PLX Technology PEX 8618, Quick Start Manual

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1.1 Transmitter
A PCI Express Transmitter is typically a differential CML driver that transmits an 8b/10b encoded bit-stream
across the channel to the Receiver. The minimum differential voltage swing (V
TX-DIFF-PP
) of the Transmitter is
800 mV at both 2.5 GT/s and 5.0 GT/s. The DC common mode voltage can be anywhere between 0 and 3.6V;
hence, AC-coupling capacitors are required to isolate the Transmitter’s DC component from the Receiver’s fixed
0V DC common mode voltage. The AC-coupling capacitor values must range between 75 nF and 200 nF, to
ensure that the lower frequency components of the 8b/10b encoded data are not affected.
Figure 2 illustrates
what a generic PCI Express differential signal looks like, as compared to a single-ended signal.
Note: The swing values listed in Figure 2 (400 mV and 800 mV) do not reflect default PLX register values.
PCI Express Transmitters are required to support de-emphasis. The role of de-emphasis is to reduce the amount
of energy used to transmit multiple successive bits of the same polarity (
that is, non-transition bits), compared to
the amount of energy used to transmit a set of transition bits (0 -> 1 or 1 -> 0). Transition bits have higher
frequency components than non-transition bits and are, therefore, more distorted by the low-pass channel. This
effect is also known as
Inter-Symbol Interference (ISI), which is a source of deterministic jitter in the system.
The PCI Express Base Specification, Revision 2.0 defines two de-emphasis levels for devices running at
5.0 GT/s, 3.0 to 4.0 dB and 5.5 to 6.5 dB. The desired de-emphasis level for a given Link is advertised by the
downstream Ports of a switch during Link recovery. Endpoints and switch upstream capture this value and Set
their de-emphasis level, accordingly. Longer Links should use 6.0 dB, whereas shorter Links can use the 3.5 dB
level.
The standard de-emphasis level is selectable by way of the PEX 8618 Link Control 2 register Selectable De-
Emphasis bit (Configuration register, offset 98h[bit 6]).
TXp
PEX 8618 Quick Start Hardware Design Guide – Version 1.2
Copyright © 2009 by PLX Technology, Inc. All rights reserved.
2
Figure 2. Single-Ended versus Differential Voltage
In addition to supporting the standard de-emphasis levels, the PEX 8618 has a number of programmable
registers to control the Transmitter’s characteristics,
such as drive level and de-emphasis. Registers at offsets
B98h to BA4h are the SerDes Drive Level registers. Registers at offsets BA8h to BB4h are the Post-Cursor
Emphasis Level registers. The SerDes Drive Level and Post-Cursor Emphasis Level registers work in
conjunction, to determine the transition and non-transition bits driver swing and de-emphasis ratio. The PLX driver
is implemented as a two-tap driver. When transition bits are transmitted, the
SerDes Drive Level and Post-
Cursor Emphasis Level register levels are added together; for non-transition bits, the two values are subtracted.
Using Equation 1 , Example 1 presents a calculation of what the drive level and de-emphasis level would be for a
given set of register values.
TXn
VTX-DC-CM
TXp - TXn
400mV
800mV
0V