ID 683082
Date 9/26/2022
Public

2.3.1.3. Use Synchronized Asynchronous Reset

To avoid potential problems associated with purely synchronous resets and purely asynchronous resets, you can use synchronized asynchronous resets. Synchronized asynchronous resets combine the advantages of synchronous and asynchronous resets.

These resets are asynchronously asserted and synchronously deasserted. This takes effect almost instantaneously, and ensures that no datapath for speed is involved. Also, the circuit is synchronous for timing analysis and is resistant to noise.

The following example shows a method for implementing the synchronized asynchronous reset. You should use synchronizer registers in a similar manner as synchronous resets. However, the asynchronous reset input is gated directly to the CLRN pin of the synchronizer registers and immediately asserts the resulting reset. When the reset is deasserted, logic “1” is clocked through the synchronizers to synchronously deassert the resulting reset.

Figure 21. Schematic of Synchronized Asynchronous Reset

The following example shows the equivalent Verilog HDL code. Use the active edge of the reset in the sensitivity list for the blocks.

Verilog HDL Code for Synchronized Asynchronous Reset


module sync_async_reset (
input    clock,
input    reset_n,
input    data_a,
input    data_b,
output   out_a,
output   out_b
);
reg     reg1, reg2;
reg     reg3, reg4;
assign  out_a    = reg1;
assign  out_b    = reg2;
assign  rst_n    = reg4;
always @ (posedge clock, negedge reset_n)
begin
if (!reset_n)
begin
reg3     <= 1’b0;
reg4     <= 1’b0;
end
else
begin
reg3     <= 1’b1;
reg4     <= reg3;
end
end
always @ (posedge clock, negedge rst_n)
begin
if (!rst_n)
begin
reg1     <= 1’b0;
reg2     <= 1;b0;
end
else
begin
reg1     <= data_a;
reg2     <= data_b;
end
end
endmodule  // sync_async_reset


To minimize the metastability effect between the two synchronization registers, and to increase the MTBF, the registers should be located as close as possible in the device to minimize routing delay. If possible, locate the registers in the same logic array block (LAB). The input reset signal (reset_n) must be excluded with a set_false_path command:

set_false_path -from [get_ports {reset_n}] -to [all_registers]

The set_false_path command used with the specified constraint excludes unnecessary input timing reports that would otherwise result from specifying an input delay on the reset pin.

The instantaneous assertion of synchronized asynchronous resets is susceptible to noise and runt pulses. If possible, you should debounce the asynchronous reset and filter the reset before it enters the device. The circuit ensures that the synchronized asynchronous reset is at least one full clock period in length. To extend this time to n clock periods, you must increase the number of synchronizer registers to n + 1. You must connect the asynchronous input reset (reset_n) to the CLRN pin of all the synchronizer registers to maintain the asynchronous assertion of the synchronized asynchronous reset.