Open Source

Clock distribution in high speed board

Khodifad Pankaj

eInfochips Training and Research Academy

Sola, Ahmedabad

ABSTRACT

Clock signals toggle faster than any other signals in a digital system. For every data transition some clock must transition twice, completing a full cycle. Not only are clocks the fastest signals, they are also the most heavily loaded. Clocks connect to every flip-flop in a system, while individual data wires fan out to only a few devices each. Because they are so fast and heavily loaded, clock signals deserve special attention [1].This paper examines clock drivers, special clock routing rule, and peculiar circuits used to improve the distribution of clock signals.

I.INTRODUCTION

One of the most carefully engineered components of a synchronous digital system is the clock distribution network. The clock signal provides the temporal frame of reference by which data is transferred. Thus, the tightest control of the clock is vital to correct operation of the system. Making this design task more difficult is the fact that the clock signal typically has the most capacitive loading, the highest fan-out, the longest distance to travel, and certainly the highest switching frequency of any signal in the system. Compounding the problem further is the need for very clean and sharp transitions on the clock signal, so that its edges are detected simultaneously across the device. Industry trends in process technology and digital system design are making the clock distribution design both more demanding and a more significant factor in overall system performance. As technology scales, the interconnect widths become smaller, increasing the interconnect resistance. Digital systems are also steadily increasing in frequency of operation, nearly doubling this parameter every two years[2].The increase in interconnect resistance coupled with the demand for faster systems has elevated the significance of the clock distribution network on system performance.

II. TIMING MARGIN

The circuit in Figure 1 is a 2-bit ring counter, also called a switch-tail counter. When clocked at low speeds, the bit pattern at Q1 repeats forever (...00110011...). As we raise the clock frequency in Figure 1, the circuit emits the same pattern until at some high frequency the circuit fails. The circuit fails because of a lack of setup time for flip-flop 2. At the failure frequency, each transition at Q1 emerges from gate G too late to meet the setup time requirement of D2. Figure 2 diagrams this failure mode. When clocked at or beyond the failure frequency, the circuit no longer produces an 0011 output sequence. This type of failure is called a timing margin failure.

Figure 1. 2-bit Ring Counter

Figure 2.Timing Analysis of 2bit Ring Counter

The timing margin is defined in this circuit as the amount of time remaining between

(1) The time when signals actually emerge from gate G

(2) The time when signals at D2 must be valid to meet the setup requirement of flip-flop 2.

The timing margin measures the slack, or excess time, remaining in each clock cycle. A system with a big timing margin on every circuit can usually run at a higher clock speed without error.

As the clock speed in Figure 1 approaches its failure frequency, the timing margin drops to zero. Never operate a circuit near its failure frequency. Reduce the maximum operating speed for any circuit somewhat below the failure frequency, leaving a small positive timing margin under all operating conditions. A positive timing margin protects your circuit against signal crosstalk which may slightly perturb the edge transition times, general miscalculations that often occur when counting logic delays, and later minor changes in the board design or layout [3].

Many designers aim for a positive timing margin equal to about one gate delay. When working with slow logic families, this rule of thumb allots more timing margin than when working with fast logic families. This keeps the timing margin fixed as a percentage of delay over a wide range of designs. You will have to decide how much excess timing margin is acceptable.

The timing margin depends on both the delay of logic paths and the clock interval. Either too long a delay or too short a clock interval can cause a timing margin failure. As explained in the next section, differential delays between the clock signals CLKI and CLK2 can also cause a timing margin failure.

III. CLOCK SKEW

Let's take a closer look at timing margins. Figure 3 dissects our ring counter circuit, showing the components of timing margin analysis. We seek the worst-case timing margin. Figure 3 calculates the latest possible time of arrival for pulses emerging from gate G, comparing that to the earliest possible arrival time required by the setup conditions of flip-flop 2. The latest possible arrival time for a pulse coming through gate G is

In above Equation we use maximum delay times for all elements. We also assume that the clock pulse of interest occurs at time zero; no absolute time reference appears in Equation. The pulse from G gets clocked into flip-flop 2 on the next clock pulse. This clock occurs at time TCLK and propagates through path C2 to input CLK2. The earliest possible arrival for the next clock at CLK2 is Flip-flop 2 requires a valid input at least Tsetup seconds before this CLK2. The arrival time required by flip-flop 2 is [1].

