Digital System Resets
Designing a reset architecture for a digital device such as an ASIC, FPGA, CPLD, etc. can be challenging. Resets are a common culprit of metastability and unpredictable behavior. Here I will discuss various reset architectures and how to properly use them.
Before you can begin to understand resets you must first understand flip-flops. Flip-flops are the basic building block of all digital synchronous circuits. Flip-flops are used to hold state between clock edges. Flip-flops come in MANY varieties. Flip-flops usually have between 0 and 2 signals that represent some sort of “reset”. The 3 most common flip-flops are shown below (clock enables not shown):
Flip-flops don’t actually need any reset logic built-in. External logic such as multiplexers can be used to emulate all the functionality of internal reset logic. However, adding reset logic to the flip-flop directly greatly reduces the overall logic footprint.
Asynchronous Resettable Flip-Flop:
An asynchronous reset scheme enables a flip-flop to inherit a value when a specific signal is active. The two asynchronous signal names are typically referred to as “preset” and “clear”. Using positive logic, when the “preset” line is high, the output of the flip-flop is immediately forced high independent of the clock’s state and the input data. Likewise, when the “clear” line is high, the output is forced low.
This waveform shows a simple asynchronous reset process. On the first rising clock edge, the output ‘Q’ is set low because the input ‘D’ is low. On the second rising clock edge, the output now goes high as a result of the input. Between clock 2 and 3 the asynchronous clear signal goes high. As soon as the signal reaches a full logic level 1, the output of the flip-flop is immediately forced low.
Synchronous Resettable Flip-Flop:
A synchronous reset scheme enables a flip-flop to inherit a value when a specific signal is active during an active clock edge. The two synchronous signal names are typically referred to as “set” and “reset”. Using positive logic and positive clock edges, when the “set” line is high during a positive clock edge, the flip-flop is forced high independent of the input data. Likewise, when the “reset” line is high during a positive clock edge, the flip-flop is forced low.
This waveform shows a simple synchronous reset process. On the first rising clock edge, the output ‘Q’ is set low because the input ‘D’ is low. On the second rising clock edge, the output now goes high as a result of the input. Between clock 2 and 3 the synchronous reset signal goes high. This change does not effect the flip-flop output value until the third rising clock edge. At this point the output is driven low even though the input signal is still high.
What Needs to be Reset?
A good reset design approach is “reset only what needs it”. Things that need to be reset are flip-flops that must be put in a known state. Common examples are: finite state machine flip-flops; incrementing or decrementing counters; and control pipelines.
In general, data paths do not need to be reset. Adding a reset to a large data path can cause excessive resource usage and routing delays. Take care when deciding which flop-flops need to be reset.
Before deciding what reset architecture to use, let’s first define the advantages and disadvantages of the two styles.
- Flip-flops immediately take the value of reset without dependence on a clock edge.
- No signal synchronization needed for asynchronous input reset signals (like a push button reset).
- Coming out of reset often causes metastability.
- Chip-wide asynchronous resets cause modules to come out of reset at different times due to inconsistent delay paths.
Advantages of Synchronous Resets:
- All modules come out of reset at the same time and timing assumptions can safely be made about module interfaces.
- All clock/reset timing is taken care of by standard synthesis.
- Designs with large area will use excessive routing resources while trying to meet timing constraints.
- Relies on the existence of a clock. Signals won’t be reset until an active clock edge.
Note: this topic applies to all types of digital devices. Each device type (FPGA, ASIC, etc.) will have optimal setups, but understanding your options will help you decide how to safely reset your device.
The Asynchronous Reset Problem:
For asynchronous resets, going into a reset state isn’t a problem. When software tools are synthesizing, and placing components, asynchronous resets are a simple task because they are not related to a specific clock and have no timing constraints.
