DS12885S

The DS12885S is a parallel-interface real-time clock/calendar manufactured by Maxim Integrated (now part of Analog Devices), designed to be a pin-compatible and functionally equivalent replacement for the original Dallas Semiconductor DS1285 — a de facto standard RTC in IBM PC/AT architecture and countless embedded systems. The DS12885 provides a complete hardware calendar that tracks seconds, minutes, hours, day-of-week, day-of-month, month, and year, with automatic end-of-month adjustment including leap year correction that remains valid through the year 2099. Time may be represented in either binary or BCD format, and the clock can be configured for 12-hour operation with AM/PM indicator or 24-hour military format. A daylight saving time option is also provided. Beyond basic timekeeping, the DS12885 includes 114 bytes of non-volatile static RAM (NVSRAM) that is maintained by the backup battery when main power is removed — useful for storing system configuration parameters that must survive power loss. The device supports three independently maskable interrupt sources sharing a single interrupt output pin: a time-of-day alarm (programmable from once per second to once per day), a periodic interrupt (with 13 selectable rates from 122 microseconds to 500 milliseconds), and an update-end interrupt that signals when the clock has completed its once-per-second update cycle. A programmable square-wave output provides 15 selectable frequencies ranging from 1.8 mHz to 250 kHz. The multiplexed byte-wide parallel interface supports both Intel and Motorola bus timing modes (selected by the MOT pin), making it compatible with a wide range of microprocessor families. Automatic power-fail detection circuitry monitors VCC and switches the device to backup battery operation when the primary supply drops below the power-fail threshold, ensuring uninterrupted timekeeping and RAM data retention. Unlike the DS12887 module version which integrates the crystal and battery in the package, the DS12885S uses an external 32.768 kHz crystal and external lithium coin cell backup battery, giving the system designer flexibility in crystal accuracy and battery capacity selection. Packaged in a 24-pin SOIC with RoHS3 compliance and UL recognition, the DS12885S remains a reliable choice for cost-sensitive, space-constrained embedded RTC applications.

The DS12885 is a CMOS real-time clock that uses a 32.768 kHz crystal oscillator as its timebase. The crystal oscillator circuit divides the 32.768 kHz input down to a precise 1 Hz pulse (one per second), which drives the seconds counter. The seconds counter increments from 0 to 59, then rolls over and increments the minutes counter, and this cascading continues through minutes, hours, days, day-of-week, date, month, and year counters. The calendar logic automatically handles months with different numbers of days (28, 29, 30, or 31 days) and applies leap year correction — years divisible by 4 are leap years, with the year 2000 correctly identified as a leap year and 2100 as not (though the device only operates through 2099). The counters can be read or written through the parallel interface by addressing the appropriate register. The device uses a multiplexed address/data bus interface: the address is latched on the falling edge of the AS (Address Strobe) signal, and then data is transferred during the read or write cycle. The MOT pin selects between Intel bus timing (MOT=0, using /RD and /WR signals) and Motorola bus timing (MOT=1, using R/W and DS signals), providing compatibility with both major microprocessor bus standards. Three types of interrupts are supported: the alarm interrupt triggers when the current time matches the programmed alarm time (each alarm byte can be set to ‘don’t care’ for periodic alarms ranging from every second to once per day); the periodic interrupt triggers at a selectable rate determined by the RS3-RS0 bits in Register A; and the update-ended interrupt triggers once per second after the clock counters have been updated, allowing software to read the time without risk of catching the counters mid-update. The square-wave output (SQW pin) generates a continuous square wave at the same rate selected for the periodic interrupt. Power-fail detection is handled by a precision comparator that monitors VCC. When VCC drops below the power-fail threshold (typically 4.25V), the device switches to battery operation: the external interface is disabled, the clock and RAM are maintained by the backup battery (connected to VBAT), and power consumption drops to minimal levels. When VCC returns above the threshold, the device switches back to main power and normal operation resumes.

The DS12885S is available in a 24-pin SOIC package. Key pin assignments:

Note: Some pin functions are mode-dependent (Intel vs Motorola). Refer to the official DS12885 datasheet for complete specifications and recommended external components.

• Embedded Systems & Single-Board Computers: Used as the system RTC in embedded motherboards, single-board computers (SBCs), and industrial PCs to maintain time-of-day and system configuration data through power cycles
• Networking Equipment: Found in routers, switches, hubs, bridges, and network appliances where accurate timestamping of events, logs, and scheduled operations requires a battery-backed real-time clock
• Security & Access Control: Used in alarm systems, access control panels, CCTV DVRs, and building automation for time-stamped event logging, scheduled arming/disarming, and time-based access rules
• Utility Meters: Deployed in electric, gas, and water smart meters for time-of-use metering, scheduled data logging, and time-stamped tamper detection records
• POS Terminals & Cash Registers: Used in point-of-sale systems, cash registers, and vending machines for transaction time-stamping, daily reporting, and time-based pricing functions
• Industrial Automation & PLCs: Found in programmable logic controllers, industrial controllers, and data loggers for time-stamping I/O events, scheduled control operations, and production data recording
• Medical Equipment: Used in patient monitors, diagnostic devices, and medical instrumentation for time-stamping patient data, scheduling drug delivery, and maintaining device configuration
• Telecommunications Equipment: Deployed in telecom switches, PBX systems, and VoIP gateways for call record time-stamping, scheduled maintenance, and system time reference