so I've been trying to solve some questions in the book Free Range VHDL. The question I have problems with is:
Provide a VHDL behavioral model of the D flip-flop. The S and R inputs are an active low asynchronous preset and clear. If both the S and R inputs are asserted simultaneously, the output of the flip-flop will toggle.
My code is:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity sds is
Port ( d,clk,s,r :in std_logic;
f: out STD_LOGIC);
end sds;
architecture Behavioral of sds is
signal sr: std_logic_vector(1 downto 0);
signal outs : std_logic;
begin
sr <= s & r; --concatenating set and reset
process(clk,sr)
begin
case sr is
when "00" => outs <= not outs; --should toggle when both set and reset are 0
when "01" => outs <= '1'; --output should be 1 when set is 0
when "10" => outs <= '0'; --output should be zero when set is 1 and reset is 0
when "11" => if rising_edge(clk) then outs <= d; --classic d flip flop
end if;
when others => outs <= '0';
end case;
end process;
f <= outs;
end Behavioral;
The code works fine on my FPGA except for the toggling part. I cannot make the output toggle when both set and reset are 0. It always produces 1. Where is my mistake in the code? Thank you all.
Best Answer
Read the Synthesis Style Guide for your FPGA.
Synthesis may use the closest type of FF available on the FPGA and there may be none that implement that precise functionality.
Also, look at the synthesised circuit, identify the FF that was used, and read its description (paying attention to what it does with S and R both asserted).
Not all valid VHDL is synthesisable, and the synthesis guide is there to tell you what can or can't be implemented.
A good useful guide is to follow the supported patterns for synchronous processes, as described in the synthesis style guide. These will work : deviating from them may sometimes work, but will not always be well supported by the tools.
And of course, a different synthesis tool or a different FPGA may deliver different results.