RapidIO
•
The peak-to-peak differential signal of the transmitter output or receiver input, is 2 × (A – B) volts.
TD or RD
A V
TD or RD
B V
Figure 35. Differential Peak-to-Peak Voltage of Transmitter or Receiver
To illustrate these definitions using numerical values, consider the case where a LVDS transmitter has a
common mode voltage of 1.2 V and each signal has a swing that goes between 1.4 and 1.0 V. Using these
values, the peak-to-peak voltage swing of the signals TD, TD, RD, and RD is 400 mV. The differential
signal ranges between 400 and –400 mV. The peak differential signal is 400 mV, and the peak-to-peak
differential signal is 800 mV.
A timing edge is the zero-crossing of a differential signal. Each skew timing parameter on a parallel bus
is synchronously measured on two signals relative to each other in the same cycle, such as data to data,
data to clock, or clock to clock. A skew timing parameter may be relative to the edge of a signal or to the
middle of two sequential edges.
Static skew represents the timing difference between signals that does not vary over time regardless of
system activity or data pattern. Path length differences are a primary source of static skew.
Dynamic skew represents the amount of timing difference between signals that is dependent on the activity
of other signals and varies over time. Crosstalk between signals is a source of dynamic skew.
Eye diagrams and compliance masks are a useful way to visualize and specify driver and receiver
performance. This technique is used in several serial bus specifications. An example compliance mask is
shown in Figure 36. The key difference in the application of this technique for a parallel bus is that the data
is source synchronous to its bus clock while serial data is referenced to its embedded clock. Eye diagrams
reveal the quality (cleanness, openness, goodness) of a driver output or receiver input. An advantage of
using an eye diagram and a compliance mask is that it allows specifying the quality of a signal without
requiring separate specifications for effects such as rise time, duty cycle distortion, data dependent
dynamic skew, random dynamic skew, etc. This allows the individual semiconductor manufacturer
maximum flexibility to trade off various performance criteria while keeping the system performance
constant.
In using the eye pattern and compliance mask approach, the quality of the signal is specified by the
compliance mask. The mask specifies the maximum permissible magnitude of the signal and the minimum
permissible eye opening. The eye diagram for the signal under test is generated according to the
specification. Compliance is determined by whether the compliance mask can be positioned over the eye
diagram such that the eye pattern falls entirely within the unshaded portion of the mask.
Serial specifications have clock encoded with the data, but the LP-LVDS physical layer defined by
RapidIO is a source synchronous parallel port so additional specifications to include effects that are not
found in serial links are required. Specifications for the effect of bit to bit timing differences caused by
static skew have been added and the eye diagrams specified are measured relative to the associated clock
in order to include clock to data effects. With the transmit output (or receiver input) eye diagram, the user
can determine if the transmitter output (or receiver input) is compliant with an oscilloscope with the
appropriate software.
MPC8540 Integrated Processor Hardware Specifications, Rev. 4
Freescale Semiconductor
57
FREESCALE [ Freescale ]
