Electronic – How to 16 buttons be connected with only 8 wires

digital-logicmatrixmultiplexerswitches

Here is the product. I understand the idea: these 16 buttons use a 4×4 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?

Best Answer

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:

Drive one row high, the others low.
Enable passive pulldowns on the column lines.
See which column lines are high. The buttons at the intersections of those column lines with the one asserted row line are pressed. The other buttons on that row are released.
Repeat back to step one, asserting the next row in sequence.

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.

Related Solutions

Switch matrix with ring counter

I'm going to suggest that you learn about shift registers. 4 pcs of a 8-bit parallel-to-serial shift register would work very well for you and would need a maximum of 3 lines to your controller. Some of those lines can even be shared with other functions if you are clever.

The CD4021 is a nice part that I use in many of my designs. You will need a pull-up resistor for each input. I use SIP bussed resistor networks - 10 pin package gives you 9 pullup resistors in a very small space.

You can also mount the shift registers very close to your switches to minimize the number of wires to the controller.

Keypad Scanner Verilog code problem with state machine and column input

This is more of an extended comment than an answer, because I can't put code examples in a comment.

Verilog gives you a lot of power to express logic in the most appropriate form; it isn't necessary to reduce everything to Boolean logic equations. For example, your 7-segment converter would be more clearly expressed as a case statement:

module seven_seg_decoder (V, Display);
  input [3:0] V; //input code
  output [0:6] Display; //output 7-seg code

  // When you want to turn on a segment, set it low
  always @(*) begin
    case (V)
    4'h0: Display <= 7'b0000001;
    4'h1: Display <= 7'b1001111;
    4'h2: Display <= 7'b0010010;
    // ... etc.
    endcase
  end
endmodule

A case statement is also a good way to express the functionality of a state machine, as well. If you want other engineers to be able to read and understand your code, you should use the highest-level constructs that make sense.

Best Answer

Related Solutions

Switch matrix with ring counter

Keypad Scanner Verilog code problem with state machine and column input

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