Trequired =TCLK +TC2min - Tsetup [2]

Where,

Trequired= elapsed time by which data from G must arrive, ns

TCLK = interval between clocks, s

TC2 min = minimum delay of path C2, s

Tsetup = worst-case setup time required by flip-flop 2, s

Figure 3.Timing Analysis showing Clock skew

Equation 2 uses the minimum delay time for path C2, which moves the required data arrival time to the early side. This would be the worst condition. Data from G must arrive before Trequired to properly set flip-flop 2. In mathematical terms, we require

Tslow < Trequired

This constraint may be expanded using Equations 1 and 2.

In words, the clock interval must exceed the flip-flop delay, the gate G delay, and the setup time. These three terms make perfect sense because all three events must occur in sequence each cycle. The last term takes more explaining. It involves the difference in clock arrival times at nodes CLK1 and CLK2. This difference is called clock skew. If the clock arrives late at flip-flop 1, then output Q1 also occurs late, and our timing margin deteriorates. If delay C2 is unusually small, flip-flop 2 gets clocked earlier, and data must be valid earlier to meet the setup time. This also deteriorates our timing margin. In either case we must increase the clock interval, slowing down system performance, to fix the problem. Clock skew always affects timing margins[1].

III. USING LOW-IMPEDANCE DRIVERS

The brute force method for low skew has two parts:

(1) Locate all clock inputs close together.

(2) Drive them from the same source.

If a system has many clock inputs that cannot be physically collocated, the simple brute force method fails. In that case, try the spider distribution network. This network, drawn in Figure 4, distributes clocks from a single source to N remote destinations. Reflections are damped by resistive terminations R at the end of each spider leg.

Using a transmission line impedance of 75 ohm, a network of three spider legs presents a 25-52 composite load to its driver. Some commercial chips drive loads that low, but not many. To service more spider legs, we need a more powerful clock driver. Two or more driver outputs connected in parallel make a convenient and simple high-powered driver. Always draw the paralleled outputs from a common integrated circuit. Outputs from the same chip have only a small skew between them and are thus unlikely to burn each other out when connected in parallel.

Figure 4.Spider Legs Clock Distribution

The clock distribution tree in Figure 5 trades quantity for power. This scheme distributes clocks through a tree network to their final destinations. Balancing the tree with equal numbers of identical gate types helps reduce clock skew.

Figure 5.Clock Tree

IV. SOURCE TERMINATION OF MULTIPLE CLOCK LINES

On the basis of Figure 6 some engineers attempt to drive multiple source-terminated lines from a single driver. This figure shows that the input impedance of a source-terminated line is twice that of an end-terminated line. Not only that, the drive current requirement drops to zero after 2T seconds, lowering the average power drain. These facts tempt us to assume that a single gate can drive multiple source-terminated lines.

Figure 6.Single Clock Driver feeding two terminated line.

If the driver output impedance were zero (it never is), there would be no cross-coupling between lines and we could simply use a separate series terminating resistor of value R = Z0 on each line. Unfortunately, the reality of finite driver impedance forces us to contemplate joint resonance. The paragraphs below show low to jointly analyze the system. Skipping ahead to the answer, multiple source termination with nonzero driver impedance works only if the lines are equally long and each end are balanced. The source-termination resistors must equal

Rs = Zo- Rdrive*N

Where,

Rs = source termination resistor, ohm

Z0 = driven line impedance, ohm

Rdrive = effective output resistance of driver, ohm

N = number of driven lines

V. CONCLUSION

Timing margin measures the slack, or excess time, remaining in each clock cycle. Timing margin protects your circuit against signal crosstalk, miscalculation of logic delays, and later minor changes in the layout. Clock skew has as much of an impact on overall operating speed as any other propagation delay. Two or more driver outputs connected in parallel make a convenient and simple high-powered driver. The total drive power required for TTL clock signals is 25 times that of ECL circuits. A single driver can service two or more source-terminated lines under restricted circumstances.

VI.REFERENCE

1. . http://en.wikipedia.org/wiki/Clock_signal

2. Low Jitter Clock Distribution Networks,Dissertation Proposal,Sean Stetson,The University of Michigan

3. High Speed Digital Design By H.W.Johnsons