Here is the product. I understand the idea: these 16 buttons use a 4x4 matrix. 4 lines for rows, 4 lines for columns, and we have 8 cables.
But:
How can this work without any multiplexer ?
Can this detect accurately presses of multiple buttons? Even if buttons use same row or same column? Example: buttons at position (2,2) (2,3), (3,2), and (3,3) pressed at the same time.
How does it work?
How can this work without any multiplexer?
It doesn't. The keypad board seems to have just switches, with maybe some diodes we can't see. However, the left board looks like it has a processor on it. Almost certainly, multiplexing is being done in firmware.
The multiplexing algorithm works something like this:
The above process is repeated fast enough so that all the buttons are checked within a time that still feels instantaneous to a human observer. The human limit of "instantaneous" in this context is about 50 ms. Even a low end microcontroller can scan a 4x4 keypad in much less time than that.
Can this detect accurately presses of multiple buttons?
Probably yes. One way is to put a diode in series with each button. Those could be on the bottom side of the board you show. With the diodes, the algorithm described above just works.
Another way is by putting resistors in series with lines and measuring their analog voltage. Even with multiple buttons shorting multiple row/column lines together, you can eventually figure out which buttons are pressed. This requires A/D inputs in the micro, not just digital inputs as when diodes are used.
Without diodes... you'll get phantom rectangles.
For instance in your example (2,2) (2,3), (3,2), and (3,3); you chose an ironic example by choosing a rectangle. If you actually press all four, it will work. But if you press any three corners of the rectangle, the fourth corner will also appear to be pressed, even though it's not.
That keypad is clearly meant for keyboard data entry, where the user convention is one keypress at a time. You notice there are no "shift" keys on a gas station credit card entry keypad, for instance.
However, if you made S1 a shift key, and S3 an alt-shift key such that users might reasonably hold both while also pressing S13, then S15 would also appear to be pressed.
Here's how to do it without continuous scan:
I have done this with Assembly Language on bare Atmel AVR microcontrollers.
For simplicity:
Keypad Rows: use Arduino Pins 4,5,6 and 7 (AVR pins PD4, PD5, PD6 and PD7)
Keypad Columns: use Arduino Pins 8, 9, 10 and 11 (AVR pins PB0, PB1, PB2 and PB3)
Make Row pins inputs with pull-ups enabled.
Make Column pins Output and output zeroes to them.
Enable Pin Change Interrupts on all row and column pins. http://playground.arduino.cc/Main/PinChangeInterrupt
Pressing a button on the keypad will pull a Row pin low. The interrupt routine needs to read the row pins and find which pin is low. The upper 4 bits should be three ones and one zero. If you use a uint8_t 8 bit variable you can divide it by 16 or (var >> 4) to put the 4 bits in the lower bits of the number.
You can use a bitwise OR | operation with 240 to set the upper 4 bits to 1s and a bitwise NOT ~ operation to invert all the bits so you only have one bit set representing the row the button press was on. Store this number as 0, 1, 2 or 3.
Switch the pin configuration:
Make Column pins inputs with pull-ups enabled.
Make Row pins Output and output zeroes to them.
Read the column pins and find which pin is low. Do a similar manipulation with this value except you don't have to shift the number to the right. After the var ^ 240 and ^var you have a single bit to represent the column the key press was on. Remember to disable interrupts before reconfiguring ports as it will likely trigger unwanted interrupts. You may have to clear interrupt flags when leaving the interrupt routine to prevent duplicate interrupt handling.
