The following code is a simple state machine that determines the direction that the knob on the Spartan 3E is turning and outputs a clock signal (Count) as it turns. A & B are the inputs from the encoders on the knob, RST & CLK are the reset and clock obviously. Count is the clock signal that goes two cycle for each click of the knob. Dir is the current direction of the knob, 0 for clockwise and 1 for counter-clockwise. Be sure to de-bounce the A & B inputs from the knob as they will have lots of noise and cause the state machine to be unstable.
Code
VHDL
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library IEEE; use IEEE.STD_LOGIC_1164.ALL; entity Knob is Port ( CLK : in STD_LOGIC; RST : in STD_LOGIC; A : in STD_LOGIC; B : in STD_LOGIC; Direction : out STD_LOGIC; Count : out STD_LOGIC := '0' ); end Knob; architecture Behavioral of Knob is type State_Type is ( CW_A, CW_B, CW_C, CW_D, CCW_A, CCW_B, CCW_C, CCW_D ); signal PresentState, NextState : State_Type := CW_A; constant CCW : STD_LOGIC := '0'; constant CW : STD_LOGIC := '1'; begin process (CLK) begin if CLK'Event and CLK = '1' then if RST = '1' then PresentState <= CW_A; else PresentState <= NextState; end if; end if; end process; process (A, B, PresentState) begin case PresentState is when CW_A => if A = '0' and B = '1' then NextState <= CW_B; elsif A = '1' and B = '0' then NextState <= CCW_D; else NextState <= PresentState; end if; when CW_B => if A = '0' and B = '0' then NextState <= CW_C; elsif A = '1' and B = '1' then NextState <= CCW_A; else NextState <= PresentState; end if; when CW_C => if A = '1' and B = '0' then NextState <= CW_D; elsif A = '0' and B = '1' then NextState <= CCW_B; else NextState <= PresentState; end if; when CW_D => if A = '1' and B = '1' then NextState <= CW_A; elsif A = '0' and B = '0' then NextState <= CCW_C; else NextState <= PresentState; end if; when CCW_A => if A = '0' and B = '1' then NextState <= CW_B; elsif A = '1' and B = '0' then NextState <= CCW_D; else NextState <= PresentState; end if; when CCW_B => if A = '0' and B = '0' then NextState <= CW_C; elsif A = '1' and B = '1' then NextState <= CCW_A; else NextState <= PresentState; end if; when CCW_C => if A = '1' and B = '0' then NextState <= CW_D; elsif A = '0' and B = '1' then NextState <= CCW_B; else NextState <= PresentState; end if; when CCW_D => if A = '1' and B = '1' then NextState <= CW_A; elsif A = '0' and B = '0' then NextState <= CCW_C; else NextState <= PresentState; end if; when others => NextState <= CW_A; end case; end process; Direction <= '1' when PresentState = CW_A or PresentState = CW_B or PresentState = CW_C or PresentState = CW_D else '0'; Count <= '1' when PresentState = CW_B or PresentState = CW_C or PresentState = CCW_B or PresentState = CCW_C else '0'; end Behavioral; |
Change Log
2/28/2011: I had loads of trouble implementing this recently on hardware and ended up rewriting it so that it is easier to implement and less troublesome.
11/9/2010: Added code for VHDL module.
Hello,
This information for view the knob funtion is really interestig for me, but a need it in VHDL not in Verilog, could you say me what can I do that in VHDL Language, do you have something like this in VHDL,please?
Thank you
I have been watching posts. I always find you articles telling.
I need the same code in verilog. Or atleast the mechanism of the code so that I can write this in verilog.