The S-35390AH-T8T2U is a low-power, two-wire I²C real-time clock designed for battery-backed timekeeping in compact systems. This guide condenses the RTC datasheet into actionable engineering detail so designers can evaluate power, accuracy, pinout, and I²C timing quickly. The S-35390AH-T8T2U appears in a family of devices focused on microamp-range backup currents and automotive-capable reliability, making it a common choice for wearables, IoT endpoints, and backup clocks in industrial systems.

Point: Engineers selecting a backup-capable RTC need clear data on supply ranges, backup behavior, and register control. Evidence: The manufacturer's RTC datasheet provides recommended VCC/VBAT limits, typical active and backup currents, and full register maps. Explanation: This article extracts those authoritative parameters and pairs them with practical PCB wiring, timing test steps, and a concise troubleshooting checklist so you can validate the part in prototype and production with confidence.

Background: What the S-35390AH-T8T2U is and where it fits

Product overview and target applications

Point: The S-35390AH-T8T2U is a two-wire I²C RTC with battery backup targeted at power-sensitive systems. Evidence: As stated in the RTC datasheet, it implements timekeeping/calendar registers, alarm outputs, and battery switchover to maintain clock operation when primary power is removed. Explanation: Typical applications include fitness wearables and handheld IoT devices where coin-cell backup must preserve time for months, industrial controllers needing a small, robust clock, and automotive backup where temperature and reliability specifications are tighter than consumer parts. Its differentiators versus many competitors are extremely low VBAT-mode current and automotive-grade variants in the family, which reduce maintenance frequency and increase field reliability.

Key features at a glance (bullet summary)

Point: Quick reference of headline specs speeds part selection. Evidence: The RTC datasheet lists these definitive values in the front specification tables. Explanation: Use these short bullets when comparing parts or building a BOM.

• Voltage range: recommended VCC and VBAT ranges noted in the datasheet (typical VCC ~1.8–5.5V depending on variant).
• Low backup current: VBAT-mode current in the single-digit microamp range for long battery life.
• Battery input: dedicated VBAT pin supporting coin cells or supercaps.
• Alarm/interrupt: INT output with maskable alarms and periodic interrupts.
• Temperature range: wide operating range for industrial/automotive use—see the environmental section of the datasheet for exact limits.
• Package: compact TSSOP package; consult the mechanical drawing for land pattern and thermal notes.

S-35390AH-T8T2U — Electrical specifications and performance

Power rails, consumption, and battery backup behavior

Point: Understanding VCC/VBAT ranges and current profiles is essential for battery life estimates. Evidence: The RTC datasheet gives recommended supply ranges and measured currents: typical active-mode currents (tens to hundreds of microamps depending on I²C traffic) and VBAT backup currents (single-digit microamps or lower). Explanation: Design VCC within the recommended range for reliable I²C interface timing; route VBAT from a coin cell or supercap sized to hold the backup-mode current for the desired retention period. Battery switchover is automatic—on VCC loss the internal switch enables VBAT to sustain the RTC core. For best results, use a low-impedance VBAT source and avoid placing large decoupling directly on VBAT that would draw extra current during switchover testing.

Timekeeping accuracy and clock/calendar functions

Point: Oscillator source and accuracy determine drift and calendar reliability. Evidence: The device uses an internal crystal oscillator (or external 32.768 kHz crystal per datasheet guidance) and provides typical ppm figures and temperature drift curves in the specifications. Explanation: Use a quality 32.768 kHz crystal with recommended load capacitance to meet the stated ppm. Expect temperature-induced drift; consult the datasheet thermal tables to quantify drift across the operating range. The clock/calendar supports leap-year handling and standard calendar rollovers; alarms can be configured for seconds, minutes, hours, day, date, month, and year matches with maskable fields for periodic wake-ups.

Environmental, reliability, and compliance data

Point: Know the operating environmental windows and solder/packaging limits for assembly and field use. Evidence: The RTC datasheet lists operating temperature, humidity tolerance, solder profile, and any automotive qualifications for specific variants. Explanation: For automotive or industrial usage, verify the specific H-series or automotive-rated part variant and consult thermal derating curves to ensure timing stability across temperature. Follow the recommended reflow profile for the TSSOP package, and adhere to floor-life and moisture sensitivity levels noted by the manufacturer to avoid solder-related failures.

Pinout & package: exact pin functions and mechanical details

Pin function breakdown (SDA, SCL, VCC, VBAT, GND, INT/OSC, etc.)

Point: Correct pin mapping and signal conventions avoid early integration problems. Evidence: The pin table in the RTC datasheet maps each TSSOP pin to signals such as SDA, SCL, VCC, VBAT, GND, INT, and any oscillator pins. Explanation: Implement SDA and SCL as open-drain lines with external pull-up resistors to the logic VCC. VBAT must be connected to a dedicated backup source; do not tie VBAT to VCC directly. INT is typically open-drain (or push-pull per variant) and can be used to wake a host MCU—observe polarity settings in control registers. When referencing the pinout in your schematic, use the datasheet pin table as the authoritative mapping and confirm pin numbers against the mechanical drawing before PCB sign-off.

