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Series termination

Series termination, or source termination, is a technique used 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.

ARM recommends that all outputs from the target system be simulated to ensure that a reliable signal is delivered to the DSTREAM 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 DSTREAM is 50Ω.

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

Table 2-18 Typical series terminating resistor values

Driver strength Typical series terminator  
32mA 39Ω Best signal integrity, highest speed
24mA 33Ω  
16mA 27Ω  
12mA 22Ω  
8mA 15Ω  
6mA 10Ω Worst signal integrity, lowest speed

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