I've already done the datapath for my system using a synchronous reset and now I have to control everything using state machines. What I'm confused about right now is whether or not the reset signal should be tied to a separate state which then sends a reset signal to all the processes or have the reset signal tied to the control and data-path processes.

I'm not under any timing, space or speed constraints but I would like to know if one method is always better than the other or if certain conditions apply. Any advice or direction would be appreciated.

EDIT: I've looked at this whitepaper on the topic of resets but I'm still not sure.


1 Answer 1


In an FSM it is a good idea to two states, one for the asynchronous reset and one for the reset and define them so there is no room for the synthesizer to generate hardware without them.

Use an asynchronous reset if the clock is not available at power up (which is determined by your power and clock design) this will allow the state machine to be set to a known state while the clock is not running. If your designing the schematic, make sure you follow guidelines for generating a clean async signal (if it doesn't have a clean rise time this can create metastablity problems).

You should always have a synchronous reset to memory elements to set the element to a known state.

The code (for memory) look like this (for VHDL). I'm too lazy to write an FSM so I'll copy this counter in to illustrate (just change the counter logic to an FSM (FSM's are very similar to counters):

  if ARST_L = '0'  then
    DAC_COUNT <= 0;
  elsif rising_edge(SYS_CLK) then
    if DAC_START_H = '1'  or DAC_RST_H = '1'  then
      DAC_COUNT <= conv_std_logic_vector(10#0#, 5);
    elsif (DAC_COUNT < conv_std_logic_vector(10#21#, 5) )     then
      DAC_COUNT <= DAC_COUNT + 1;
    end if;
  end if;
end process;

ARST_L is an asynchronous reset, and is not gated by the clock. In this particular design it goes high as the power comes on, thus resting everything to a known state.

DAC_RST_H is a clocked reset, and sets the counter (or FSM to a known state).

  • \$\begingroup\$ Shouldn't the third line be if ARST_L='1'? \$\endgroup\$
    – SRR
    Mar 28, 2019 at 22:52
  • 1
    \$\begingroup\$ No, you want the async reset to be low until the voltage comes up to an acceptable level. There is no way to get a a '1' if the system voltage hasn't ramped up to its nominal value \$\endgroup\$
    – Voltage Spike
    Mar 29, 2019 at 5:57

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