I am totally confused among these 4 terms: always_ff, always_comb, always_latch and always. How and for what purpose can these be used?
1 Answer
always is the main type of process from Verilog, the other is an initial which is ran once at the start of a simulation.
always_ff @(posedge clk) :
Represents a flip-flop (ff), the process is triggered (executed) on every positive edge of the clock. This replaces always @(posedge clk). This is the only type where non-blocking (<=) assignments should be used, as this mimics the way a flip-flop transfers data.
always_ff @(posedge clk) begin a <= b; end always_latch : is for representing latches.
Usage would be :
always_latch begin if (enable) begin a_latch = something; end //No else clause so a_latch's value //is not always defined, so it holds its value end This replaces :
always @* begin if (enable) begin a_latch = something; end //No else clause so a_latch's value //is not always defined, so it holds its value end always_comb:
Is for combinatorial logic, it is replacement for always @* when you do not want a latch. Now we can now differentiate our design intent between when we want and do not want latches.
The SystemVerilog names always_ff, always_latch and always_comb have stricter criteria for when they are triggered, this means the chance for RTL to Gate level (post synthesis) mismatch is reduced. It does mean the are not 100% equivalent to there always @ counter part and may change some simulation behaviour.
