[blog] Synchronous versus asynchronous reset

Reset is about getting a system back to a known initial state. Temporary data is flushed. When I say data I mean 0s and 1s. Everything will be put to a known state so the circuit starts up. Sounds important.

When we go for synchronous or asynchronous reset, active low, our HDL code will look like one of these:

module synch_reset_pipeline(in1, in2, ..., out1, out2,...);
// #1 module with synchronous reset
always @(posedge clk) 
begin
  if (!rst) begin
   // reset logic: put this module to known state 
  end
  else begin
  // functional logic: processes data
  end
end
endmodule
module asynch_reset_pipeline(in1, in2, ..., out1, out2,...);
// #2 module with asynchronous reset
always @(posedge clk or negedge rst) 
begin
  if (!rst) begin
   // reset logic 
  end
  else begin
  // functional logic
  end
end
endmodule

To simplify, firstly, think about a single sequential element (a flip-flop). Then, imagine it is starting up, catching the very first data signal arriving on the input and propagating it to other pipeline stages. How would be a desirable reset signal? I would say:

  • it resets the circuit to a known state
  • not prone to any metastability
  • the duty cycle is just long enough to meet the above criteria

Given the above bullets what is the go for approach? Whatever fits the system better, on area, timing and power. On module #1, we are telling the tool that the reset signal is under the domain of the clock, and the reset logic will take place when reset level is low. So when the clock positive edge triggers and reset is asserted, the flops that are connected to that logic will reset. Therefore, the reset signal is pretty much another data signal (like: assign current_input = (!rst_sel) ? reset_value : next_input), selecting between the next value to be sampled or the reset data value, when a reset happens. And we should make sure it will be filtered from glitches and never prone to any meta-stability – but this is valid to any other external input too. It cannot take the fastest way through the datapath when asserted, otherwise it might reset the logic violating setup requirements, and therefore put it to a unknown state, that will be sampled and propagated. The deassertion by its turn need to respect the hold time of the signals that were propagated through the pipeline. As data and reset are on the same clock domain, the work relies pretty much on the clock tree synthesis. It costs more circuitry in general. (Although flops are smaller, they say)

On #2, the asynchronous reset is a high priority interruption, per say. The or inside the sensitivity list changes everything. Whenever reset is low (asserted) it comes to bring everything down and up again. If the pulse has a very low duty cycle, maybe the flop will not be stable yet, and will put the the circuit to an unknown state when reset is released; maybe a very bad combination (boom). That is, we need to model this asynchronous path to respect a time after asserting and before deasserting the signal, so we make sure every pipeline stage is at a known state. To prevent timing violation when asserting, we need to make sure the reset will take place respecting the (worst) removal time, so the initial data to be sampled is valid. For deasserting, we need to make sure the clock samples a stable value, by respecting the (worst) recovery time. Besides, you need to tell the tool that the signal that multiplex the input to your flops between operation data and reset data is a false path – that is the clock must ignore the impact of this signal on the data path, and not make any efforts to support setup and hold timings of the sequential elements. You will take care by yourself by controlling the reset behavior. A technique could be to model the buffers safely enough to charge and discharge on a determined relative time, so the reset removal and recovery time are always safe. That is you fix a positive delay for reset assertion and a negative delay for deassertion, regarding the active clock edge. You will need to synchronize the input reset signal to a common in both aproaches to mitigate metastability. Asynchronous reset costs less circuitry, but is also more complex: a very critical asynchronous control signal is on the game.