It may seem silly to implement a register module rather than let the HDL infer it, but doing so separates data and control at the most basic level, including various kinds of resets, which are part of control. This separation of data and control allows us to simplify the control logic and reduce the need for some routing resources.
On FPGAs, the initial state of registers is set in the configuration bitstream and applied by special power-on reset circuitry. The initial state of a design is available "for free" and can be returned to at run-time, which removes the need for that control and data logic.
The asynchronous reset is not implemented here as its existence prevents register retiming, even if tied to zero. This limitation complicates design and reduces performance as we would have to manually place registers to properly pipeline logic. If you absolutely need an asynchronous reset for ASIC implementation or for some critical registers, use the Register_areset instead.
If you need to clear the register during normal operation, use the synchronous clear input. This may create extra logic, but that logic gets folded into other logic feeding data to the register, and would have been necessary anyway but present as another case in the surrounding logic. Having a clear input allows us to get to the initial power-on-reset state without complicating the design.
Let's begin with the usual front matter:
`default_nettype none module Register #( parameter WORD_WIDTH = 0, parameter RESET_VALUE = 0 ) ( input wire clock, input wire clock_enable, input wire clear, input wire [WORD_WIDTH-1:0] data_in, output reg [WORD_WIDTH-1:0] data_out ); initial begin data_out = RESET_VALUE; end
Here, we use the "last assignment wins" idiom (See
Resets) to implement reset. This is also one
place where we cannot use ternary operators, else the last assignment for
data_out <= (clear == 1'b1) ? RESET_VALUE : data_out;) would
override any previous assignment with the current value of
clear is not asserted!
always @(posedge clock) begin if (clock_enable == 1'b1) begin data_out <= data_in; end if (clear == 1'b1) begin data_out <= RESET_VALUE; end end endmodule
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