Keypad Scanner help!

Hi, welcome to the dark side of VHDL if you are a software programmer !!! ;-)

--- Quote Start ---

VHDL is NOT a programming language. It describes hardware.

--- Quote End ---

1) Do you want you code to be synthesized (and to be put on FPGA) ?

If so, forget the clauses "after", they are simply ignored.

That why only the 3rd part of your code is "active", the others are simply optimized away, because Keypad_write will always be "1101" (You could find that in Quartus warning messages)

2) Make a really synchronous process, in fact a D Flip Flop with Enable like that :

PROCESS (clock_50)
BEGIN
IF rising_edge(clock_50) THEN -- clock signal
     IF pulse_10us = '1' then -- enable signal
          -- your sequential synchronous statements here
     END IF;
END IF;
-- absolutely nothing here
END PROCESS;

3) You need a signal that supplies 1 pulse whose width = clock_50 period, whose period is 10 us.

You can obtain that with a synchronous counter (quite easy to design, good exercise).

4) You need a one-hot counter ("0001" then "0010" then "0100" then "1000") simply by rotate one hot bit each 10us (= the above Enable signal).

that you simply invert for "keyboard_write"

5) IMPORTANT

You don't cover all possible cases !!

6) Don't use multiple IF statements in a process, because the signals are affected by the last statement

In your case, you should use

CASE Keypad_write IS
    WHEN "0111" => 
        If (Keypad_read = "0111" ) THEN
            LED<= "00000001";
        ELSIf (Keypad_read = "1011") THEN
            LED<= "00000100";
        ELSIf (Keypad_read = "1101") THEN
            LED<= "00000111";
        END IF;
    WHEN .... => ....
    WHEN OTHERS => ....
END CASE;

Be strongly aware that there is a latence of 10us on Keyboard_write, so you need to insert one 10us Enable D Flip Flop on Keyboard_read