Keypad Scanner help!

powerful you have become !

why is the 10 us delay necessary?

First, Keyboard_read is asynchronous, to keep the logic synchronous, you need to synchronize the inputs (by adding a DFF (D Flip Flop))

Better is to put at least 2 DFF on inputs to prevent metastability.

Because, In the process, " Keypad_write <= "0111"; " means that Keypad_write is updated at the end or process and NOT immediately.

So when you read "Keypad_read", your read the state of your keypad when keypad_write is in the state BEFORE "0111".

To read "Keypad_read" with correct Keypad_write, you have to wait the next clock.

Examples are available :

http://www.coe.montana.edu/ee/lameres/courses/eele367_spring13/misc_handouts/chapter_08_notes.pdf

http://www.cems.uwe.ac.uk/~ngunton/worksheets/keypad.vhd

EDIT:

Some tricks:

PROCESS (clock_50)
BEGIN
If (clock_50'EVENT AND clock_50 = '1' ) THEN
	-- it is a count down here
        -- produce pulse when reach 0
	If (count = 0) THEN -- little better to compare to 0
               count<= 500;
                pulse_10us <= '1';
        ELSE
                count <= count -1;
                 pulse_10us <= '0'
	END IF;
END IF;
END PROCESS;

Usually, you miss the last ELSE statement in IF...THEN...ELSIF...ELSE....END IF;

This missing can produce lacthes or combinatory loops which are very bad.