TSSOP package dimensions and footprint notes

Point: Mechanical accuracy prevents assembly and thermal issues. Evidence: The mechanical drawing in the datasheet specifies package dimensions, lead pitch, and recommended land pattern. Explanation: Implement the recommended pad dimensions and solder mask expansions exactly; small deviations in TSSOP land pattern can cause tombstoning or insufficient solder fillet. Pay attention to exposed pad recommendations (if present) and note distance from adjacent components for pick-and-place tooling clearances. For thermal relief, follow the manufacturer’s guidance to ensure consistent solder joints in mass production.

Recommended PCB wiring and decoupling

Point: Proper decoupling and routing preserve timekeeping and minimize noise. Evidence: Electrical notes in the datasheet advise decoupling VCC close to the device and separate VBAT routing. Explanation: Place a 0.1 µF ceramic decoupling capacitor from VCC to ground within 1–2 mm of the VCC pin, plus a 1 µF bulk cap as needed. Route VBAT with short traces and avoid routing high-frequency signals nearby; do not place large decoupling or load-switch circuits on VBAT unless required. For SDA/SCL, use 4.7k–10k pull-ups for 3.3V systems as a starting point; lower values may be needed at longer bus lengths or higher speeds. Consider small series resistors and common-mode ESD protection on the bus if the device is exposed to external connectors.

Registers, I²C protocol and timing (practical data deep-dive)

I²C addressing, read/write sequences and register map overview

Point: Correct addressing and pointer management are essential for robust communications. Evidence: The datasheet specifies the 7-bit device address and the register pointer behavior, including auto-increment for burst reads/writes. Explanation: Use the documented device address and perform a write of the register pointer before read sequences as required. The register map exposes timekeeping registers (seconds, minutes, hours), calendar registers (day, date, month, year), alarm registers, control/status, and calibration registers. Implement read-modify-write patterns carefully for control bits and clear interrupt flags explicitly after servicing alarms to avoid repeated triggers.

Timing diagrams and electrical timing constraints

Point: Adhering to I²C timing prevents bus errors and NACKs. Evidence: Timing parameters such as setup/hold times, SCL high/low minimums, and bus modes are given in the datasheet. Explanation: Support standard (100 kHz) and fast (400 kHz) modes per datasheet limits; check the specified tSU, tHD, and tF/tR values when designing pull-ups and bus capacitance. Capture write, read, and burst timing diagrams in your validation plan and use a logic analyzer to verify correct ACK/NACK behavior during address and data phases. If you see intermittent NACKs, measure bus rise times and reduce pull-up resistance or add series termination as necessary.

Power sequencing and battery switching timing

Point: Understanding VBAT takeover timing ensures uninterrupted timekeeping during power glitches. Evidence: The datasheet explains behavior during VCC dropouts and the time domain for switchover to VBAT. Explanation: Test power loss scenarios by intentionally cycling VCC while monitoring VBAT and RTC registers to confirm continuous seconds increments and that the oscillator remains running. Verify that control register writes complete before applying VCC off sequences where possible. Include a test to measure VBAT-mode current during takeover to confirm that actual backup current matches datasheet typical values.

Integration & implementation guide (wiring, code patterns, examples)

Basic wiring examples (single-supply and battery-backed)

Point: Clear wiring reduces integration errors. Evidence: Reference wiring conventions in the RTC datasheet and standard I²C practice. Explanation: For single-supply operation, route VCC, GND, SDA, and SCL with pull-ups to VCC. For battery-backed designs, connect VBAT to a coin cell holder or supercap with polarity protection; ensure VBAT never exceeds the absolute maximum in the datasheet. Use a diode or ideal-diode arrangement if simultaneous charging from VCC is required, but follow the datasheet recommendations for allowed VBAT charging schemes. Numbered wiring checklist: 1) VCC to device VCC pin with 0.1 µF cap; 2) GND to common ground; 3) VBAT to backup source; 4) SDA/SCL with recommended pull-ups; 5) INT to MCU interrupt pin with optional level-shifting.

Example register sequences / pseudocode for common tasks

Point: Pseudocode accelerates firmware bring-up. Evidence: Register addresses and bit definitions are in the RTC datasheet register map. Explanation: High-level examples: 1) Set time: write register pointer to seconds, then burst-write BCD bytes for seconds→year. 2) Read time: write pointer to seconds, then burst-read N bytes, convert BCD to binary. 3) Enable alarm: set alarm registers, set mask bits, set the alarm-enable bit in control register, clear flag after trigger. Always check status bits and clear interrupt flags explicitly. These sequences avoid race conditions and repeated interrupts.

