digital clock

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Digital clock
A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Usually, digital clocks are associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism. (Analog clocks are driven mechanically or electronically.) The biggest digital clock is the Lichtzeitpegel ("Light Time Level") on the television tower Rheinturm Düsseldorf, Germany.

Construction
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 is 12 hour format (with some indication of AM or PM). Some clocks can display either time mode according to the owner's preference. Emulations of analog-style faces often use an LCD screen, and these are also sometimes described as "digital".

Displays

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.


High-Level View
Here is a quick overview of the components of a digital clock at a high level.
At the heart of the clock there is a piece that can generate an accurate 60-hertz (Hz, oscillations per second) signal. There are two ways to generate this signal:
1. The signal can be extracted from the 60-Hz oscillations in a normal power line. Many clocks that get their power from a wall socket use this technique because it is cheap and easy. The 60-Hz signal on the power line is reasonably accurate for this purpose.
2. The signal can be generated using a crystal oscillator. Obviously, any battery-operated clock or wristwatch will use this technique instead. It takes more parts, but is generally much more accurate.
The 60-Hz signal is divided down using a counter. When building your own clock, a typical TTL part to use is a 7490 decade counter. This part can be configured to divide by any number between 2 and 10, and generates a binary number as output. So you take your 60-Hz time base, divide it by 10, divide it by 6 and now you have a 1-Hz (1 oscillation per second) signal. This 1-Hz signal is perfect for driving the "second hand" portion of the display. So far, the clock looks like this in a block diagram: