- Analog Ladders
- Using a single analog input pin, an analog value is read in which represents which button has been pressed.
Advantage: Only One Analog Input pin is needed for 8, 12, or more keys.
- Analog Matrix
- Similar in use to the analog ladder method, this method is used in conjunction with the common matrix style switching arrangement keypad.
Advantage: Only One analog Input pin is needed, and standard matrix keypad is used.
- Scanned Matrix
- This is a traditional method whereby a keyboard matrix is scanned by software to identify the row and column of a pressed key.
Advantage: Cheap and conventional keypad hardware solution.
- Diode Logic
- This is a simple method providing a keycode by using wired-or logic implemented with just diodes.
Advantage: Simple software and hardware. 2 pins for upto 3 keys, 3 pins for upto 7 keys, 4 pins for upto 15 keys, 5 pins for upto 31 keys.
- Encoded
- There are many forms of keyboard encoding, from Using a multiplexer, a priority encoder, or a DTMF decoder, generally these resolve the input key to a keycode, and sometimes also another line signals that a keycode is available.
Advantage: Software can be simple, versatile range of interface options.
- Schmitt I/P
- This method uses a schmitt trigger buffered I/O pin to read in an analog value by use of a timing loop. (The analog ladder and matrix circuits can be altered for use with a schmitt trigger I/P.)
Advantage: One Schmitt-buffered I/O pin, simple hardware.
- Shift Register
- There are a few ways in which 8, 16, or just about any other number of digital inputs can be read using 2 or 3 digital pins. One pin is an input, and one or two pins are outputs, depending on the method used.
Advantage: Simple hardware, and only 2 or 3 pins required.
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