of clock pulses to some reference date. Since any good clock in your electronics should operate whether or not the device is in use, you also need a persistent power source for your timekeeping module.

Instead of wiring up a crystal or an oscillator circuit to an MCU, you can use a real-time clock (RTC) module in your board to provide accurate timekeeping. These small chips provide a simple way to track time in a digital system and feedback the data to an MCU over a standard low-speed interface. Here are some applications that will need an RTC module circuit and some popular options for these circuits.

RTC Module Circuit Design

RTC modules are very simple and come in small footprints. They also have very low power consumption as they generally only need to read a clock pulse from a crystal or oscillator circuit. The best RTC modules will provide at least a decade of accurate timekeeping on a single coin cell backup battery, and they can run on the main system power when available. This simple capability might be built into an MCU, but not all systems need a large MCU with an integrated RTC circuit.

The schematic below shows a simple RTC module circuit with a popular IC, the DS1307 from Maxim Integrated. Unreferenced clock pulses are fed to this module from a 32.768 kHz clock, and the clock pulses are referenced to a specific date internally in the DS1307. From there, the data can be queried by an external MCU over I2C and stored in the MCU’s memory. This is basically the same process that other MCUs with an internal oscillator and RTC circuit would use to track time as part of their firmware.

Example RTC module circuit for use as a system clock for an ATTiny85 MCU.

In the above schematic, the only other noteworthy component is the ATTiny85 MCU. This MCU has a small DIP footprint, internal EEPROM with 100,000 erase/write cycles, internal Flash, and internal SRAM. This particular implementation is part of a power monitoring and tracking circuit for a client project, but it illustrates some particular cases where a larger MCU is not necessary, and a smaller MCU, backup battery, and RTD module circuit will provide the functionality you need.

Why Use an RTC Module Circuit versus an MCU?

The power product I mentioned above does not require a large MCU for a number of reasons. Some common reasons are listed below, and they might apply to your next system as well.

System only tags and stores data. This is a typical task for sensor node boards, where data might be tagged with a timestamp and some other criteria triggered by logic circuits. The data can then be stored in Flash via SPI. This doesn’t require a heavy-duty MCU as long as the MCU has an SPI bus interface or other low-speed interface (e.g., I2C in the above example).

Core functions are built into other integrated circuits. In the power monitoring board example I’ve referenced above, we didn’t need to integrate the logic for sensing and control loops into a single MCU. This would have saved some space, but building out the firmware would be like reinventing the wheel. Many of these functions are built into COTS power management ICs.

Intermittent power. If the device ever needs to go offline, or you expect power access will be intermittent, then you can ensure you’ll continue tracking time in your system thanks to a battery backup. RTC module circuits can easily interface with a coin cell battery and can run for multiple years without charging or replacement. This also means the system won’t need an additional regulator/management unit to work on battery power. In contrast, using an MCU for timekeeping on battery power wastes battery life, so it’s worth it to just use a small RTC module circuit regardless.

Popular RTC Components

Maxim Integrated, DS1307 and DS3231

These two RTC modules are arguably the most popular RTC modules for a range of devices. The DS1307 IC (shown in the schematic above) is a minimal component that only includes I2C interface, square wave output, backup battery management controller, and inputs for an external oscillator. The DS3231 RTC module is a more powerful version that still communicates via I2C, but it contains an integrated oscillator, reset function through an external pin, and square wave/32.768 kHz oscillator outputs.

DS3231 application circuit. From the DS3231 datasheet.

Maxim Integrated, MAX31341BEWC

This particular RTC module is Maxim’s smallest RTC component to date, offering ultra-low current consumption of 180 nA. This component also features integrated alarm functions alongside standard timekeeping functions that are accessed via I2C. The component can also be locked to a reference clock with various standard frequencies, allowing the clock accuracy to be determined by the external source. Target applications include medical devices, wearables, telematics, and similar areas.

MAX31341 application RTC module circuit with external MCU. From the MAX31341 datasheet.

Renesas, ISL12057IUZ

The ISL12057IUZ from Renesas offers similar capabilities as the MAX31341, but it is marketed for industrial, automotive, and other application areas with harsh environments. As with the other components listed above, it provides square wave output and data is accessed over an I2C interface. The component can support an external 32.768 kHz oscillator with 6 pF load capacitance through an integrated inverting amplifier.

ISL12057IUZ application RTC module circuit. From the ISL12057IUZ datasheet.

Other Components for Time Tracking

Aside from the RTC module itself, you’ll need some other components to provide stable power, configure the digital bus to read data, and interface with an external controller. These components include:

• MCU or FPGA

• Batteries and battery holders

• Passives (resistors and capacitors)

If you’re designing a mobile device, IoT product, or other system that needs timekeeping with an RTC module circuit, you can find these components and any more with the advanced search and filtration features in Octopart. When you use Octopart’s electronics search engine, you’ll have access to distributor data and parts specifications, and it’s all freely accessible in a user-friendly interface. Take a look at our integrated circuits page to find the components you need.

Stay up-to-date with our latest articles by signing up for our newsletter.