I'm adding a series of "Overviews" for readers who may not be coming from a hardware background.
What's the big deal with 7-segment LED displays? Well, it's not a big deal, but it's not completely trivial either.
Suppose you have a single-digit 7-segment LED display. One digit can display 0-9 decimal, or 0x0 to 0xF hexadecimal. Suppose you are only interested in displaying 0 or 1. The interface is not a single input pin that you set to either GND for 0 or VCC for 1, with all other pins set to GND. Instead, 7-segment displays have an interface as follows:
What's the big deal with 7-segment LED displays? Well, it's not a big deal, but it's not completely trivial either.
Suppose you have a single-digit 7-segment LED display. One digit can display 0-9 decimal, or 0x0 to 0xF hexadecimal. Suppose you are only interested in displaying 0 or 1. The interface is not a single input pin that you set to either GND for 0 or VCC for 1, with all other pins set to GND. Instead, 7-segment displays have an interface as follows:
There are 7 input pins, named A-G, each of which controls one segment. There is usually also a pin controlling the decimal point, typically named DP, but we'll ignore this for simplicity. To display a 0, you'll need to enable pins A,B,C,D,E,F, but disable pin G. To display a 1, you'll need to enable pins B and C while disabling all other pins. There's some logic or lookup involved. It gets more involved as you expand your display capability from 0..1 to 0..9 or 0x0..0xF.
For four digit 7-segment LED displays, the A-G pins (and DP as well) are shared among the four digits, and 4 more pins indicate which digit the current A-G pins' values should be "sent to." It's simpler, if at any given time, only one of the four digit selection pins is enabled, with 3 disabled. You'd have to rotate among the four (like software concept of "round robin") pins. Now there is not just logic or lookup but state as well. Furthermore, these 4 "selector-pins" with shared A-G configuration can be in the form of "common anode" or "common cathode." More details at Wikipedia.
For four digit 7-segment LED displays, the A-G pins (and DP as well) are shared among the four digits, and 4 more pins indicate which digit the current A-G pins' values should be "sent to." It's simpler, if at any given time, only one of the four digit selection pins is enabled, with 3 disabled. You'd have to rotate among the four (like software concept of "round robin") pins. Now there is not just logic or lookup but state as well. Furthermore, these 4 "selector-pins" with shared A-G configuration can be in the form of "common anode" or "common cathode." More details at Wikipedia.
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