WHAT IS CLOCK SKEW? HOW CLOCK SKEW WORKS?
The preceding timing analysis example is simplified for ease of presentation by assuming that the source and destination flops in a logic path are driven by the same clock signal. Although a synchronous circuit uses a common clock for all flops, there are small, nonzero variances in clock timing at individual flops.
Wiring delay variances are one source of this nonideal behavior. When a clock source drives two flops, the two wires that connect to each flop’s clock input are usually not identical in length. This length inequality causes one flop’s clock to arrive slightly before or after the other flop’s clock.
Clock skew is the term used to characterize differences in edge timing between multiple clock inputs. Skew caused by wiring delay variance can be effectively minimized by designing a circuit so that clock distribution wires are matched in length.
A more troublesome source of clock skew arises when there are too many clock loads to be driven by a single source. Multiple clock drivers are necessary in these situations, with small variations in electrical characteristics between each driver.
These driver variances result in clock skew across all the flops in a synchronous design. As might be expected, clock skew usually reduces the frequency at which a synchronous circuit can operate.
Clock skew is subtracted from the nominal clock period for setup time analysis purposes, because the worst-case scenario shown in Fig. 1.17 must be considered. This scenario uses the same logic circuit in Fig. 1.16 but shows two separate clocks with 1 ns of skew between them.
The worst timing occurs when the destination flop’s clock arrives before that of the source flop, thereby reducing the amount of time available for the D-input to stabilize. Instead of the circuit having zero margin with a 20-ns period, clock skew increases the minimum period to 21 ns.
The extra 1 ns compensates for the clock skew to restore a minimum source to destination period time of 20 ns. A slower circuit such as this one is not very sensitive to clock skew, especially after backing off to 40 MHz for timing margin as shown previously.
Digital systems that run at relatively low frequencies may not be affected by clock skew, because they often have substantial margins built into their timing analyses. As clock speeds increase, the margin decreases to the point at which clock skew and interconnect delay become important limiting factors in system design.
Hold time compliance can become more difficult in the presence of clock skew. The basic problem occurs when clock skew reduces the source flop’s apparent tCO from the destination flop’s perspective, causing the destination’s input to change before tH is satisfied.
Such problems are more prone in high-speed systems, but slower systems are not immune. Figure 1.18 shows a timing diagram for a circuit with 1 ns of clock skew where two flops are connected by a short wire with nearly zero propagation delay.
The flops have tCO = 2 ns and tH = 1.5 ns. A scenario like this may be experienced when connecting two chips that are next to each other on a circuit board. In the absence of clock skew, the destination flop’s input would change tCO after the rising clock edge, exceeding tH by 0.5 ns.
The worst-case clock skew causes the source flop clock to arrive before that of the destination flop, resulting in an input change just 1 ns after the rising clock edge and violating tH. Solutions to skew-induced tH violations include reducing the skew or increasing the delay between source and destination.
Unfortunately, increasing a signal’s propagation delay may cause tSU violations in high-speed systems. Hold time may not be a problem in slower circuits, because slower circuits often have paths between flops with sufficiently long propagation delays to offset clock skew problems.
However, even slow circuits can experience hold-time problems if flops are connected with wires or components that have small propagation delays. It is also important to remember that hold-time compliance is not a function of clock period but of clock skew, tCO, and tH. Therefore, a slow system that uses fast components may have problems if the clock skew exceeds the difference between tCO and tH.
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