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VHDL (VHSIC-HDL, Very High-Speed Integrated Circuit Hardware Description Language) is a hardware description language used in electronic des...

Friday 30 April 2010

Positive and Negative Edge Detector Circuit

Positive and negative edge detection is a common requirement in microprocessors. One application could be to detect edge/level triggered events on certain GPIO inputs. Here i will show you a simple circuit which is use to detect Positive as well negative edges.


library ieee;
use ieee.std_logic_1164.all;
entity edge is
   port (
    inp        : in  std_logic;         -- inpit
    clk        : in  std_logic;         -- clock
    rst        : in  std_logic;         -- reset
    edge_op : out std_logic);        -- setected edge output
 end edge;

architecture edge_ar of edge is
  signal sig1 : std_logic;              -- signal from 1st flop
  signal sig2 : std_logic;              -- signal from 2nd flop

begin  -- edge_ar
   edge : process(clk, rst)
    if rst = '1' then
      sig1 <= '0';
      sig2 <= '0';
    elsif clk'event and clk = '1' then
      sig1 <= inp;
      sig2 <= sig1;
    end if;
  end process edge;

  edge_op <= sig1 xor sig2;

end edge_ar;

Saturday 3 April 2010

What is Clock Skew?

Given two sequentially-adjacent registers, Ri and Rj, and an equipotential clock distribution network, the clock skew between these two registers is defined as

Tskew-i,j = Tci - Tcj

where Tci and Tcj are the clock delays from the clock source to the registers Ri and Rj, respectively.