component

PCF8523 Real-Time Clock

Public Unreviewed

by Ray

Low-power I2C real-time clock and calendar with battery backup and alarm.

rtcreal-time-clocki2ccalendarlow-powernxpsoic-8
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README

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PCF8523

NXP · Real-Time Clock (RTC) · SOIC-8

The PCF8523 is a low-power CMOS real-time clock and calendar with an I2C interface. It tracks seconds through year with leap-year compensation, runs from a backup battery when main power is off, and provides programmable alarm, timer, and clock-output functions. It suits battery-backed timekeeping in metering, logging, and embedded systems.

Pinout

Pin Name Type Function
1 OSCI input 32.768 kHz crystal oscillator input; high-impedance node. Connect to one terminal of the 32.768 kHz quartz crystal; internal load capacitors are selectable (7 pF or 12.5 pF via CAP_SEL). Keep wire length between quartz and package minimal.
2 OSCO output 32.768 kHz crystal oscillator output; high-impedance node. Connect to the other terminal of the 32.768 kHz quartz crystal.
3 VBAT power_in Battery supply voltage input. Feeds the on-chip battery switch-over circuit which automatically supplies the RTC from the backup battery when VDD drops.
4 VSS power_in Ground supply voltage (0 V reference).
5 SDA bidirectional I²C-bus serial data input/output. Open-drain; requires external pull-up resistor. Supports Fast-mode Plus up to 1 MHz.
6 SCL input I²C-bus serial clock input. Requires external pull-up resistor. Supports Fast-mode Plus up to 1 MHz.
7 INT1/CLKOUT output Interrupt 1 / clock output (open-drain). Selectable via register: either drives the active-LOW INT1 interrupt (alarm/timer/second) or the programmable CLKOUT (32.768 kHz / 16.384 kHz / 8.192 kHz / 4.096 kHz / 1.024 kHz / 32 Hz / 1 Hz). Requires external pull-up.
8 VDD power_in Main supply voltage input. Operating range 1.0 V to 5.5 V; typical backup current 150 nA at VDD = 3.0 V, Tamb = 25 °C.

Key specifications

Parameter Value
Type I2C real-time clock / calendar
Interface I2C (up to 1 MHz)
Timekeeping Seconds to year, leap-year compensated
Backup Battery switch-over
Features Alarm, timer, programmable clock output
Supply voltage 1.8 V to 5.5 V
Operating temperature -40 C to 85 C

Ordering variants

Same part, different packaging or reel option.

  • PCF8523T
  • PCF8523T/1
  • PCF8523TK
  • PCF8523TK/1
  • PCF8523TS
  • PCF8523TS/1
  • PCF8523U/12AA/1

CAD (KiCad official)

  • Symbol: pcf8523.kicad_sym
  • Footprint: pcf8523.kicad_mod
  • 3D model: SOIC-8 body from KiCad Package_SO/SOIC-8_3.9x4.9mm_P1.27mm.

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

  • Datasheet: NXP datasheet (78 pp, sha256 409cb5ca8b0d04bc, retrieved 2026-08-17, tier: manufacturer)
  • Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
  • CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF