■ BCM5221 Product Application Note
■ BCM5220 7/7/00
Broadcom Corporation
Page 7 5221/5220-AN01 Product Application Note, Revision R
The SD+/- traces do not require special impedance considerations due to the static nature of signal detect. Note that the
actual potential of the 3.3V rail connected to the SD- voltage divider should match the 3.3V potential as measured on the
fiber transceiver power pins (VDD as measured at the transceiver power pins may be less than nominal if series inductive
filtering is used, which may be recommended by the fiber transceiver manufacturer).
Configuring the BCM5221 100Base-FX operation is accomplished simply by connecting the SD+ input to a 100Base-FX
compliant transceiver and by setting the potential of the SD- pin as shown in Figure 3. There is no way to configure for
100Base-FX operation via software.
Hewlett Packard (http://www.hp.com/HP-COMP/fiber/sg/) and Siemens (http://w2.siemens.de/semiconductor/products/37/
37651.htm) both manufacture high quality 100BASE-FX transceivers.
SD+/-. Assuming the use of a 3.3V fiber transceiver, take care to level shift the SD+ signal from the fiber transceiver to the
BCM5221 properly. Because the SD signal from the transceiver is essentially static, there is no need to treat the SD+ trace
as a transmission line. A simple level shifter is required and its placement relative to either the BCM5221 or the fiber optic
transceiver is non-critical. As with any interface, trace lengths should be kept to a minimum wherever possible. The SD- input
of the BCM5221 requires an external bias of 1.7V to ensure that the differential SD+/- input functions properly.
RD+/- . Because the RD+/- inputs of the BCM5221 are internally biased, and because typical 3.3V fiber transceivers nor-
mally source PECL with a Vbb (center of the voltage swing) of approximately 2.0V, level shifting is required. Additionally, if
the system layout requires that the RD+/- trace lengths are more than approximately one inch, take care to set up the termi-
nation to account for transmission line effects as well (as indicated in Figure 3).
TD+/- . The BCM5221 employs current-sink outputs to transmit the 100Base-FX signaling to the fiber transceiver so a spe-
cial level shifting and impedance matching termination network must be implemented. The slight mismatch in the final volt-
age divider for the TD+ versus TD- ensures that the TD+/- input of the fiber transceiver will not switch due to noise that might
be present when the BCM5221 is quiet.
SERIAL 10M (7-WIRE INTERFACE)
The BCM5221 will interface to most MicroProcessors via serial 10M across the MII (also referred to as 7-Wire interface).
This mode can only be enabled via the MDIO interface by writing a 1 to bit 1 of register 1Eh.
Figure 4 on page 8 illustrates the connection between the BCM5221 and a typical µP. Note that this figure represents con-
nection with the more popular Motorola µPs (such as the MPC850, 860, etc).
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