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Series termination

Series termination, or source termination, is a technique that is used in point-to-point signaling, to ensure that no excessive overshoot or ringing occurs.

To achieve series termination, use a series resistor to reduce the source voltage, by approximately 50%, as it is transmitted by the driver. When the signal reaches the end of the transmission line, the high impedance of the receiver causes a reflection that reverts the signal to its original amplitude. When the reflection returns to the series terminating resistor, the potential across the resistor drops to zero, preventing any further current from entering the transmission line. The receiver observes a perfect 100% logic transition, without any overshoot or ringing.

To ensure that a reliable signal is delivered to the DSTREAM-ST unit, Arm recommends that all outputs from the target system are simulated, and, if necessary, series terminated. Some overshoot or undershoot is acceptable, but Arm recommends ensuring this is kept less than ~0.5V. Above ~0.5V, the clamping diodes at the receivers start to cause high transient currents, which then cause increased crosstalk, radio emissions, and target power usage.

The target signal impedance for DSTREAM-ST is 50Ω.

When the outputs cannot be simulated, typical series terminating resistor values are:

Typical series terminating resistor values

Driver strength

Typical series terminator

Notes

32mA

39Ω

Best signal integrity, highest speed

24mA

33Ω

-

16mA

27Ω

-

12mA

22Ω

-

8mA

15Ω

-

6mA

10Ω

Worst signal integrity, lowest speed

Some types of IC use impedance matched outputs to improve their signal integrity. Impedance matched outputs are commonly achieved by using weaker drive transistors to slow down the edge transitions. Using weaker drive transistors limits the data throughput of the driver.

To achieve the highest data rates with the best signal integrity, Arm recommends using:

  • A fast and strong driver.

  • An appropriate series terminating resistor.

If you determine that series terminating resistors are not required, as a backup option, Arm recommends that 0Ω links are placed close to the driver.

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

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