Application of serial clock DS1305 in field self-contained equipment

Field observation instruments often need to operate for extended periods in remote environments, relying entirely on their own power sources—hence the term "self-contained." While some devices can harness natural energy like solar or wind power, many cannot. This makes efficient power management a critical aspect of developing such systems.

This article explores the working principle of the DS1305 serial real-time clock chip and its application in self-contained field instruments. Energy efficiency is a major concern when designing these devices, as they may need to function continuously or intermittently for days, even years. Most rely on solar or chemical power, but for equipment placed deep underground or on the ocean floor, only chemical batteries are viable.

Since environmental changes are typically slow, these instruments can remain dormant most of the time, waking up only at specific intervals. By integrating an energy-saving power management unit, power consumption can be significantly reduced, making the device more compact and longer-lasting. In a project under the National Marine 863 Program 818, we used this approach. The system needed to collect data at scheduled times and respond to unexpected commands. We chose the DS1305 to control the timing, allowing the host to wake up, perform the task, and then shut down again, minimizing power usage.

The DS1305 itself is a low-power device, so the entire system consumes minimal energy. With fewer chemical batteries, it can operate for long periods in the field without recharging.

1. Basic Working Principle of the DS1305 Real-Time Clock

1.1 Main Features of the DS1305

The DS1305 is a serial real-time clock with the following key features:

• Calendar and clock functions (years, months, days, hours, minutes, seconds, and weeks up to 2100)
• 96 bytes of non-volatile RAM for user data storage
• Two programmable alarm outputs
• Dual power supply (main and backup battery)
• Operating voltage: 2.5V to 5.5V
• Operating temperature range: -40°C to +85°C

1.2 Pin Configuration and Functions

The DS1305 comes in DIP and TSSOP packages. Here's a brief description of its main pins:

• VCCl: Main power input
• Vcc2: Backup power (rechargeable battery)
• Vbat: 3V battery input
• X1/X2: Connect to a 32.768 kHz crystal
• INTO/INT1: Interrupt outputs (active low)
• SDI/SDO: SPI data input/output
• CS: Chip select
• SCLK: Serial clock input
• SERMODE: Interface mode selection
• PF: Power failure output

1.3 Internal Structure

The internal structure includes power control, serial interface logic, shift registers, calendar logic, and non-volatile RAM. This allows the DS1305 to maintain accurate timekeeping while consuming very little power.

1.4 Register Map

The DS1305 has a well-defined register map for the calendar, clock, and user RAM. Each register address corresponds to a specific function, enabling precise control over timekeeping and alarms.

1.5 Register Definitions

Key registers include the Control Register, Status Register, and Trickle Charge Register. These allow users to configure the device’s behavior, including alarm settings, interrupt control, and battery charging options.

1.6 Interface Modes

The DS1305 supports both SPI and 3-wire modes. Multi-byte transfers are also possible, making it flexible for various applications. Timing diagrams show how data is read from or written to the device.

2. Interface with AT89C2051

The DS1305 can easily interface with microcontrollers like the AT89C2051. The SPI and 3-wire modes provide a simple and reliable way to communicate with the clock chip.

3. Application in Marine Environmental Monitoring

In marine monitoring systems, the DS1305 is used to manage power efficiently. By setting up timed alarms, the system can wake up the CPU at specific intervals, collect data, and then return to sleep. This reduces overall power consumption and extends the life of the device.

Using the DS1305’s programmable alarm feature, we designed a power control unit that could generate interrupts at regular intervals. This allowed the main system to activate only when needed, saving energy and improving reliability. The solution was successfully implemented in a project focused on suspended sediment monitoring in the ocean, proving its effectiveness in real-world conditions.