NXP Semiconductors PCF2123U/5GA/1,015

Overview

PCF2123U/5GA/1,015 from NXP Semiconductors is a CMOS real-time clock (RTC) and calendar IC optimized for ultra-low-power battery-backed applications. It features SPI interface (6.25 Mbit/s), 32.768 kHz crystal oscillator with integrated 7 pF load capacitors, programmable alarm/timer with open-drain interrupt output, and ±128 ppm fine-tuning via offset register. It delivers timekeeping down to seconds resolution across years in systems like smart meters and portable medical devices.

For engineers reviewing the PCF2123U/5GA/1,015 datasheet, PCF2123U/5GA/1,015 pinout, PCF2123U/5GA/1,015 application, or PCF2123U/5GA/1,015 equivalent, this page provides verified functional specifications, bare-die terminal mapping, low-power design implications of quartz series resistance and timer clock selection, BCD-encoded time registers with OS flag monitoring, and validated alternatives for RTC subsystem replacement.

Technical Context

The PCF2123U/5GA/1,015 implements a fully autonomous RTC core driven by an on-chip 32.768 kHz oscillator with integrated 7 pF load capacitance - eliminating external capacitors. Its 16-register memory map includes BCD-coded time/date registers (02h–08h), alarm registers (09h–0Ch), offset calibration (0Dh), and timer control (0Eh–0Fh), all accessed via 3-wire SPI with CE, SCL, SDI, and SDO lines.

It supports dual interrupt modes: alarm-triggered (AF flag + AIE enable) and periodic (1-second or 1-minute via MI/SI bits), both routed to the open-drain INT pin. The watchdog function and software reset (0x58 to Control_1) provide system-level reliability, while the OS flag in the Seconds register (bit 7) enables hardware-verified oscillator health monitoring during brown-out conditions.

Key Specifications

Pinout & Package

PCF2123U/5GA/1,015 is supplied as an unsawn wafer die (bare die) with 12 bonding pads. No leadframe or molded package is present; it requires flip-chip or wire-bond assembly onto custom substrates. Thermal and electrical isolation of the die paddle (VSS-connected) and substrate (VSS-connected) is mandatory per NXP mechanical guidelines.

Key Features

Applications

Equivalent & Alternatives

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

Compared with DS3231M and RV-3028-C7, PCF2123U/5GA/1,015 uniquely combines SPI interface, integrated 7 pF load capacitance, and 100 nA backup current - making it optimal for cost-sensitive, space-constrained, SPI-based metering and portable designs where moderate accuracy (±128 ppm before calibration) is acceptable.

Availability

PCF2123U/5GA/1,015 is available at Aetrix Electronics and suitable for smart energy metering, portable medical devices, and industrial IoT sensors requiring stable component supply across extended product lifecycles.

Supply support for PCF2123U/5GA/1,015 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 PCF2123 product line was engineered specifically for ultra-low-power timekeeping in battery-operated utility and medical equipment, emphasizing oscillator stability, BCD register simplicity, and minimal external component count.

FAQ

What is the backup current specification for PCF2123U/5GA/1,015 at 2.0 V and 25 °C?

The PCF2123U/5GA/1,015 exhibits a typical backup current of 100 nA under those conditions, as confirmed in Section 2 of the NXP datasheet Rev. 6. This value assumes the oscillator is running with a quartz having Rs ≤ 15 kΩ and all timers disabled. Actual current may increase with higher quartz series resistance or active timer clocks - refer to Figures 5 and 6 for Rs- and clock-dependent IDD curves. PCF2123U/5GA/1,015 achieves this ultra-low draw via optimized CMOS design and automatic clock gating during SPI idle states.

Does PCF2123U/5GA/1,015 include integrated load capacitors for the 32.768 kHz crystal?

Yes, PCF2123U/5GA/1,015 integrates two 7 pF load capacitors internally - one on OSCI and one on OSCO - eliminating the need for external capacitors. This is explicitly stated in Section 2 ("Features and benefits") and verified in Figure 1 (Block diagram) and Table 4 (Pin description). PCF2123U/5GA/1,015 thus reduces BOM count and layout sensitivity, especially critical in compact wearable and metering designs where crystal placement and stray capacitance impact startup reliability.

How does the OS (Oscillator Stop) flag in PCF2123U/5GA/1,015 work, and how is it cleared?

The OS flag resides in bit 7 of the Seconds register (02h) and is set automatically whenever the 32.768 kHz oscillator stops - including during power-on stabilization (200 ms–2 s), brown-out, or physical crystal disconnection. It remains set until explicitly cleared by firmware writing '0' to that bit. If the flag cannot be cleared, the oscillator is non-functional. PCF2123U/5GA/1,015 uses this flag to provide hardware-verified time integrity, essential for regulatory-compliant data logging in medical and utility applications.

Can PCF2123U/5GA/1,015 generate a 1 Hz interrupt signal, and how is it configured?

Yes, PCF2123U/5GA/1,015 can generate a precise 1 Hz interrupt via the second interrupt (SI) bit in Control_2 register (01h). Setting SI = 1 enables the interrupt, and the INT pin asserts active-LOW each second. This is distinct from the countdown timer and requires no register programming beyond Control_2. PCF2123U/5GA/1,015 guarantees synchronization to the RTC's 1 Hz tick, making it ideal for driving LED blink patterns or MCU wake-up events without software timing loops.

What is the purpose of the offset register (0Dh) in PCF2123U/5GA/1,015, and what is its adjustment range?

The offset register (0Dh) in PCF2123U/5GA/1,015 provides ±128 ppm frequency tuning in 0.4 ppm increments (7-bit signed value) to compensate for crystal aging, temperature drift, or initial tolerance. This allows field calibration without hardware changes. The MODE bit selects coarse/fine adjustment, and values are applied to the oscillator divider chain. PCF2123U/5GA/1,015 uses this register to maintain long-term accuracy in environments where external calibration infrastructure is unavailable - such as sealed smart meters or implantable devices.