Serial RapidIO
17.3 Signal Definitions
LP-serial links use differential signaling. This section defines terms used in the description and
specification of differential signals. Figure 51 shows how the signals are defined. The figures show
waveforms for either a transmitter output (TD and TD) or a receiver input (RD and RD). Each signal
swings between A volts and B volts where A > B. Using these waveforms, the definitions are as follows:
1. The transmitter output signals and the receiver input signals TD, TD, RD, and RD each have a
peak-to-peak swing of A – B volts.
2. The differential output signal of the transmitter, V , is defined as V – V .
TD
OD
TD
3. The differential input signal of the receiver, V , is defined as V – V .
RD
ID
RD
4. The differential output signal of the transmitter and the differential input signal of the receiver
each range from A – B to –(A – B) volts.
5. The peak value of the differential transmitter output signal and the differential receiver input
signal is A – B volts.
6. The peak-to-peak value of the differential transmitter output signal and the differential receiver
input signal is 2 × (A – B) volts.
TD or RD
A Volts
TD or RD
B Volts
Differential Peak-to-Peak = 2 × (A – B)
Figure 51. Differential Peak–Peak Voltage of Transmitter or Receiver
To illustrate these definitions using real values, consider the case of a CML (current mode logic)
transmitter that has a common mode voltage of 2.25 V and each of its outputs, TD and TD, has a swing
that goes between 2.5 and 2.0 V. Using these values, the peak-to-peak voltage swing of the signals TD and
TD is 500 mVp-p. The differential output signal ranges between 500 and –500 mV. The peak differential
voltage is 500 mV. The peak-to-peak differential voltage is 1000 mVp-p.
17.4 Equalization
With the use of high speed serial links, the interconnect media will cause degradation of the signal at the
receiver. Effects such as Inter-Symbol Interference (ISI) or data dependent jitter are produced. This loss
can be large enough to degrade the eye opening at the receiver beyond what is allowed in the specification.
To negate a portion of these effects, equalization can be used. The most common equalization techniques
that can be used are:
•
A passive high pass filter network placed at the receiver. This is often referred to as passive
equalization.
•
The use of active circuits in the receiver. This is often referred to as adaptive equalization.
MPC8548E PowerQUICC™ III Integrated Processor Hardware Specifications, Rev. 6
78
Freescale Semiconductor
FREESCALE [ Freescale ]
