2.9 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, and, if necessary, series
terminated to ensure that a reliable signal is delivered to the DSTREAM-ST unit. 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-ST is 50Ω.
The following table lists some typical series terminating resistor values for instances when the outputs
cannot be simulated.
Table 2-8 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
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.
2 ARM
®
DSTREAM-ST target interface connections
2.9 Series termination
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