I'm looking for a clean way to declare Verilog/SystemVerilog types with a parameterised bit width. This is what I've got so far and was wondering if there is a better way to do it. I've looked through the system functions in the LRM 1800-2009 and -2017. The closest I could find is $bits
, but I would like something like $minbits
. Have I overlooked something?
In VHDL, it's done by simply specifying the range
:
signal counter: integer range 0 to MAX_COUNT;
...and the compiler will calculate the minimum bit width to hold that range
.
For the parameter values of 20 ns and 125 ms, the counter should be 23 bits with MAX_COUNT being 6,250,000.
module Debounce
#(
parameter CLOCK_PERIOD_ns = 20, // nanoseconds.
parameter DEBOUNCE_PERIOD_ms = 125 // milliseconds.
)
. . .
function int MinBitWidth([1023:0] value);
begin
for (MinBitWidth = 0; value > 0; MinBitWidth = MinBitWidth + 1)
begin
value = value >> 1;
end
end
endfunction
localparam MAX_COUNT_32BITS = DEBOUNCE_PERIOD_ms * 1_000_000 / CLOCK_PERIOD_ns; // Default type of 32-bits.
localparam COUNTER_BITS = MinBitWidth(MAX_COUNT_32BITS); // Calculate actual bit width needed.
typedef logic [COUNTER_BITS - 1 : 0] TCounter;
localparam TCounter MAX_COUNT = MAX_COUNT_32BITS; // Assign to a type of the actual bit width (truncation warning from Quartus).
localparam TCounter ONE = 1;
TCounter counter;
. . .
always @(posedge clock)
begin
. . .
if (counter == MAX_COUNT_32BITS - 1) // Synthesises a 32-bit comparer no matter how many bits are needed with unused bits tied to ground.
. . .
if (counter == MAX_COUNT - ONE) // Synthesises a 23-bit comparer as expected.
. . .
counter <= counter + 1; // Synthesises a 23-bit counter as expected.
. . .
counter <= counter + ONE; // Synthesises a 23-bit counter as expected.
Incorrect Algorithm
I considered $clog2
which is the correct way to obtain an address bus width from a RAM depth parameter. However, this is not the same as the minimum bit width of a value. Let me explain...
Consider a value of 4 which is 100 base-2 (3 bits wide).
The $clog2
algorithm calculates a value of 2, which is incorrect. It should be 3. The reason for this miscalculation is because $clog2
subtracts 1 from the value before it starts to compute the number of bits, i.e. 4 becomes 3, then it calculates the minimum bit width of the value 3, giving 2 bits. While this is mathematically correct for the ceiling of log base-2, it is not the bit width of the original value.
Here is the clogb2
algorithm from the LRM:
function integer clogb2;
input [31:0] value;
begin
value = value - 1; // GOTCHA!
for (clogb2 = 0; value > 0; clogb2 = clogb2 + 1) begin
value = value >> 1;
end
end
endfunction
Correct Algorithm
The correct algorithm is to calculate the minimum bit width of the original value, which is the algorithm given by @jonathan-mayer in his first answer before he edited it.
Here is the correct algorithm as a function:
function integer MinBitWidth;
input [1023:0] value;
begin
for (MinBitWidth = 0; value > 0; MinBitWidth = MinBitWidth + 1)
begin
value = value >> 1;
end
end
endfunction