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26 DESIGN007 MAGAZINE I JULY 2020 capacitance of the tri-state load basically form an AC termination reducing reflections. On-die termination (ODT) is implemented with several combinations of resistors on the later versions of DDR memory. Designers can use a combination of transistors which have different values of turn-on resistance. In the case of DDR2, there are three kinds of internal resistors: 150-ohm, 75-ohm, and 50-ohm. The internal on-die termination values in DDR3 are 120 ohms, 60 ohms, 40 ohms, and so forth. But for devices that do not incorporate ODT, dual- series terminations suffice. Key Points • The three most common termination strategies are series, end, and differential. • Transmission line termination is necessary when the round trip propagation time of the signal is equal to or greater than the transition (rise or fall) time of the driver. • The objective of the clock is to provide circuit timing and thereby coordinate the activity within the system. • For clock lines with multiple receivers, it is best to route to the receiver that is the farthest away from the driver first, and then match that delay when routing to the other receivers. • The clock signal should have the longest delay of all so that the data signals have time to settle before they are clocked. • Star routing is ideal for distributing a clock to multiple loads in low- to medium-speed designs. • Clock signals generally have a fast rise time and hence are noisy due to high harmonic content; as a result, they must be isolated from the rest of the circuitry. • The propagation speed of microstrip is always faster than stripline. Figure 4: Comparison of the termination of both ends (blue) vs. the termination at either end.