A fully asynchronous reset is one that both asserts and de-asserts a flip-flop asynchronously. Here, asynchronous reset refers to the situation where the reset net is tied to the asynchronous reset pin of the flip-flop. Additionally, the reset assertion and de-assertion is performed without any knowledge of the clock. This type of reset is very common but is very dangerous if the module boundary represents the FPGA boundary.
The biggest problem with the asynchronous reset circuit described above is that, it will work most of the time. However, if the edge of the reset deassertion is too close to the clock edge and violate the reset recovery time, then the output of FF goes to metastable. The reset recovery time is a type of setup timing condition on a flip-flop that defines the
minimum amount of time between the de-assertion of reset and the next rising clock edge as shown in Figure.
It is important to note that reset recovery time violations
only occur on the de-assertion of reset and not the assertion. Therefore, fully asynchronous resets are not recommended.
SYNCHRONOUS RESET
The most obvious solution to the problem introduced in the preceding section is to fully synchronize the reset signal as you would any asynchronous signal.
The advantage to this type of topology is that the reset presented to all functional flip-flops is fully synchronous to the clock and will always meet the reset recovery time. The interesting thing about this reset topology is actually not the deassertion of reset for recovery time but rather the assertion In the previous section, it was noted that the assertion of reset is not of interest, but that is true only for asynchronous resets and not necessarily with synchronous resets. Consider the scenario illustrated in Figure.
Consider the scenario where the clock is running sufficiently slow, the reset is not captured due to the absence of a rising clock edge during the assertion of the reset signal. The result is that the flip-flops within this domain are never reset.
Fully synchronous resets may fail to capture the reset signal itself (failure of assertion) depending on the nature of the clock.
For this reason, fully synchronous resets are not recommended unless the capture of the reset signal (reset assertion) can be guaranteed by design.
Asynchronous Assertion, Synchronous De-assertionA third approach that captures the best of both techniques is a method that asserts all resets asynchronously but de-asserts them synchronously.
In Figure, the registers in the reset circuit are asynchronously reset via the external signal, and all functional registers are reset at the same time. This occurs asynchronous with the clock, which does not need to be running at the time of the reset. When the external reset de-asserts, the clock local to that domain must toggle twice before the functional registers are taken out of reset. Note that the functional registers are taken out of reset only when the clock begins to toggle and is done so synchronously.
A reset circuit that asserts asynchronously and de-asserts synchronously generally provides a more reliable reset than fully synchronous or fully asynchronous resets.
The advantage to this type of topology is that the reset presented to all functional flip-flops is fully synchronous to the clock and will always meet the reset recovery time. The interesting thing about this reset topology is actually not the deassertion of reset for recovery time but rather the assertion In the previous section, it was noted that the assertion of reset is not of interest, but that is true only for asynchronous resets and not necessarily with synchronous resets. Consider the scenario illustrated in Figure.
Consider the scenario where the clock is running sufficiently slow, the reset is not captured due to the absence of a rising clock edge during the assertion of the reset signal. The result is that the flip-flops within this domain are never reset.
In Figure, the registers in the reset circuit are asynchronously reset via the external signal, and all functional registers are reset at the same time. This occurs asynchronous with the clock, which does not need to be running at the time of the reset. When the external reset de-asserts, the clock local to that domain must toggle twice before the functional registers are taken out of reset. Note that the functional registers are taken out of reset only when the clock begins to toggle and is done so synchronously.
