Which display: LCD or LED?

In many cases you want a microcontroller to show its measurements or other numerical data. So you need some kind of display, numeric or alpha-numeric.

Of course there are many different kinds of liquid crystal displays (LCD) and 7-segment light emitting diode (LED) displays. While LC displays offer the standard ASCII set, some characters cannot be displayed with 7-segment displays.
We are discussing only the pros and cons of the standard types. Most LC displays are equipped with the controller HD44780 (Hitachi) which can be operated in 4-bit and 8-bit mode. Though the 8-bit mode offers higher programming speed most people prefer the 4-bit mode to save 4 pins.

Ambient light Can be read easily in total darkness but hard to read in bright sunlight. You need to adjust contrast. When used in a dark environment you need to activate backlight (if available).
Readability Wide selection of colors and sizes, can be read from all angles and far distances. Most LC displays offer a digit height of 4 mm so you can hardly manage to read them from a distance of more than 2 meters.
Power Might need more power than the controller itself. Needs nearly no power at all except for the backlight.
Number of pins You need 7 pins for the segments (plus 1 pin for the decimal point if needed).
If you want to control more than one digit you have to install an ISR to switch from one digit to the other in a very fast mode. So, with multiplexing you need 1 pin per digit additionally.
4 pins for data plus 3 pins for control (you also need to connect the power supply). For programming, just include the LCD library.
Speed The LEDs can be operated at processor speed (although you won't notice that). So, if speed matters you have to go for a LED display.
But mind you: multi-digit displays will be controlled in a multiplex way, so you will loose some speed.
Due to the physics of the devices and the serial kind of data transmission the response of LCDs is relatively slow.
Additional remarks If you are not using extra hardware (except the resistors for the segments of course) you can use either common cathode or common anode types. But if you use drivers like ULN2803, UDN2981 or decoders like SN74LS154 to drive the digits you have to use the common cathode types.  

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