Real-time clock (RTC)

01

Budget the time error from the crystal

Abracon's AB26T 32.768 kHz watch crystal datasheet gives a frequency tolerance of ±20 ppm at 25 °C, a turnover temperature of +25 °C ± 5 °C, a temperature coefficient of −0.034 ± 0.006 ppm/°C², first-year ageing of ±3 ppm at 25 °C, a load capacitance of 12.5 pF (typ.) and 35 kΩ maximum series resistance, for operation from −10 °C to +60 °C as standard. [2] Its ABS07 surface-mount crystal lists −0.036 to −0.040 ppm/°C². [3]

Editorial calculation: with a parabolic coefficient of −0.034 ppm/°C² and turnover at 25 °C, the crystal runs 0.034 × (0 − 25)² ≈ 21 ppm slow at 0 °C and about 42 ppm slow at −10 °C or +60 °C, on top of its ±20 ppm room-temperature tolerance. Each 1 ppm is 0.0864 s per day, so 42 ppm is about 3.6 s a day, or roughly 1.8 minutes in 30 days. [2]

TI's BQ32000 data sheet quotes a pre-calibration accuracy of ±35 ppm (typ.) at 25 °C with a reference board and a specific surface-mount crystal, and notes that board design and crystal selection change the result. [1] It specifies a crystal load capacitance of 12 pF and series resistance up to 70 kΩ, and says no external load capacitor should be used when the crystal's CL is 12 pF. [1] Editorial note: match the crystal's load capacitance to the RTC and ask for accuracy across your temperature range, not only at 25 °C.

Give the supplier the accuracy needed (seconds per day or month) over your temperature range, supply voltage, backup source (coin cell, supercapacitor, rechargeable) and expected time on backup, interface (I²C or SPI), alarms and interrupts needed, package or module format, operating temperature and qualification (industrial, automotive).

02

Plan the backup supply and calibration

The BQ32000 runs from 3 to 3.6 V, draws up to 100 μA from VCC, switches automatically to a backup supply (with an integrated trickle charger for rechargeable backup), keeps time with the backup supply down to 1.4 V, and draws 1.2 μA (typ.), 1.5 μA maximum from a 3 V backup with the oscillator running at 25 °C. [1] It operates from −40 °C to 125 °C. [1]

Its programmable calibration adjusts the clock from −63 ppm to +126 ppm by periodically adjusting the width of the 1 Hz clock, which can be measured on the IRQ pin; the effect is observable only every eight or sixteen minutes. An OF (oscillator fail) flag shows whether the clock has stopped. [1]

Editorial calculation: at the 1.5 μA maximum backup current, one year on backup draws 1.5 μA × 8,760 h ≈ 13 mAh from the backup source, before self-discharge and temperature effects. Compare it with the usable capacity of the cell or capacitor you choose.

03

Choose between an RTC with a built-in compensated crystal and one with an external crystal

Seiko Epson's Epson Crystal Device site lists its "RX8900CE" RTC module with a built-in 32.768 kHz crystal unit and a Digital Temperature Compensated Crystal Oscillator (DTCXO), an I²C-Bus interface at 400 kHz, an interface voltage range of 2.5 V to 5.5 V, a temperature-compensated voltage range of 2.0 V to 5.5 V and a clock supply voltage range of 1.6 V to 5.5 V, with selectable clock outputs of 32.768 kHz, 1024 Hz and 1 Hz. [4]

Renesas lists the ISL1208 as an I2C real-time clock/calendar with automatic backup to battery or super capacitor, power failure detection, on-chip oscillator compensation and the "Same Pin Out as ST M41Txx and Maxim DS13xx Devices". [5] With a part like this the error budget still depends on the external crystal's tolerance, temperature coefficient and ageing, which Abracon's data sheets give for its AB26T and ABS07 crystals [2] [3].

Epson moves the crystal into the package and compensates it digitally for temperature [4], so the crystal parameters are no longer a separate purchase line. The page reviewed does not state a ppm figure, so ask Epson or its distributor for the accuracy over your temperature range from the RX8900CE data sheet before comparing it with a crystal-plus-calibration design such as the BQ32000 [1].

Editorial calculation: 1 ppm is about 2.6 s in a 30-day month (30 × 86,400 s × 10⁻⁶); a 400 nA battery current is one-third of the 1.2 µA (typ.) backup current listed for the BQ32000. [5] [1]

Values are from the Epson and Renesas product pages; their data sheets were not reviewed. Units for the ISL1208 compensation range are not shown on the parameter table.

04

Analogue chip makers and frequency-control makers

The companies shown below fall into two groups: semiconductor makers listing RTC ICs, and crystal and oscillator makers listing RTC modules or chips.

Listings include RTC ICs among analogue and automotive-grade semiconductors, and RTC modules or chips alongside quartz crystals, TCXOs and oscillators from frequency-control makers. Ask each for accuracy over temperature, backup current and crystal requirements.

05

Crystal, counters, calibration, backup switchover, interface and flags

All rows except the crystal follow the BQ32000 data sheet; the crystal row follows Abracon.

06

Accuracy, supply, backup current and package for the quotation

The accuracy you need and your temperature range come from your product. TI, Abracon and Renesas figures appear in the middle column as examples.

07

Send the accuracy, power and interface data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Accuracy: [seconds per day or month; temperature range]
Power: [supply; backup source; time on backup]
Interface: [I2C or SPI; alarms; interrupts]
Crystal: [included or external; CL; tolerance]
Format: [IC package or module]
Grade: [industrial; automotive]

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