Series termination

Series (or source) termination is a technique employed in point-to-point signaling to ensure that no excessive overshoot or ringing occurs. This is achieved by reducing the source voltage by approximately 50% close to the driver. When the signal reaches the end of the transmission line, the high impedance of the receiver causes a reflection which approximately doubles the signal back to its original amplitude. When the reflection returns to the series terminating resistor, the potential across the resistor drops to zero which prevents any more current from entering the transmission line. From the perspective of the receiver, this gives a perfect 100% logic transition without any overshoot or ringing.

It is recommended that all outputs from the target system be simulated to ensure that a reliable signal is delivered to the RealView® ICE probe. Some overshoot/undershoot is acceptable but it is recommended to keep this below ~0.5V. Beyond this point, the clamping diodes at the receivers will start to cause high transient currents which in turn cause increased crosstalk, radio emissions and target power usage.

The target signal impedance for use with RealView ICE is 50Ω.

The following table lists some typical series terminating resistor values for instances when the outputs cannot be simulated.

Table 6. Typical series terminating resistor values

Driver strengthTypical series terminator 
32mA39ΩBest signal integrity, highest speed
24mA33Ω 
16mA27Ω 
12mA22Ω 
8mA15Ω 
6mA10ΩWorst signal integrity, lowest speed

Some types of IC use “impedance matched” outputs to improve their signal integrity. This is usually achieved by using weaker drive transistors to slow down the edge transitions. This has the side effect of limiting the data throughput of the driver.

To achieve the highest data rates with the best signal integrity, it is recommended to use a strong and fast driver and appropriate series terminating resistor.

If it is determined that series terminating resistors are not required, it is recommended that 0Ω links be placed close to the driver as a fall-back option.

When series terminating multiple signals, it is common to use small quad resistor packages. This saves board space and reduces parasitic effects without much risk of placement or tombstoning issues during production.

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