Linear Feedback Shift Register
This example instantiates a linear feedback shift register that provides
a high-speed counter. Some performance attributes are sacrificed in order to gain the high-speed functionality.
The linear feedback shift
register counts through 2^WIDTH-1 states.
A state consisting of only 1s is illegal; if the counter contains all 1s, it
cannot leave that state unless the counter is cleared or another value is loaded. The maximum
width of fast_cnt is 16 so the maximum number of states it can count to
is 65535.
The states of the counter are not sequential and the states change depending on the
specified counter width. For example, in the Graphic Editor file shown below, the width is set to 4, so the counter goes through the following 15 states:
0000, 0001, 0011, 0111, 1110, 1101, 1011, 0110, 1100, 1001, 0010, 0101,
1010, 0100, and 1000. If the width is set to a different value, then the counter will go through a different sequence of states.
This design should be targeted for a FLEX device. Although it has certain
drawbacks when compared to other counters,
it will run much faster than an lpm_counter of the same width in a
FLEX device.
The Graphic Editor and AHDL files for this example are provided below.
To download the Graphic Editor file, click on the link below with the right mouse button and choose Save Link As from the drop-down list box.
For more information on using this example in your project, go to:
top.gdf
fast_cnt.tdf
INCLUDE "lpm_xor.inc";
PARAMETERS
(
WIDTH
);
SUBDESIGN fast_cnt
(
data[WIDTH-1..0] : INPUT = GND;
clock : INPUT;
clk_en : INPUT = VCC; -- Clock Enable
load, aclr : INPUT = GND;
q[WIDTH-1..0] : OUTPUT;
)
VARIABLE
counter[WIDTH-1..0] : DFFE;
BEGIN
ASSERT (WIDTH > 1 )
REPORT "Value of 'Width' must be greater than 1"
SEVERITY ERROR;
ASSERT (width <= 16)
REPORT "Value of 'Width' must be equal to or less than 16"
SEVERITY ERROR;
counter[].clk = clock;
counter[].clrn = !aclr;
counter[].ena = clk_en;
IF (WIDTH == 2) GENERATE
IF load THEN counter[] = data[]; ELSE
counter1 = counter0;
counter0 = counter0 XNOR counter1;
END IF;
END GENERATE;
IF (WIDTH == 3) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[2..1] = counter[1..0];
counter0 = counter2 XNOR counter1;
END IF;
END GENERATE;
IF (WIDTH == 4) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[3..1] = counter[2..0];
counter0 = counter2 XNOR counter3;
END IF;
END GENERATE;
IF (WIDTH == 5) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[4..1] = counter[3..0];
counter0 = counter4 XNOR counter2;
END IF;
END GENERATE;
IF (WIDTH == 6) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[5..1] = counter[4..0];
counter0 = counter5 XNOR counter4;
END IF;
END GENERATE;
IF (WIDTH == 7) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[6..1] = counter[5..0];
counter0 = counter6 XNOR counter5;
END IF;
END GENERATE;
IF (WIDTH == 8) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[7..1] = counter[6..0];
!counter0 = lpm_xor(counter7, counter5, counter4, counter3)
with (LPM_WIDTH = 1, LPM_SIZE = 4);
END IF;
END GENERATE;
IF (WIDTH == 9) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[8..1] = counter[7..0];
counter0 = counter8 xnor counter4;
END IF;
END GENERATE;
IF (WIDTH == 10) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[9..1] = counter[8..0];
counter0 = counter9 XNOR counter6;
END IF;
END GENERATE;
IF (WIDTH == 11) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[10..1] = counter[9..0];
counter0 = counter10 XNOR counter8;
END IF;
END GENERATE;
IF (WIDTH == 12) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[11..1] = counter[10..0];
!counter0 = lpm_xor (counter11, counter10, counter7, counter5)
WITH (LPM_WIDTH = 1, LPM_SIZE = 4);
END IF;
END GENERATE;
IF (WIDTH == 13) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[12..1] = counter[11..0];
!counter0 = lpm_xor (counter12, counter11, counter9, counter8)
WITH (LPM_WIDTH = 1, LPM_SIZE = 4);
END IF;
END GENERATE;
IF (WIDTH == 14) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[13..1] = counter[12..0];
!counter0 = lpm_xor (counter13, counter12, counter7, counter3)
WITH (LPM_WIDTH = 1, LPM_SIZE = 4);
END IF;
END GENERATE;
IF (WIDTH == 15) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[14..1] = counter[13..0];
counter0 = counter14 XNOR counter13;
END IF;
END GENERATE;
IF (WIDTH == 16) GENERATE
IF load THEN counter[] = data[]; ELSE
counter[15..1] = counter[14..0];
!counter0 = lpm_xor(counter15, counter14, counter12, counter3)
WITH (LPM_WIDTH = 1, LPM_SIZE = 4);
END IF;
END GENERATE;
q[] = counter[];
END;
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