Question

I have written a VHDL code which implements the functionality of a PWM Controller. I have simulated it successfully and the results are as expected. I also checked the syntax for synthesis but it dint showed any error. When I went for synthesizing it using XILINX ISE 12.4 it’s not synthesizing and the error states

“ERROR:Xst:827 – line 67: Signal tmp_PC cannot be synthesized, bad synchronous description. The description style you are using to describe a synchronous element (register, memory, etc.) is not supported in the current software release.”

--library UNISIM;
--use UNISIM.VComponents.all;

entity CONTROLLER is
PORT(

    CLK:    IN  STD_LOGIC;
    VOUT: IN  STD_LOGIC;
    M1:   OUT STD_LOGIC:='0';
    M2:   OUT STD_LOGIC:='0'

);


end CONTROLLER;

architecture Behavioral of CONTROLLER is

    SIGNAL VREF:        STD_LOGIC_VECTOR(7 DOWNTO 0):="01000000";
    SIGNAL V_ERR:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL PWM:         STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL PWM_NEW:         STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL COUNT:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL COUNT2:          STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL TEMP1:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL TEMP2:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL TEMP3:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL FEED_BACK:       STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL REG:         STD_LOGIC_VECTOR(7 DOWNTO 0):="00000000";
    SIGNAL PWM_COUNT:       STD_LOGIC_VECTOR(7 DOWNTO 0):="10000000";

BEGIN

PROCESS(CLK)

BEGIN

    IF(RISING_EDGE(CLK) AND COUNT2<"10000000")THEN
            COUNT <= COUNT+'1';
    END IF;

    IF(RISING_EDGE(CLK) AND COUNT2>="10000000")THEN
        COUNT <= COUNT+'1';
    END IF;

    IF (COUNT>"00000101" AND COUNT<"01111000") THEN
        IF(RISING_EDGE(CLK))THEN
            IF (VOUT='0') THEN
                FEED_BACK<= FEED_BACK+'1';
            END IF;
        END IF;
    END IF;   

    IF (COUNT>"01111000" AND COUNT<"01111100")THEN
        REG<=FEED_BACK;
        TEMP1<=VREF-REG;
        IF(TEMP1>"01000000") THEN
           TEMP2<=TEMP1+"11111111";
            V_ERR<=TEMP2+'1';
        END IF;
        IF (TEMP1<"01000000") THEN
                V_ERR<=TEMP1;
        END IF;

        PWM<=V_ERR+VREF;

        IF (PWM>"11000000")THEN
             PWM<="11000000";
        IF(PWM<"00001010")THEN
            PWM<="00001010";

        END IF;
        END IF;

    END IF;

    PWM_NEW<= PWM;

   IF (RISING_EDGE(CLK))THEN
        IF(COUNT="01111111")THEN
            COUNT<="00000000";
            FEED_BACK<="00000000";
        END IF;
    END IF;

    IF(RISING_EDGE(CLK))THEN
        COUNT2 <= COUNT2+ '1';
    END IF;

   IF(COUNT>"00000000" AND COUNT<("00000010"))THEN
        IF(RISING_EDGE(CLK)) THEN
            M1<='0';
            M2<='0';
        END IF;
   END IF;

    IF(COUNT>("00000010") AND COUNT<("00000010"+PWM_NEW))THEN
        IF(RISING_EDGE(CLK)) THEN
            M1<='1';
            M2<='0';
        END IF;
   END IF;

    IF(COUNT>("00000010"+PWM_NEW) AND COUNT<("00000100"+PWM_NEW))THEN
        IF ( RISING_EDGE(CLK)) THEN
            M1<='0';
            M2<='0';
        END IF;
    END IF;

    IF(COUNT>("00000100"+PWM_NEW) AND COUNT<("10000000"))THEN
        IF (RISING_EDGE(CLK)) THEN
            M1<='0';
            M2<='1';
        END IF;
    END IF;

    IF (COUNT=("10000000"))THEN
        IF (RISING_EDGE(CLK)) THEN
            COUNT2<="10000001";
        END IF;
    END IF;

END PROCESS;
end Behavioral;`

I tried looking up the error message and got different answers. The possible reasons appear to
1: Improper “IF” nesting which is not according to the synthesis template.
2: The use of “risisng_edge(clk)” instead of usual “(clk’event and clk=’1′)”.

I am still not totally sure what could be the exact problem. It would be really helpful if someone could suggest the possible errors I am overlooking.

Answer 1

In order to be recognized by synthesis tools, your process must have a single if rising_edge(clk) block.

It should be easy to adapt your code, except for the block with reg <= feed_back;

If this specific section models an asynchronous behavior, then move it to a combinational process.

Regarding the possible causes 1. and 2. you listed, your code is ok with respect to both : the nesting is ok (syntactically) and your usage of rising_edgeis ok.