Asynchronous resets create a problem when the reset signal is being deactivated. If the reset is released near an active clock edge the results of that clock cycle are unknown. The following waveform shows this scenario:
At the start of the second clock cycle the clock rises and the clear signal falls. What should the flip-flop be set to? Will the input ‘D’ win the fight or will the clear signal? The answer is that we don’t know. Not knowing the state of a signal will certainly cause issues. An even bigger problem is the violation of the setup and hold time requirements of the flip-flop. Violating these requirements results in metastability.
Consider a state machine that has 3 states and is one-hot encoded with 001, 010, and 100. Now consider the asynchronous deactivation problem. What if bits 1 and 2 got reset but bit 3 did not? The state could then be 101 and the circuit’s logic would consider the state machine to be in two states simultaneously. Obviously this would kill the design.
Some designers attempt to overcome this problem by first synchronizing the reset to the appropriate clock domain then using it as a synchronous reset. If this new synchronous reset is used globally, you’ve effectively converted your design to a synchronous reset architecture. If the new reset signal is only used locally, you’ll create problems due to not knowing exactly when adjacent modules are in or out of reset.
The Synchronous Reset Problem:
Unlike asynchronous resets, synchronous resets must travel between flip-flops in one clock period. During synthesis and place & route, the software tools will ensure that each reset signal will arrive at its destination before the active clock edge that it triggers on. This may seem like a good thing because the designer now doesn’t need to worry about violating the setup and hold times of the flip-flops being reset. This is true, but only on a small scale.
Synchronous resets, specifically global synchronous resets, create routing problems that lead to sub-optimal timing results. Using a global synchronous reset effectively means that every block must see the same reset signal every clock cycle. Routing one signal to all locations of a chip in one clock cycle requires a massive amount of routing resources or, depending on the clock speed and die size, is impossible.
Consider a large design with 3 major sub-designs. Each sub-design must communicate with all other sub-designs so it is important to know that each block comes out of reset on the same clock cycle. This is the main idea behind a global synchronous reset.
The small red block is a module that synchronizes the input reset to the clock in order to provide a synchronous reset to the rest of the chip. Now consider the results if all 3 blocks directly use the reset as a synchronous reset. All flip-flops using the reset signal will draw current from reset source. For a large design, the fanout of this structure will cause most designs to fail static timing analysis.
From our discussion thus far, it’s apparent that working with synchronous resets is easier because the software tools will provide proper timing. The first thing we need to do is synchronize the asynchronous input reset to our clock domain. The synchronizer below outputs a reset that activates asynchronously and deactivates synchronously. Using this style of synchronizer gives us the advantages of asynchronous resets and the safety of synchronous resets.
Now that we have a good reset signal we need to spread it across the chip efficiently. We will create ‘M’ parallel reset pipelines of ‘N’ flip-flops. ‘M’ is is the number of major blocks the design contains. ‘N’ is determined according to clock speed and die size. It needs to be high enough such that each reset pipeline can meet timing while delivering the reset to the desired location.
This figure shows M=3 and N=6. The 3 separate pipelines are of equal length so each of the 3 blocks will receive the reset at the same time. The 6 pipeline stages allow the place & route tools to easily make it across the chip while still meeting timing. The pipeline stages work just like the synchronizer in that they produce a asynchronous reset assertion and a synchronous reset deassertion.
After the HDL is in place to generate the circuits described above, synthesis and timing constraints must be used in order for this reset architecture to work.
- A synthesis directive must be placed on all flip-flops in the pipeline stages informing the synthesizer to keep all flip-flop instances. By default, the synthesizer will see that the pipeline stages are parallel versions of each other and “optimize” them away. For Synopsys constraints, the “syn_keep” directive will perform this task.
- In order to use the advantage of the asynchronously asserted reset, the reset must be used asynchronously in the HDL. Because it is asynchronous, the synthesizer will assume no timing dependencies relative to the clock. However, we must guarantee that the deassertion of the reset is synchronous. A place & route constraint must be placed between all stages of the pipeline and between the last stage and its destination. The constraint must ensure that the reset reaches its destination without violating the setup and hold times of the input flip-flops. If the reset is used synchronously, this step can be skipped.