Interrupts, alarms and power-mode usage patterns

Point: Use INT to minimize MCU wake time and maximize battery life. Evidence: The datasheet documents INT behavior, polarity control, and alarm masking. Explanation: Configure alarms for the coarsest resolution needed (e.g., minute vs. second) to minimize wake frequency. Use the RTC’s interrupt output to wake the host; after wake, read and clear the flag before entering sleep again. When maximizing battery life, reduce I²C poll frequency and rely on INT-driven events for infrequent wakeups.

Troubleshooting, testing and best practices (actionable checklist)

Common integration issues and fixes

Point: Early test failures usually come from wiring, pull-ups, or VBAT miswiring. Evidence: Typical field issues mirror the scenarios highlighted in the datasheet application notes. Explanation: Checklist fixes: 1) I²C NACKs — verify pull-ups, address, and bus voltage; 2) Oscillator not running — confirm crystal type, load caps, and oscillator pins per datasheet; 3) VBAT not supplying — confirm polarity, absence of heavy loads on VBAT, and proper switchover tests. Follow step-by-step isolation: measure pins, confirm register reads, and observe INT behavior under controlled test vectors.

Measurement and validation checklist

Point: A concise test plan ensures reliable qualification. Evidence: The datasheet supplies values to compare against during validation. Explanation: Key test steps: 1) Verify VCC and VBAT voltages and decoupling. 2) Measure standby current with VCC removed (VBAT only) to confirm microamp-level draw. 3) Confirm continuous timekeeping across VCC dropouts by logging seconds increments over extended periods. 4) Validate alarm triggers and INT timing with a logic analyzer. Record deviations from datasheet typicals and iterate on layout or component changes accordingly.

Design tips for reliability and manufacturability

Point: Small layout and BOM choices reduce yield loss. Evidence: Soldering notes and footprint guidance in the datasheet guide assembly practice. Explanation: Use manufacturer-recommended land patterns, include ESD protection on I²C lines in exposed designs, and select crystal vendors with consistent quality. In automated assembly, ensure paste volume and stencil apertures match the TSSOP pads to prevent bridging. Consider secondary sourcing for crystals and capacitors to avoid supply chain disruptions.

Summary

Point: The S-35390AH-T8T2U delivers low backup current, flexible I²C control, and a compact TSSOP pinout well suited to battery-backed timekeeping in wearables, IoT, and industrial designs. Evidence: Key specifications drawn from the RTC datasheet highlight VBAT-mode microamp currents, a full clock/calendar with maskable alarms, and a defined pinout for SDA/SCL, VCC, VBAT, GND, and INT. Explanation: Follow the pinout and power-wiring rules—proper decoupling, VBAT routing, and I²C pull-ups—then validate with the provided measurement checklist before committing to production. Download and consult the official RTC datasheet and run the validation checklist during prototype and qualification stages to ensure the selected part meets your application requirements. S-35390AH-T8T2U

Key summary

• Low backup current: VBAT-mode microamp-level current enables months of battery-backed timekeeping—verify against the datasheet typical for battery sizing.
• Pinout and wiring rules: Use SDA/SCL open-drain with pull-ups, separate VBAT routing, and close VCC decoupling to avoid common integration errors.
• I²C timing and registers: Follow the datasheet’s register map and timing constraints for reliable reads/writes and alarm configuration.
• Validation checklist: Measure VBAT standby current, confirm continuous seconds over VCC dropouts, and validate alarms with a logic analyzer before production.

Common questions & answers

What are the recommended VBAT and VCC ranges for the S-35390AH-T8T2U?

Answer: Consult the RTC datasheet’s electrical characteristics for exact limits; typical VCC ranges support common logic voltages and VBAT accepts small coin cells or supercaps within specified maximums. Ensure VBAT polarity is correct and avoid placing large capacitors directly on VBAT. For battery-backed designs, validate that VBAT-mode current aligns with your retention requirements and size the coin cell accordingly.

How should I wire the SDA and SCL lines to avoid I²C errors with this RTC?

Answer: Use open-drain connections for SDA and SCL with pull-up resistors to VCC (start with 4.7 kΩ for 3.3V systems). Keep pull-ups close to the master device, control bus capacitance by minimizing trace length and stubs, and add series resistors (22–100 Ω) if ringing or overshoot occurs. Validate bus timing against the datasheet and use a logic analyzer to check ACK/NACK behavior during address and data phases.

How can I verify the RTC keeps time across power loss?

Answer: Perform a controlled power-cycling test: set a known time, remove VCC while VBAT stays connected, and log second increments via VBAT operation for an extended period. Measure VBAT current during this test and compare to the datasheet’s VBAT-mode typical current. Also test alarm firing and INT signaling during and after switchover to ensure uninterrupted timekeeping and correct interrupt behavior.