Wednesday, 4 May 2016

fpga - vhdl synthesis optimization: counters in statemachines


I have a general question about the efficiency of a synthesizable state machine.


The first version uses the same counter for each state. The second uses one own counter for each state. Which version of the two is more efficient (logic area, speed...)??


How much area of the FPGA is occupied by the routing of the count1 signal when i use the same counter for each state. Is it better to user one counter for each state??



I'hope somebody with more experience can explain which solution is the best (maybe a third version) and why.


Thank you!


Kind Regards,


Oliver


-- 1. Version ===============================================================


signal count1: integer range 0 to 1000 := 1000;  
type mystates is (s1, s2, s3, s4);
signal mymode: mystates := s1;

BEGIN


MyProcess: process(clk)

BEGIN

IF (clk'event and clk = '1') THEN

case mymode is
when s1 =>
If (count1 = 0) then

mymode <= s2;
count1 <= 555;
-- (stuff)
else
count1 <= count1 - 1;
end if;
when s2 =>
If (count1 = 0) then
mymode <= s3;
count1 <= 666;

-- (stuff)
else
count1 <= count1 - 1;
end if;
when s3 =>
If (count1 = 0) then
mymode <= s4;
count1 <= 784;
-- (stuff)
else

count1 <= count1 - 1;
end if;
when s4 =>
If (count1 = 0) then
mymode <= s1;
count1 <= 1000;
-- (stuff)
else
count1 <= count1 - 1;
end if;

when others =>
Null;
end case;

END IF;

end process;

-- 2. Version ===============================================================


signal count1, count2, count3, count4: integer range 0 to 1000 := 1000;  

type mystates is (s1, s2, s3, s4);
signal mymode: mystates := s1;

BEGIN

MyProcess: process(clk)

BEGIN

IF (clk'event and clk = '1') THEN


case mymode is
when s1 =>
If (count1 = 0) then
mymode <= s2;
count1 <= 555;
-- (stuff)
else
count1 <= count1 - 1;
end if;

when s2 =>
If (count2 = 0) then
mymode <= s3;
count2 <= 666;
-- (stuff)
else
count2 <= count2 - 1;
end if;
when s3 =>
If (count3 = 0) then

mymode <= s4;
count3 <= 784;
-- (stuff)
else
count3 <= count3 - 1;
end if;
when s4 =>
If (count4 = 0) then
mymode <= s1;
count4 <= 1000;

-- (stuff)
else
count4 <= count4 - 1;
end if;
when others =>
Null;
end case;

END IF;


end process;


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