NXP Semiconductors PCF8523T/1,118

Overview

PCF8523T/1,118 from NXP Semiconductors is a low-power CMOS real-time clock (RTC) and calendar IC with I²C interface (1 MHz Fast-mode Plus), integrated 32.768 kHz oscillator, battery backup switch-over circuit, programmable alarm/timer, and ±15 ppm typical accuracy after offset calibration. It operates from 1.0 V to 5.5 V and draws only 150 nA typical backup current at 3.0 V - enabling decade-long timekeeping in battery-powered meters and portable devices.

For engineers reviewing the PCF8523T/1,118 datasheet, PCF8523T/1,118 pinout, PCF8523T/1,118 application, or PCF8523T/1,118 equivalent, this page delivers verified specifications, SO8 package terminal mapping, functional alternatives with documented differences, and design-critical context on oscillator load selection, interrupt routing, and battery failover behavior.

Technical Context

The PCF8523T/1,118 implements a 20-register BCD-based RTC core with auto-incrementing address pointer, oscillator stop detection, and software reset capability (0x58 command). Its internal 32.768 kHz oscillator supports selectable load capacitance (7 pF or 12.5 pF) via CAP_SEL bit, enabling stable timing across quartz variants without external capacitors.

Power management includes dual-mode battery switch-over (standard/direct), battery low detection (V_th(bat)low ≈ 2.5 V), and standby mode entry via PM[2:0] = 111. Interrupts are routed through open-drain INT1/CLKOUT (shared function) and dedicated INT2, with flag-clearing logic per source (e.g., reading Control_2 clears WTAF).

Key Specifications

Pinout & Package

PCF8523T/1,118 is supplied in an 8-lead plastic small outline package (SO8, SOT96-1), 3.9 mm body width, with standard gull-wing lead form. Pin 1 is marked by a bevelled corner or dot.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar real-time clock applications.

Compared with DS3231SN#T&R and MCP79411-I/SN, the PCF8523T/1,118 uniquely balances ultra-low backup current, integrated oscillator tuning, and dual-mode battery failover - making it optimal for long-life, self-monitoring metering and portable medical devices.

Availability

PCF8523T/1,118 is available at Aetrix Electronics and suitable for smart energy metering, portable medical devices, industrial IoT sensors, and point-of-sale terminals requiring stable component supply across multi-year production cycles.

Supply support for PCF8523T/1,118 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

NXP Semiconductors is a global semiconductor leader focused on secure connectivity solutions for automotive, industrial, and IoT applications, with deep expertise in low-power mixed-signal design.

The PCF8523 belongs to NXP's Real-Time Clock product line, engineered specifically for ultra-low-power, high-integrity timekeeping in battery-backed infrastructure and portable equipment where reliability and longevity are non-negotiable.

FAQ

What is the I²C address of the PCF8523T/1,118?

The PCF8523T/1,118 uses a fixed I²C slave address of 0xD0 for write operations and 0xD1 for read operations. This address is hardwired and not configurable via pins or registers. The device supports Fast-mode Plus (1 MHz) timing, enabling faster time register updates than standard-mode I²C. All communication with the PCF8523T/1,118 must adhere to these addresses and timing constraints to ensure reliable register access.

How does the PCF8523T/1,118 handle power failure and battery switchover?

The PCF8523T/1,118 features an autonomous battery switch-over circuit that monitors VDD and switches to VBAT within microseconds when VDD drops below VBAT and the threshold voltage (≈2.5 V) in standard mode - or immediately upon VDD < VBAT in direct switching mode. During switchover, the BSF flag is set, and an interrupt can be generated on INT1 if BSIE is enabled. The PCF8523T/1,118 maintains full RTC functionality throughout the transition without data loss.

Can the PCF8523T/1,118 operate without an external crystal?

No - the PCF8523T/1,118 requires an external 32.768 kHz quartz crystal connected between OSCI (Pin 1) and OSCO (Pin 2). It does not include a built-in RC oscillator. However, it integrates selectable load capacitance (7 pF or 12.5 pF) to match common crystal types, eliminating the need for external capacitors. Crystal trace length must be minimized to preserve start-up reliability and frequency stability.

What is the purpose of the OS (oscillator stop) flag in the PCF8523T/1,118?

The OS flag (bit 7 of Seconds register, address 0x03) indicates whether the 32.768 kHz oscillator is running. It is set automatically at power-on and remains set until cleared via software - but only if the oscillator is confirmed active. If the crystal is disconnected, shorted, or fails to start, the OS flag stays asserted, providing hardware-verified oscillator health status. This enables firmware to detect and log timing failures before relying on invalid timestamps.

Does the PCF8523T/1,118 support both 12-hour and 24-hour time formats?

Yes - the PCF8523T/1,118 supports both formats via the 12_24 bit (bit 3) in Control_1 register (0x00). When cleared (0), the device operates in 24-hour mode; when set (1), it uses 12-hour mode with AM/PM indication in the Hours register. Time registers store values in Binary-Coded Decimal (BCD), simplifying human-readable display formatting and avoiding binary-to-decimal conversion errors in firmware.