NXP Semiconductors PCF2123BS/1,518

Overview

PCF2123BS/1,518 from NXP Semiconductors is a low-power SPI Real-Time Clock (RTC) and calendar IC with 32.768 kHz crystal oscillator, 6.25 Mbit/s SPI interface, open-drain INT/CLKOUT pins, programmable offset register for ±127 ppm fine-tuning, and integrated 7 pF load capacitors. It delivers year/month/day/hour/minute/second/weekday timekeeping in BCD format and supports wake-up via alarm or timer interrupt in battery-powered metering and embedded timing applications.

For engineers reviewing the PCF2123BS/1,518 datasheet, PCF2123BS/1,518 pinout, PCF2123BS/1,518 application, or PCF2123BS/1,518 equivalent, this page provides verified package mapping (HVQFN16), confirmed 16-register SPI address space, validated low-power operation (100 nA typical at 2.0 V), exact alarm/timer interrupt behavior, and real-world design implications of BCD time registers and OS flag monitoring.

Technical Context

The PCF2123BS/1,518 implements a fully autonomous RTC using an internal 32.768 kHz oscillator with integrated 7 pF load capacitors - eliminating external capacitors - and a 16-stage divider chain feeding BCD-coded time registers (02h–08h). Its SPI interface operates at up to 6.25 Mbit/s with separate SDI/SDO lines and active-HIGH CE control, supporting auto-incrementing register reads/writes to prevent carry corruption during time updates.

Alarm functionality uses four independent BCD-coded registers (09h–0Ch) with per-field enable bits (AE_M/AE_H/AE_D/AE_W); matching triggers AF flag in Control_2 (01h), which - when AIE=1 - drives the open-drain INT pin. The countdown timer (0Eh/0Fh) offers four selectable clock sources (4.096 kHz to 1/60 Hz), enabling precise intervals from 244 µs to 4.25 hours, while pre-defined second/minute interrupts (SI/MI bits) provide fixed periodic wake-up without timer register configuration.

Key Specifications

Pinout & Package

HVQFN16 package (SOT758-1), 3 × 3 × 0.85 mm body, exposed thermal pad connected to VSS. Pin 1 index located at top-left corner; pins numbered counter-clockwise.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar RTC applications.

Compared with DS3231SN#T&R and MCP79412-I/SN, the PCF2123BS/1,518 uniquely combines SPI interface scalability, integrated 7 pF load capacitors, and ±127 ppm offset tuning - making it optimal for compact, high-volume, battery-backed timing systems where layout simplicity and field recalibration matter.

Availability

PCF2123BS/1,518 is available at Aetrix Electronics and suitable for smart metering, battery-powered IoT sensors, industrial HMIs, medical wearables, and low-standby-power timing applications requiring stable component supply across long production lifecycles.

Supply support for PCF2123BS/1,518 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 markets, with deep expertise in low-power mixed-signal design and timing IP.

The PCF2123BS/1,518 belongs to NXP's ultra-low-power RTC product line, engineered specifically for energy-constrained applications demanding decade-long backup operation, robust oscillator monitoring, and minimal external component count.

FAQ

What is the backup current specification for PCF2123BS/1,518 at 2.0 V?

The PCF2123BS/1,518 draws 100 nA typical backup current at VDD = 2.0 V and Tamb = 25 °C. This ultra-low quiescent current enables over 10 years of operation on a standard CR2032 coin cell in continuous backup mode, assuming 220 mAh capacity and 50% depth-of-discharge margin. The actual runtime depends on crystal series resistance and ambient temperature, as shown in Figure 5 of the datasheet.

Does PCF2123BS/1,518 require external load capacitors for the 32.768 kHz crystal?

No, the PCF2123BS/1,518 integrates two 7 pF load capacitors internally, eliminating the need for external capacitors. This reduces BOM cost and PCB area while improving manufacturing yield. The crystal must be rated for CL = 7 pF and mounted with minimal trace length between OSCI and OSCO pins to ensure reliable start-up and low jitter performance.

How does the OS flag in the Seconds register function for PCF2123BS/1,518?

The OS (Oscillator Stop) flag is bit 7 of the Seconds register (02h). It is set automatically when the oscillator stops due to low VDD, crystal disconnection, or excessive series resistance. The flag remains set until cleared by firmware - providing a hardware-monitored method to detect timing failures. If the OS flag cannot be cleared, the oscillator is not running, indicating a fault condition that requires corrective action before relying on time data.

Can PCF2123BS/1,518 generate a 1-second interrupt without configuring the countdown timer?

Yes, the PCF2123BS/1,518 supports a dedicated 1-second interrupt via the SI (Second Interrupt) bit in Control_2 register (01h). When SI = 1 and AIE = 0, the INT pin pulses once per second regardless of alarm or timer register settings. This pre-defined function requires no timer register writes and avoids the complexity of configuring CTD[1:0] and TE bits - simplifying firmware for periodic wake-up tasks.

What is the maximum SPI clock frequency supported by PCF2123BS/1,518?

The PCF2123BS/1,518 supports SPI communication at up to 6.25 Mbit/s, corresponding to a maximum SCL frequency of 3.125 MHz (since data is sampled on each SCL edge). This high-speed interface enables rapid time register reads/writes and efficient alarm/timer reconfiguration - critical for systems requiring frequent time updates or low-latency interrupt response.