Digital clocks typically use the 50 or 60
hertz oscillation of
AC power or a 32,768 hertz
crystal oscillator as in a
quartz clock to keep time. Most
digital clocks display the hour of the day in
24-hour format; in the United States and a few other countries, a more commonly used hour sequence option is
12-hour format[citation needed] (with some indication of AM or PM). Emulations of analog-style faces often use an
LCD screen, and these are also sometimes described as "digital".
Displays
A digital clock's display changing numbers
An LCD battery-operated clock without alarm
To represent the time, most digital clocks use a
seven-segment LED,
VFD, or
LCD display for each of four
digits.
They generally also include other elements to indicate whether the time
is AM or PM, whether or not an alarm is set, and so on.
Setting
A premium digital clock radio with digital tuning
A basic digital clock radio with analog tuning
If people find difficulty in setting the time in some designs of
digital clocks in electronic devices where the clock is not a critical
function, they may not be set at all, displaying the default after
powered on, 00:00 or 12:00.
[5][6]
Digital clocks that run on mains electricity and have no battery must
be reset every time the power is cut off or if they are moved. Even if
power is cut off for a second, most clocks will still have to be reset.
This is a particular problem with
alarm clocks that have no "battery" backup, because even a very brief
power outage during the night usually results in the clock failing to trigger the alarm in the morning.
To reduce the problem, many devices designed to operate on household electricity incorporate a
battery
backup to maintain the time during power outages and during times of
disconnection from the power supply. More recently, some devices
incorporate a method for automatically setting the time, such as using a
broadcast
radio time signal from an
atomic clock, getting the time from an existing
satellite television or
computer
connection, or by being set at the factory and then maintaining the
time from then on with a
quartz movement powered by an internal
rechargeable battery.
Uses
A digital clock built into an oven
- Because digital clocks can be very small and inexpensive devices
that enhance the popularity of product designs, they are often
incorporated into all kinds of devices such as cars, radios, televisions, microwave ovens, standard ovens,
computers and cell phones. Sometimes their usefulness is disputed: a
common complaint is that when time has to be set to Daylight Saving
Time, many household clocks have to be readjusted. The incorporation of
automatic synchronisation by a radio time signal is reducing this
problem (see Radio clock).