Plenty of products need to know the time even when they are not connected: data loggers, meters, access control, appliances and anything that timestamps an event. The real-time clock IC is a small part that causes a disproportionate number of support questions, usually because of the backup supply rather than the chip. Here is how to choose and what to check.
1. The three decisions
- Accuracy — do you need minutes per year, or is drift of seconds per day acceptable? This decides whether you need integrated temperature compensation.
- Interface — I²C for two-pin simplicity, SPI where you prefer it; the RTC itself is slow either way.
- Integration — an RTC with an internal crystal and TCXO removes layout risk; a discrete-crystal part is cheaper but demands careful board design.
2. RTC parts we stock
| Part | Notes | Link |
|---|---|---|
| DS3231SN | High-accuracy RTC with integrated TCXO | DS3231SN |
| DS1307ZN+ | Classic I2C RTC, external 32.768 kHz crystal | DS1307ZN+ |
| PCF8563T/F4 | Low-power I2C RTC with alarm and timer | PCF8563T/F4 |
| Si52147-A01AGMR | Clock generator for multi-output clock distribution | Si52147-A01AGMR |
3. The backup supply is the real design task
An RTC that loses time on power-down is almost never a faulty IC. Work through this:
- Confirm there is a backup source — coin cell, supercapacitor — on the correct pin.
- Check the isolation: a diode or trickle-charge circuit must let the backup supply the RTC when main power drops, without draining back into the main rail.
- Verify the backup voltage is within the RTC's specified range. Too low and the oscillator stops; too high and you damage the part.
- Measure the backup current. A leaky path can flatten a coin cell in weeks instead of years.
4. Layout notes for crystal-based RTCs
If you use an RTC with an external 32.768 kHz crystal, the layout is part of the design: keep the crystal and its load capacitors as close to the IC as possible, keep other traces away from that node, and guard it from thermal gradients. A crystal that is electrically correct but badly placed will still drift.
Selection checklist
- Decide the accuracy target first; it chooses the family.
- Pick the interface that fits your spare pins.
- Design and test the backup path explicitly—do not assume it works.
- For discrete-crystal parts, treat the crystal layout as a first-class design task.
- Confirm the alarm and interrupt features actually match what your firmware expects.
Browse the interface and timing device range and crystals and oscillators. Send your accuracy and interface requirement for a quote.
Frequently asked questions
Which is more accurate, the DS3231 or the DS1307?
The DS3231 uses an integrated temperature-compensated crystal oscillator (TCXO) and holds accuracy to a few minutes per year. The DS1307 relies on an external crystal and is far more sensitive to temperature, drifting noticeably over a wide range. If long-term accuracy matters, the DS3231 is the better choice.
Why does my RTC lose time when the product is powered off?
Almost always a backup-supply problem. The RTC needs a coin cell or supercapacitor on its backup pin, and the trickle-charge or diode arrangement must actually connect it when main power drops. Check the backup path and the cell voltage before blaming the part.
Do I need an external crystal with an RTC IC?
Some RTCs integrate the crystal and compensation; others require a 32.768 kHz crystal and load capacitors. Integrated parts remove the layout sensitivity but cost more. Discrete-crystal parts need careful layout: keep the crystal close, guard it from noise and match the load capacitance.
What is the difference between I2C and SPI RTCs?
Functionally they are the same timekeeping device with a different host interface. Choose based on what your microcontroller has spare. I2C uses two pins and shares the bus; SPI is faster but uses more pins. For an RTC, speed almost never matters, so I2C is usually the practical choice.
About us & trademarks: JTDZ Tech (Shenzhen Jingteng Electronics Technology Co., Ltd.) is an independent distributor of electronic components. We are not an authorised distributor or representative of the manufacturers named in this article. Manufacturer and part names are used for indicative purposes only, to help identify the components discussed; all trademarks remain the property of their respective owners. Always verify specifications against the official datasheet before design-in or purchase.
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