I am trying to build pwm in wave form .meaning for exmaple I want the pwm go for 5 rising edge on and then 5 rising edges off . But for some reason when I write :

    variable counter_test : integer  range 0 to 10 ; 

meaning that variable counter_test should run from 0 to 10 ,so in the waveform it runs from 0 to 15 (my guess is that it runs to 15 becouse 10 in binary form is 1010 ,so it has 4 bits and 2^4-1=15 ) .My question is simple why it runs till 15 and not 10 as I wrote ?

library IEEE;

entity Clock_Divider is     
        port ( 
                clk_in_27Mhz  : in std_logic ;  
                clr  : in std_logic;
                clk_out_960Hz: out std_logic
        end Clock_Divider;

architecture logic_clock_divider  of Clock_Divider is 


        variable counter_test : integer  range 0 to 9:=0; 


            if (clk_in_27Mhz'event and clk_in_27Mhz='1') then 
                    if (  0< counter_test and counter_test<4 )  then 
                    elsif (  4<counter_test and counter_test<9) then 

                    end if;

            end if;
        end process;

    end logic_clock_divider ;

and the waveform is : enter image description here

why the variable counter_test does not reset at 9 and keeps till 15 ? what is wrong with my code ? and how to fix this problem ?


1 Answer 1


There's just nothing that would reset the counter_test back to 0.

VHDL's integer ranges aren't magic, they don't automagically wrap around. The range only seems to specify the values that are to be expected to fit.

Many tools don't do the range-checking by default, my guess is it does impair the performance a bit. It's often possible to enable this, however.

Synthesis tools usually don't rangecheck, there's really no way for them to know beforehand, and there's no way for the synthesized design to report any "runtime errors".


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