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--- fpga:vhdl [2021/12/02 07:48] – created utedass
+++ fpga:vhdl [2025/06/13 12:33] (current) – utedass
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 ====== VHDL ======
+[[https://opencores.org/|opencores]] -- Mostly verilog? Might find something useful
-[[https://www.ics.uci.edu/~jmoorkan/vhdlref/Synario%20VHDL%20Manual.pdf|Synario VHDL Manual]]
+===== Things is should write down =====
+  * Math-functions for determining bit width (IEEE.math_real.all)
+  * Generic notation
+  * natural and conversion to integer and real
+  * Defining functions
+  * Entity instantiation [[https://stackoverflow.com/questions/36229368/component-instantiation-vs-entity-instantiation-in-vhdl|Stack overflow]]
+  * State machines using type
+  * Some simulation specific stuff like after 3ps, assert and until rising_edge(clk)
+  * High impedance value of std_logic ('Z'), only for ports
+  * Naming conventions once I have settled for one
+  * Generate statement
+  * Immediate values syntax
+===== Conversions =====
+<code vhdl>
+library ieee;
+use ieee.std_logic_1164.all; -- std_logic_vector
+use ieee.numeric_std.all; -- signed and unsigned
+-- numbers, math operations allowed
+signal my_int : integer; -- 32 bit 2-complement
+signal my_int_ranged : integer -1 to 23; -- Limited range. Simulator might enforce, synthesiser might not
+signal my_uint : unsigned (7 downto 0);
+signal my_sint : signed (7 downto 0) := x"0a"; -- Default values might not synthesize, depends on target
+-- loose bits, math operations not allowed
+signal my_bit : std_logid;
+signal my_bits : std_logic_vector (7 downto 0);
+-- Integer assignements
+my_int <= 1337; -- Decimal
+my_int <= 16#beef#; -- Hex (base 16)
+my_int <= 2#101010; -- Binary (base 2)
+-- std_logic assignement
+my_bit <= '0';
+-- Assignments to std_logic_vector (also works on signed and unsigned)
+-- All assignments must match in width
+my_bits <= (others => '0'); -- Set all bits to '0'
+my_bits(3 downto 0) <= (others => '1'); -- Set lowest 4 bits to '1'
+my_bits <= "10101011"; -- Set with bit array
+my_bits <= x"ab"; -- Set with hex
+my_bits <= '1' & '0' & "10" & x"b"; -- Concatenate different notations
+-- conversions between integers
+my_uint <= to_unsigned(my_int, 8); -- Must specify bit length
+my_sint <= to_signed(my_int, 8);
+my_int <= to_integer(my_uint);
+my_int <= to_integer(my_sint);
+-- casts between binary representations, lenght must match
+my_uint <= unsigned(my_sint);
+my_uint <= unsigned(my_bits);
+my_sint <= signed(my_uint);
+my_sint <= signed(my_bits);
+my_bits <= std_logic_vector(my_uint);
+my_bits <= std_logic_vector(my_sint);
+-- From ASCII code value
+my_ascii_code <= std_logic_vector(to_unsigned(character'pos('A'), my_ascii_code'length));
+-- From number immediate values to std_logic_vector
+my_immediate_hex <= std_logic_vector(to_unsigned(16#150#, my_immediate_hex'length));
+my_immediate_dec <= std_logic_vector(to_unsigned(10#150#, my_immediate_dec'length));
+</code>
+<code vhdl>
+-- Char to unsigned
+tx_char <= to_unsigned(character'pos('a'), 8);
+</code>
+===== Textio =====
+Can be used in simulation.
+<code vhdl>
+use std.textion.all;
+-- Further down...
+  process is
+    variable l : line;
+  begin
+    write (l, String'("Hello world!"));
+    writeline (output, l);
+    wait;
+  end process;
+</code>
+===== Other things =====
+<code vhdl>
+report "this is a message"; -- severity note
+-- or:
+report "this is a serious message" severity warning; -- Possible values: note, warning, error, failure
+</code>