Issue link: https://iconnect007.uberflip.com/i/1267313
JULY 2020 I DESIGN007 MAGAZINE 25 that the reflected pulse sees the internal driver source impedance and the resistor in series (usually totaling 50 ohms). Since this matches the transmission line, it completely absorbs the reflected energy. However, data flows in both directions from the CPU to the memory, when writing to memory, then back from the memory to the CPU for reading the memory data. Where do we place the series terminator in a point-to-point configuration—at one end of the data-trace, or maybe in the center? Out of habit—or perhaps for fear of doing the wrong thing—one would usually put the termination close to the CPU rather than the load. The simulated waveforms of the impact of having a termination resistor close to the driver, close to the load, and also in the middle of the transmission line show little distinction. However, there is a better unconventional solution: placing a termination resistor at each end of the transmission line (Figure 3). Fig- ure 4 presents an improved eye diagram using this solution for both the read/write cycles. The blue waveform is the termination at both ends—the red and green waveforms are at either end of the transmission line. Having a resistor at both ends of the trans- mission line, close to the driver and load, is an elegant solution as the resistor and input ment and routing processes. Online design rule checks (DRCs) will warn the designer when a constraint is violated. On a multilayer PCB, clock signals should be routed on a stripline (inner layer) sandwiched between two solid reference planes to reduce radiation. The spacing between the signal trace and the return planes should be as small as possible to increase coupling and reduce the loop area. The three constraints to keep in mind include: 1. Route clock signals between the planes, fanout out close to the driver (200 mils) dropping to an inner layer, and route back up to the load again with a short fanout. A series terminator is required for each load. 2. Use the same reference plane (GND if pos- sible) for the return signal, as this reduces the loop area and hence radiation. 3. Minimize crosstalk to other signals by keeping a distance of at least three times the trace width to sensitive signals. 2. Bi-Directional Data Termination With series termination, the resistor needs to be close to the source (Figure 2). This ensures Figure 2: Typical series termination with a resistor close to the driver. Figure 3: Series terminations at both ends (simulated in HyperLynx).