metastability would not be a concern because all timing conditions for the flip-flops would be met. If the data changes in this region, as shown the figure. When you check for the hold time, no matter how long you wait, the assert will not fail.-- check hold time wait for t_h; assert intern'delayed'stable(t_h + t_su) This change in testbench (similar to what you have done for setup violation) should solve the problem. In other words, each flip-flop (or any sequential element, in general) needs some time for the data to remain stable before the clock edge arrives, such that it can reliably capture the data. Setup Time. This article assumes that the reader has at least a basic understanding of what a … Similarly hold time is the amount of time, data has to remain stable after a clock edge has appeared. The blue area represents the t h or Hold Time. This usually involves the output of the flip-flip taking on an intermediate voltage level, but there’s also the possibility that it starts to oscillate. The time signifies the minimum duration of data stability before the arrival of rising/falling clock edge. These constraints dictate the max and min delays of a computational logic between flip-flops. The register output then is available after a specified cloc k-to-output delay (tCO). We will cover the basics of STA: setup time and hold time constraints. Setup and Hold Times nSetup Time: the amount of time the synchronous input (D) must be stable before the active edge of the clock nHold Time: the amount of time the synchronous input (D) must be stable after the active edge of the clock nIf either is violated correct operation of the FF is not guaranteed nMetastability can result. It then needs to be stable until at least the hold time after the clock edge. Timing violations lead to metastability. DELAY FLIP-FLOP (DFF) METASTABILITY IMPACT ON CLOCK AND DATA RECOVERY (CDR) AND PHASE-LOCKED LOOP (PLL) CIRCUITS by Alfred Sargezisardrud Modeling delay flip-flops for binary (e.g., Alexander) phase detectors requires paying close attention to three important timing parameters: setup time, hold time, and clock edge-to-output (or briefly C2Q time). Metastability is an important source of errors in computer systems [Cha73].
{Back to Digital Logic metastability Index} Setup time- Setup time is measured at the input of the flip-flop with respect to rising/falling edge of the clock to the flop. Resolve time (among others) has to be looked up, via the data sheet (if it's provided). In this case, it may violate the flip-flop’s setup or hold violation times and cause the little scamp to enter a metastable condition. This article explains what setup and hold times are and how they are used inside of an FPGA.
As a rule: The faster the flip flop used, the better the MTBF for a given circuit. DELAY FLIP-FLOP (DFF) METASTABILITY IMPACT ON CLOCK AND DATA RECOVERY (CDR) AND PHASE-LOCKED LOOP (PLL) CIRCUITS by Alfred Sargezisardrud Modeling delay flip-flops for binary (e.g., Alexander) phase detectors requires paying close attention to three important timing parameters: setup time, hold time, and clock edge-to-output (or briefly C2Q time). The amount of time the synchronous input (D) must be stable after the active edge of the clock. This duration is known as setup time. As illustrated below, metastability can happen to registers when their setup or hold times are violated; that is, if the data input changes within the capture window. Setup, Hold time & metastability of a flop. By deasserting le after input d changes, the hold check should fail.
The new data needs to reach the second FF before the next clock edge by at least the setup time. As a result, the output of the register may enter a metastable state, which involves oscillating between logic 0 and 1 values. If either setup time or hold time violates, correct operation of FF is not guaranteed . So together they define a "setup-hold-window", in which data has to remain stable. What are the cases in which metastability occurs? transition to an outptu state within the guaranteed propagation time of the device, people start talking about meta-stability. Hold time is similar to setup time, but it deals with events after a clock edge occurs. Hold time is the minimum amount of time required for the input to a Flip-Flop to be stable after a clock edge. When the clock skew/slew is too much (rise and fall time are more than the tolerable values). Setup time and Hold time are important concepts to understand for every digital designer. Metastability Characterization Report for Microsemi Flash FPGAs Introduction Whenever asynchronous data is registered by a clocked flip-flop, there is a probability of setup or hold time violation on that flip-flop. If the flip-flop's setup time is 20 ns, it means that data has to be stable atleast 20ns before the capturing clock-edge. In previous post, we discussed setup time and hold time definitions. If any of the constraint is not met, we call it as timing violation. Also, we know from theory two important concepts, "setup time" and "hold time". Setup time is defined as the minimum amount of time before the clock's active edge that the data must be stable for it to be latched correctly. We must also consider what happens if the setup and hold times on a register are not met.
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