The DS1339 serial real-time clock (RTC) is a low-power clock/calendar chip with two programmable calendar alarms and one programmable square wave output. Address and data are transferred serially over the I2C bus. The clock/calendar can provide seconds, minutes, hours, day, month, and year information. For months with fewer than 31 days, adjustments are made automatically to the last day of the month, including corrections for leap years. The clock can work in 24-hour mode or 12-hour mode with AM/PM indicator. The DS1339 has an internal power-sensing circuit that detects power failure and automatically switches to backup power.

operate

The DS1339 operates as a slave device on a serial bus. Gain access by implementing a START condition and provide a device ID followed by data. Subsequent registers can be accessed sequentially until a STOP condition is executed. The device is fully accessible and can write and read data greater than VPF at VCC. However, when VCC is below VPF, the internal clock registers are blocked from any access. If VPF is less than VBACKUP, the device power is switched from VCC to VBACKUP when VCC falls below VPF. If VPF is greater than VBACKUP, the device power is switched from VCC to VBACKUP when VCC falls below VBACKUP. This register is maintained from the VBACKUP source until VCC returns to the nominal level. The block diagram in shows the main elements of a serial real-time clock.

block diagram

oscillator circuit

The DS1339 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or capacitors to operate. The table below lists several crystal parameters for external crystals. The figure below shows a functional schematic of an oscillator circuit. When using crystals, the startup time is usually less than 1 second with the specified characteristics.

accurate clock

The accuracy of the clock depends on the accuracy of the crystal and the accuracy of matching between the capacitive load of the oscillator circuit and the capacitive load of the trimmed crystal. Crystal frequency drift due to additional temperature drift adds to the error. External circuit noise coupling into the oscillator circuit can cause the clock to run fast. The figure below shows a typical PC board layout for isolation

The DS1339C package contains a Quartz crystal tuning fork crystal. Pick and place equipment can be used, but precautions should be taken to ensure excessive shock is avoided. Ultrasonic cleaning should be avoided to prevent damage to the crystal. Avoid running signal traces under the package unless a ground plane is placed between the package and the signal traces. All NC (No Connect) pins must be grounded. Leaded SO packages can be reflowed as long as the peak temperature does not exceed 240 °C. Peak reflow temperature (≥ 230°C) duration should not exceed 10 seconds and total time above 200°C should not exceed 40 seconds (30 seconds nominal). Exposure to reflow temperature is limited to a maximum of 2 times. The RoHS SO package can be assembled using lead-free reflow soldering using a JEDEC J-STD-020 reflow profile.