Product Application Note ■ BCM5221
7/7/00 ■ BCM5220
Broadcom Corporation
5221/5220-AN01 Product Application Note, Revision R Page 8
Figure 4: BCM5221 Serial 10 (7-Wire) Interface
AUTO-MDI/MDIX CROSSOVER
In most 10/100Base-TX connections, one end of the link is configured as an MDI (Medium Dependent Interface) crossover
so that each transceiver’s transmitter is connected to the other’s receiver. This allows the end user to install straight-through
cables. However, there are many instances where cross-over cables are required and this has caused significant confusion
and downtime in the field. The BCM5221 contains the ability to perform Auto-MDI/MDIX crossover on chip, thus eliminating
the need for crossover cables or cross-wired (MDIX) ports.
During auto-negotiation and 10/100BASE-TX operation, the BCM5221 normally transmits on TD± and receives on RD±.
When connected via a straight-through cable to another device that does not perform the Auto-MDI/MDIX crossover, the
BCM5221 automatically switches its transmitter to RD± and its receiver to TD± to communicate with the remote device. If
two devices are connected that both have Auto-MDI/MDIX crossover capability, then a random algorithm determines which
end performs the crossover function.
The Auto-MDI/MDIX crossover feature is a function of auto-negotiation. If the BCM5221 is configured not to perform Auto-
Negotiation, the feature does not work, and a specific cable is required to ensure the Transmitter at one end of the cable is
connected with the Receiver at the other end of the cable. This feature is enabled by default, but can be disabled by setting
RENA
RXDV
TXD[0]
TXD
RXD
RXD[0]
CRS
TXENTENA
CLSN COL
RXER
COL
TCLK
RCLK
BCM5221
Motorola
µ
µµ
µ
P
IMPORTANT
: Setup and Hold timing
must be carefully considered in order
to ensure proper data transfer timing
across the MII in both directions.
TXC
RXC
TXD[3:1]
RXD[3:1]
n/c
n/c
n/c
n/c
n/c
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