Micrel, Inc. KS8721BL/SL
June 2009
16
M9999-062509-1.3
10/100 BASE-T
Media Dependent Interface
10/10
BASE-T
Media Dependent Interface
Transmit Pair
Receive Pair
Transmit Pair
Receive Pair
Modular Connector (RJ45)
HUB
(Repeater or Switch)
Modular Connector (RJ45)
HUB
(Repeater or Switch)
Figure 2. Crossover Cable
Power Management
The KS8721BL/SL offers the following modes for power management:
• Power-Down Mode: This mode can be achieved by writing to Register 0.11 or pulling pin 30 PD# low.
• Power-Saving Mode: This mode can be disabled by writing to Register 1fh.10. The KS8721BL/SL turns off
everything except for the Energy Detect and PLL circuits when the cable is not installed. In other words, the
KS8721BL/SL shuts down most of the internal circuits to save power if there is no link. Power-saving mode is in
the most effective state when auto-negotiation mode is enabled.
100BT FX Mode
Please contact your local field application engineer (FAE) for a reference schematic on fiber connection.
100BT FX mode is activated when FXSD/FXEN is higher than 0.6V (this pin has a default pull down). Under this mode,
the auto-negotiation and auto-MDI-X features are disabled.
In fiber operation, the FXSD pin should connect to the signal detect (SD) output of the fiber module. The internal
threshold of FXSD is around 1/2 V
DD
±50mV (1.25V ±0.05V). Above this level, the fiber signal is considered detected.
The operation is summarized in the following table:
FXSD/FXEN Condition
Less than 0.6V 100TX mode
Less than 1.25V,
but greater than 0.6V
FX mode
No signal detected
FEF generated
Greater than 1.25
FX mode
Signal detected
Table 1. 100BT FX Mode
To ensure proper operation, the swing of fiber module SD should cover the threshold variation. A resistive voltage
divider is recommended to adjust the SD voltage range.
Far End Fault (FEF), repetition of a special pattern which consists of 84-one and 1-zero, is generated under “FX mode
with no signal detected.” The purpose of FEF is to notify the sender of a faulty link. When receiving an FEF, the LINK
will go down to indicate a fault, even with fiber signal detected. The transmitter is not affected by receiving an FEF and