KS8721B/BT
Micrel
Power Management
The KS8721B/BT 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 KS8721B/BT will then turn off
everything except for the Energy Detect and PLL circuits when the cable is not installed. In other words, the
KS8721B/BT will shutdown most of the internal circuits to save power if there is no link. Power saving mode will be
in his most effective state when auto-negotiation mode is enable.
100BT FX Mode
100BT FX mode is activated when FXSD/FXEN is higher 0.6V (This pin has a default pull down). Under this mode, the auto-
negotiation and auto-MDIX features are disabled.
In fiber operation FXSD pin should connect to the SD (signal detect) output of the fiber module. The internal threshold of FXSD
is around 1/2 V
±50mV (1.25V ±0.05V). Above this level, it is considered fiber signal detected, and the operation is
DD
summarized in the following table:
FXSD/FXEN
Condition
Less than 0.6V
100TX mode
Less than 1.25V,
FX mode
but greater than 0.6V
No signal detected
FEF generated
Greater than 1.25
FX mode
signal detected
Table 1. 100BT FX Mode
To ensure a 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.
FEF (Far End Fault), 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 a FEF, the LINK will go down
toindicateafault,evenwithfibersignaldetected.ThetransmitterdoesnotaffectbyreceivingaFEFandstillsendsoutitsnormal
transmit pattern from MAC. FEF can be disabled by strapping pin 27 low. Refer to “Strapping Options” section.
Media Converter Operation
KS8721B/BT is capable of performing media conversion with 2 parts in a back to back RMII loop-back mode as indicated in
the diagram. Both parts are in RMII mode and with RMII BTB asserted (pin 21 and 22 strapped high). One part is operating
at TX mode and the other in FX mode. Both parts can share a common 50MHz oscillator.
Under this operation, auto-negotiation on the TX side will prohibit 10baseT link up. TXD2, active High, can disable transmitter
and set it at tri-state. RXD2 serves as energy detection can indicate if there is line signal detected. TXD3 should tied low and
RXD3 let float. Please contact Micrel FAE for Application Note.
Vcc
21 22
Pin
Rx +/-
Tx +/-
RxD
KS8721B
TxD
TxC/
Ref_CLK
OSC
50 MHz
TxC/
Ref_CLK
FTx
FRx
TxD
KS8721B
(Fiber Mode)
RxD
Pin
34
Pin
21 22
Vcc
To the SD pin of the
Fiber Module
Figure 3. Fiber Module
KS8721B/BT
16
August 2003
MICREL [ MICREL SEMICONDUCTOR ]
