Question

I am trying to convert some Verilog code that produces a slower clock from a faster clock for a UART module. The original verilog code is based on the module over at fpga4fun.com, and this is my attempt to translate it for my VHDL-based design.

entity baud_generator is
generic(
    f_clk : integer := 50000000;  -- default: 50 MHz
    baud  : integer := 115200;    -- default: 115,200 baud
    accum_width : integer := 16;
    accum_inc : integer := (baud sll accum_width) / f_clk
);
port(
    clock : in std_logic;
    reset_n : in std_logic;
    enable : in std_logic;
    baud_clock : out std_logic
);  
end entity baud_generator;

However, my compiler, Aldec-HDL, doesn’t like the following line:

 accum_inc : natural := (baud sll accum_width) / f_clk

Here is the exact error message:

 # Error: COMP96_0300: baud_generator.vhd : (20, 52): Cannot reference "f_clk" until the interface list is complete.
 # Error: COMP96_0300: baud_generator.vhd : (20, 28): Cannot reference "baud" until the interface list is complete.
 # Error: COMP96_0071: baud_generator.vhd : (20, 28): Operator "sll" is not defined for such operands.
 # Error: COMP96_0104: baud_generator.vhd : (20, 27): Undefined type of expression.
 # Error: COMP96_0077: baud_generator.vhd : (20, 27): Assignment target incompatible with right side. Expected type 'INTEGER'.

In verilog, I have something like this:

module baud_generator(
  input clock,
  input reset_n,
  input enable,
  output baud_clock
);
parameter f_clock = 50000000;
parameter baud    = 115200;
parameter accum_width = 16;
parameter accum_inc = (baud << accum_width) / f_clock;
//...
endmodule

What is it that I need to modify in that line to make the compiler happy? Is it possible to use generics chained together like that?

Answer 1

This basically says you cannot do computations with the generic values to caluclate (default values for) other generics.

Just use accum_inc as a constant, not as a generic.

Also, the SLL (shift logic left) operator is meant for bit patterns (unsigned and signed datatypes in the ieee.numeric_std and ieee.numeric_bit packages), not for integers. You can do the same by multiplying by a power of two.