In the DS1338, there is an automatic power supply selector switch. When external power is selected, the power is supplied by the external power supply. When the external power supply is shut off, a backup battery provides the power and the power switching is done automatically to ensure that the device is powered at all times. This in turn ensures that the internal clock is always ticking.
There are two main reasons why the device clock resets when the external power is shut off:
There are two main reasons why the device clock resets when the external power is shut off:
- The external power level drops too quickly. If the voltage from the external power supply drops too quickly and power supply has not switched over to battery power, the device may be un-powered momentarily, causing the clock information to be lost. The shortest power-off time for the DS1388 is 300us.
- When the external power supply powers off, there is an undershoot. If the undershoot voltage is lower than -0.3V, the device’s internal data will be lost.