Ultra Low Power RTC
Technical Specifications
Informational podcast coming soon
Designed to perform critical time-keeping functions in a range of applications, these small, highly efficient RTC/calendars fit nearly anywhere and cut power demands by as much as 60 percent. Even at low power the oscillator is robust and tolerates rough environment and spreads in quartz crystal parameters.
Technical papers
PCF2123
SPI Real time clock/calendar
The PCF2123 is a CMOS1 Real-Time Clock (RTC) and calendar optimized for low power applications. Data is transferred serially via a Serial Peripheral Interface (SPI-bus) with a maximum data rate of 6.25 Mbit/s. An alarm and timer function is also available providing the possibility to generate a wake-up signal on an interrupt pin. An offset register allows fine tuning of the clock.
PCF8523
Real-Time Clock
(RTC) and calendar
The PCF8523 is a CMOS1 Real-Time Clock (RTC) and calendar optimized for low power consumption. Data is transferred serially via an I2C-bus with a maximum data rate of 1000 kbit/s. Alarm and timer functions are available with the possibility to generate a wake-up signal on an interrupt pin. An offset register allows fine-tuning of the clock. The PCF8523 has a backup battery switch-over circuit, which detects power failures and automatically switches to the battery supply when a power failure occurs.
Technical Papers

The NXP PCF8523 and PCF2123 are CMOS devices that combine a real-time clock (RTC) and a calendar. Optimized for low power consumption, they operate at a current of less than 100 nA with a 2.0 V power supply, and consume less than 0.2 μW of power. The PCF8523 uses an I2C-bus serial interface while the PCF2123 is equipped with an SPI serial interface.

Both devices are cased in tiny leadless packages that save space in compact designs. This makes them well suited for handheld and battery-operated applications, including bloodpressure monitors and other home-use medical devices, pocket calculators, portable phones, and PDAs. They are also an excellent choice for white goods, such as washing machines, metering units, and other industrial systems. Both devices can be powered by a very small battery cell or a super-cap.

The calendar functions track year, month, weekday, and day with built-in Leap Year correction. The programmable alarm and timer function gives the option of generating a wake-up signal on an interrupt pin.

In the PCF2123, the Watchdog timer can be used to wake a microcontroller from hibernation mode or monitor microcontroller tasks independently.

RTC/calendar PCF2123 Key Features
  • Real time clock provides year, month, day, weekday, hours, minutes, and seconds based on a 32.768 kHz quartz crystal
  • Low backup current while running: typical 100 nA at VDD = 2.0 V and Tamb = 25 °C
  • Resolution: seconds to years
  • Watchdog functionality
  • Freely programmable timer and alarm with interrupt capability
  • Clock operating voltage: 1.1 V to 5.5 V
  • 3 line SPI-bus with separate, but combinable data input and output
  • Serial interface at VDD = 1.6 V to 5.5 V
  • 1 second or 1 minute interrupt output
  • Integrated oscillator load capacitors for CL = 7 pF
  • Internal Power-On Reset (POR)
  • Open-drain interrupt and clock output pins
  • Programmable offset register for frequency adjustment
Technical papers
PCF8523
RTC/calendar PCF8523 Key Features
  • Provides year, month, day, weekday, hours, minutes, and seconds based on a 32.768 kHz quartz crystal
  • Resolution: seconds to years
  • Clock operating voltage: 1.0 V to 5.5 V
  • Low backup current: typical 150 nA at VDD = 3.0 V and Tamb = 25 °C
  • 2 line bidirectional 1 MHz Fast-mode Plus (Fm+) I2C interface
  • Battery backup input pin and switch-over circuit
  • Freely programmable timer and alarm with interrupt capability
  • Selectable integrated oscillator load capacitors for CL = 7 pF or CL = 12.5 pF
  • Internal Power-On Reset (POR)
  • Open-drain interrupt or clock output pins
  • Programmable offset register for frequency adjustment
PCF2123