High Speed Parallel Trace

Sampling high-speed parallel buses requires special concepts to compensate for the effects of wave reflections, component tolerances, different trace lengths, limited pad driver capabilities, signal coupling etc.. All these effects might add up to a reduced signal amplitude, jitter and channel to channel skew. TPAs (Trace Port Analyzers) that cannot compensate for these issues might not be able to trace high speed targets, even if they support high data rates in theory. The real challenge however is not to process high speed data within a digital design, but to get the trace signals from the target into the TPA hardware without any information loss. Trace probes with AUTOFOCUS technology meet that challenge!

For short rise times transmission line theory has to be applied. As a rule of thumb use transmission line theory, if the rise time is smaller or equal than twice the propagation delay of the trace line:


For example, if the rise time of your trace port signals is 1 ns, your maximum trace line length (free of worries) would be 5 cm (2 inches). Above 5 cm (2 inches) trace signals have to be properly terminated to avoid/reduce the effects of wave reflections (over- and undershoots). Trace probes with AUTOFOCUS technology automatically terminate trace signals when the trace is active. All trace signals including the clock signal are terminated with 50% of the reference voltage provided on pin 12 of the trace mictor (labeled Trace A on the Preprocessor).



The effects of turning the signal termination on or off can be seen in the ShowFocus window, which is used to display the data eyes that were recognized by the Preprocessor.

For high trace port data rates (200+ Mbit/s) the signal shape within the Preprocessor is determined by a reference voltage, which is typically around 50% of the signal amplitude. The AUTOFOCUS hardware configuration includes an automatic setup of optimal reference voltages. Both clock and data channels will be setup individually. For the clock channel a reference voltage is chosen that guarantees a stable clock signal and in case of HalfRate (data on both clock edges) a 50/50 duty cycle. For the data channels a reference voltage is chosen that results in maximum data eye width.



Depending on the trace port data rate (automatically measured by the Preprocessor) the effort level used for the AUTOFOCUS hardware configuration will be increased or decreased. Again the effects of the reference voltage setup can be observed in the ShowFocus window:

Trace probes with AUTOFOCUS technology have the powerfull capability to recognize data eyes, calculate and setup optimal sampling points for every data channel (red lines in the picture below). The typical time resolution is around 280 ps. Below is an example for an 8-bit ETMv1.3 trace port with data rates of 312 Mbit/s in HalfRate (data on both trace clock edges). Please note, that in the example there is not even a common sampling point were the complete bus would have valid data! In fact a TPA (Trace Port Analyzer) that cannot compensate for channel to channel skew would fail for this real life example. Trace probes with AUTOFOCUS technology on the other hand are able to compensate for this skew.