Analog Devices Inc./Maxim Integrated DS1305E+T&R

Product Summary DS1305E/T&R Real-Time Clock by Analog Devices Inc./Maxim Integrated

The DS1305E/T&R is a CMOS Real-Time Clock (RTC) with clock/calendar, programmable alarms, 96 bytes of battery-backed nonvolatile (NV) RAM, and a flexible serial interface. Manufactured by Analog Devices Inc./Maxim Integrated, this device is designed for surface-mount applications and is housed in a 20-pin TSSOP package (0.173", 4.40mm width), offering a robust solution for precision timekeeping requirements in embedded systems. While this model is classified as obsolete, its features and architecture continue to influence design choices in systems where cost-efficient, reliable RTC integration is crucial.

Main Characteristics and System Advantages of DS1305E/T&R

The DS1305E/T&R manages all timekeeping functions, supporting full binary-coded decimal (BCD) real-time clock/calendar capability. The device counts seconds, minutes, hours (12/24-hour selection), date, month, day of week, and year, with automatic leap-year compensation up to the year 2100. Its dual programmable alarms, each configurable by time or day, provide flexible alert and scheduling options for system events. Additional built-in 96-byte NVSRAM allows system configurations or critical logging data to be retained across power cycles.

The device operates within a supply voltage range of 2V to 5.5V, enabling use in a variety of low- and standard-voltage systems. Dual power supply support, an optional trickle charger, and dedicated battery input extend reliability for timekeeping in the event of main power interruptions—a requirement for applications in data logging, communication equipment, and metering.

DS1305E/T&R Functional Architecture and Pin Layout

The DS1305E/T&R features 20 pins optimized for RTC functions, SPI/3-wire serial interfacing, and flexible backup options. Key pins include dual power supply lines (Vcc1 for primary, Vcc2 for backup/trickle-charging), a dedicated battery pin (VBAT), logic-level power for the interface (VccIF), two oscillator connections for a standard 32.768 kHz crystal (X1, X2), data input/output lines (SDI, SDO, SCLK), chip enable (CE), and interrupt outputs (INT0, INT1). The SERMODE pin determines the use of the SPI or standard 3-wire interface for microcontroller communication. Several no-connect (NC) pins simplify PCB routing in complex systems.

Clock, Calendar Functions, and User Memory in DS1305E/T&R

The core function of the DS1305E/T&R centers on accurate time and calendar management. RTC registers store time in BCD format, and timekeeping is maintained as long as any power supply input remains valid and the oscillator is operational. The device handles month-length variations and leap years automatically. Users must carefully initialize and configure clock registers on first power-up, ensuring the oscillator is enabled and write-protection disabled.

All calendar and time registers can be accessed byte-wise or in burst mode for fast sequential reads/writes, facilitating quick backups or multiple-register transfers during host processor activity. The 96-byte NV RAM provides a convenient, battery-backed storage resource for critical system parameters or log data, safeguarded by the same backup architecture as the clock.

DS1305E/T&R Power Supply Design and Battery Backup Solutions

A distinguishing feature of the DS1305E/T&R is its dual power supply input design (Vcc1 and Vcc2) and dedicated battery backup input (VBAT). This architecture ensures continuous operation even if the main supply fails, crucial in security, industrial, or portable data systems. The device supports connection to rechargeable backup elements (super-capacitors, NiMH/Li battery) via Vcc2, enhanced by an on-chip programmable trickle charger.

The optional trickle charger parameters are programmable, enabling fine-tuned charging profiles for various backup sources. If used with a non-rechargeable cell, the VBAT pin is used directly and must be routed appropriately to avoid reverse-charging risks. The VccIF pin allows proper voltage matching for logic-level compatibility with 3V or 5V microcontrollers—a frequent requirement in mixed-supply embedded systems.

