DS1302S+

The DS1302S+ from Analog Devices (formerly Maxim Integrated) is a trickle-charge timekeeping chip that contains a real-time clock/calendar and 31 bytes of static RAM. The device communicates with a microprocessor via a simple 3-wire serial interface (CE, I/O, SCLK), requiring only three MCU pins for full RTC functionality. The real-time clock/calendar provides seconds, minutes, hours, day, date, month, and year information, with automatic end-of-month date adjustment including leap year correction valid through 2100. The clock operates in either 24-hour or 12-hour format with an AM/PM indicator. The 31 bytes of general-purpose RAM are battery-backed, providing non-volatile storage for critical system parameters. The DS1302S+ features dual power supply pins (VCC1 for backup, VCC2 for main power), with automatic switchover when the main supply fails. The programmable trickle charger allows the main supply to charge the backup battery or supercapacitor through internal diodes and resistors, with configurable charge current (1 or 2 diodes, 2kOhm/4kOhm/8kOhm resistor). The ultra-low 300nA timekeeping current at 2.0V backup voltage means a CR2032 coin cell can maintain timekeeping for over 5 years. The write-protect bit prevents accidental modification of clock and RAM data. Burst mode allows reading or writing all 8 clock registers or all 31 RAM bytes in a single continuous transfer, minimizing MCU overhead. The DS1302S+ is the successor to the DS1202, adding dual power pins, trickle charger, and 7 additional RAM bytes.

The DS1302S+ operates as a serial-interfaced real-time clock with trickle-charge capability. Key subsystems include: (1) 32.768kHz Oscillator – an external 32.768kHz crystal connected between X1 and X2 pins provides the timebase; the internal oscillator circuit drives the crystal and divides the frequency down to 1Hz for the seconds counter; the crystal load capacitance should be 6pF for optimal accuracy; (2) Clock/Calendar Counters – a chain of BCD counters increments seconds, minutes, hours, day, date, month, and year; the month-end correction logic automatically handles months with 28, 29, 30, or 31 days, including leap years; the 12/24-hour format bit controls the hours counter mode; (3) 3-Wire Serial Interface – CE (chip enable) must be high for any data transfer; SCLK provides the clock for synchronous data transfer; I/O is a bidirectional data pin; each transfer begins with a command byte (8 bits: 7 address bits + 1 R/W bit) followed by 8 data bits; data is clocked on the rising edge of SCLK; (4) Dual Power Supply – VCC2 is the main supply (2.0-5.5V) and VCC1 is the backup supply (2.0-3.5V); an internal switch selects the higher of the two voltages; when VCC2 drops below VCC1, the device automatically switches to VCC1 for continuous operation; (5) Trickle Charger – controlled by a special register (not accessible in burst mode); TCS bits (1010) enable the charger; DS bits select 1 or 2 series diodes; RS bits select 2k/4k/8k series resistor; the maximum charge current is (VCC2 – diode drop – VCC1) / R; for example, 5V VCC2 with 1 diode and 2k resistor gives approximately (5.0-0.7)/2000 = 2.15mA maximum charge current into a supercapacitor.

• Embedded timestamp: 5V MCU system with CR2032 backup for data logging timestamps over 5+ years
• Industrial scheduling: weekly timer with 31B RAM for relay schedule parameters, supercap backup via trickle charger
• Consumer appliance: microwave/oven clock with 12/24-hour display, battery-backed through power outages
• Portable instrument: 3.3V system with rechargeable Li-Ion backup using trickle charger for continuous timekeeping