6 I
2
C Interface
The PEX 8618 also implements a two-wire I
2
C Slave interface (I
2
C Port 0). Through its I2C_SCL0 and I2C_SDA0
balls, the PEX 8618 allows an external I
2
C Master to read and write device registers through an out-of-band
mechanism. The I
2
C slave address is 0111xxx, with the lower 3 bits being determined by the values on Strapping
balls I2C_ADDR[2:0]. The simplest way to implement an I
2
C interface to the PEX 8618 is illustrated in
.
Figure 14. I
2
C Interface Block Diagram
7 PCI Express Lane Good Indicators
The PEX 8618 provides 16 Active-Low “Lane Good” Output balls for each PCI Express Lane on the device.
These Output balls can be used to indicate the status of each PEX 8618 Lane. If a given Lane Good indicator is
continually asserted, that lane is up and operating at 5 GT/s. If a given Lane Good indicator is blinking, that lane is
up and operating at 2.5 GT/s.
8
Debug Functions
(The optional Debug function is primarily intended for prototyping activities. Its use requires assistance from PLX
Technical Support.)
Two major debug functions of the PEX 8618 are External Probe mode (EPM) and SerDes Debug mode (SDM).
The EPM function is for viewing the internal state machines and control signals of the station-based modules and
the core-based module. The SDM function is for viewing the 20-bit Receive Bus (elastic buffer exit) and 20-bit
Transmit Bus of each Lane of the SerDes, in the PEX 8618. Two Strapping balls are used to enable either Debug
mode function. Pulling down the STRAP_PROBE_MODE# ball enables the EPM function. Pulling down the
STRAP_SERDES_MODE_EN# ball enables the SDM function. The EPM contains 18 inputs and 38 outputs. The
SDM contains 7 inputs and 45 outputs. When either Debug mode is enabled, the EPM and SDM inputs and
outputs are serviced by most of the General-Purpose I/O balls, the Spare balls, as well as other control and status
balls such as PEX_LANE_GOOD and STRAPPING balls.
Table 2
cross-references the ball names and their
related Debug signal names.
Notes: Inputs are marked in
blue
, outputs are marked in
red
.
The maximum frequency of Debug mode Output signals, such as PROCMON (N/C, at location T6), is 125 MHz,
with fast rise and fall time. When routing these traces to the mictor connector for scope probing, 50-ohm, single-
ended controlled-impedance traces are recommended. To service normal operation and debug functions, low-
capacitive load bus switches can be used to prevent the reflections. For example, a bus switch can be used to
separate the LED circuit from the PEX_PORT_GOODx# signals, when they are used as outputs of Debug mode
signals.
PEX 8618 Quick Start Hardware Design Guide – Version 1.2
Copyright © 2009 by PLX Technology, Inc. All rights reserved.
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