DS1305E/T&R Alarm Functions, Interrupt Features, and Interface Options

Two fully programmable time-of-day alarms can generate interrupts on user-defined criteria: seconds, minutes, hours, or days. “Mask” bits in the alarm registers allow flexible comparison schemes, enabling one or both alarms to trigger on events ranging from once-per-second to a weekly schedule. Each alarm can assert its own open-drain, active-low interrupt output (INT0, INT1), or share a common interrupt through configuration. These outputs function reliably under any valid power state, ensuring system event capture under backup or battery backup conditions.

Host microcontroller connectivity is accomplished via an SPI (modes 1 and 3) or standard 3-wire serial interface, selectable at the board level. Data transfer is handled by a clear address/data protocol, and burst mode enables efficient multi-byte data movement for both RTC and RAM registers.

Unique Registers and Trickle Charge Mechanism in DS1305E/T&R

The DS1305E/T&R incorporates specialized registers for real-time control and backup:

The Control register manages oscillator start/stop, alarm interrupt enables, and the write-protect function to safeguard configuration data.

The Status register contains interrupt request flags, which are managed by reading or writing the corresponding alarm registers.

The Trickle Charger register allows the user to enable, configure, or disable the onboard trickle charge circuit for rechargeable backup cells. Selection of resistor and diode configurations is digitally programmable per the register setting, and the charger can only be activated by a specific bit pattern (“1010”) to prevent accidental enablement.

DS1305E/T&R Packaging, Electrical Ratings, and Environmental Details

The DS1305E/T&R is offered in a compact 20-pin TSSOP package, measuring 0.173" (4.40mm) in width. Typical operating current for timekeeping ranges from 25.3μA to 81μA between 2V and 5.5V. The standard part supports a commercial temperature range (0°C to +70°C); alternative versions (e.g., DS1305EN) target industrial environments (−40°C to +85°C). The device is moisture sensitivity level (MSL) 1 rated, suitable for unrestricted storage, but it is not RoHS compliant. System designers should consider environmental regulations in procurement decisions.

DS1305E/T&R Implementation and Design Tips

Integrating the DS1305E/T&R requires careful crystal selection for the 32.768 kHz reference, paying close attention to crystal ESR, load capacitance (6 pF nominal), and PCB layout isolation to minimize noise and drift. The backup architecture mandates thoughtful selection of rechargeable or non-rechargeable elements, as well as proper system power-down protocols to ensure register state retention. For interrupt-driven architectures, external pull-up resistors are necessary on the open-drain INT0 and INT1 pins.

The serial interface maximizes compatibility with both legacy 3-wire and modern SPI implementations, simplifying system integration. The onboard NVRAM is ideally used for system-critical parameters or event logs, ensuring persistence across system resets or outages—a key benefit in applications such as remote telematics, industrial meters, and security systems.

DS1305E/T&R Alternative and Substitute Model Options

Given the DS1305E/T&R is now obsolete, engineers and buyers must evaluate replacement options to ensure product longevity and maintain timekeeping performance. They should assess follow-on or compatible RTCs from Analog Devices Inc./Maxim Integrated (e.g., DS1306, DS3234, DS1340) with similar supply, interface, pinout, and backup features. Selection should be based on supply chain continuity, package compatibility, supply voltage, interface protocols, and compliance with the latest RoHS/environmental regulations.

For best results, engineers should thoroughly cross-reference functional block diagrams, register sets, and timing characteristics, as migration between RTC families may reveal subtle differences in alarm logic, serial protocol, or NVSRAM management. Evaluation boards and migration guides are valuable tools to de-risk the transition in legacy designs.

Conclusion

The DS1305E/T&R Real-Time Clock from Analog Devices Inc./Maxim Integrated stands as a technically mature and well-documented solution for robust, backup-capable timekeeping in embedded systems. Its versatile features, including dual alarms, battery backup, NVSRAM, and a flexible serial interface, make it an exemplary reference for understanding integrated RTC requirements. While sourcing and compliance challenges now drive designers toward newer alternatives, insights from the DS1305E/T&R’s feature set and integration considerations inform best practices for RTC selection and system architecture decisions in modern designs.