■ BCM5221 Product Application Note
■ BCM5220 7/7/00
Broadcom Corporation
Page 9 5221/5220-AN01 Product Application Note, Revision R
bit 11 in Register 1Ch to a 1. During operation, the MDI state can be determined by reading bit 13 of Register 1Ch, as indi-
cated in the BCM5221 data sheet. Additionally, a manual MDI swap can be forced by setting or clearing bit 12 of Register
1Ch.
The Auto-MDI/MDIX feature is implemented in the BCM5221 in accordance with the IEEE 802.3ab specification. The Auto-
matic MDI/MDI-X state machine facilitates switching between the receiving pair and the transmitting pair respectively, prior
to the auto-negotiation mode of operation so that Fast Link Pulses (FLPs) can be transmitted and received in compliance
with Clause 28 of the auto-negotiation specification. The final state of the crossover circuit is determined by an algorithm that
controls the switching function between the two pairs (transmit and receive). The algorithm uses an 11-bit Linear Feedback
Shift Register (LFSR) to create a pseudo-random sequence with which the BCM5221 determines its initial configuration. In
the BCM5221, care is taken to ensure the LFSR is seeded with a unique value during reset. This determines the initial state
of MDI or MDIX.
Per the specification, a sample timer is implemented in a manner that allows the BCM5221 to detect either valid Link Pulses
or a Valid Link. If after 62+/-2 ms the receiver is unable to detect a valid Link Pulse or a valid Link, the LFSR is advanced,
switching the pairs. Additionally, an asynchronous timer is used in the unlikely event where the two PHYs have identical ref-
erence clocks and reset circuits. When this timer expires, it resets the state machine to the MDI State and restarts the sample
timer. This timer is free running and has no impact on the LFSR.
Auto-MDI/MDIX operates only on the transmit and receive data pairs (swapping them if required). It does not operate on the
individual wires within a given pair and therefore cannot correct for possible polarity swap issues. However, the 10BASE-T
transceiver within the BCM5221 includes polarity detection and correction to ensure proper functionality should a polarity
problem exist in the wiring. Note that 100Base-TX signaling is NOT sensitive to polarity and therefore does not strictly require
polarity detection and correction.
Specific magnetics and cable termination issues must be considered when using the BCM5221 in Auto-MDI/MDIX mode.
See the following section for further detail.
MAGNETICS
Most applications requires single channel magnetics to support implementations based on one or more BCM5221 devices.
In some applications, Quad channel magnetics may be more practical.
In any case, note that one important restriction in pairing magnetics with the BCM5221 relates to the ordering of isolation
transformer and common mode choke in the transmit signal path (and receive signal path when Auto-MDI/MDIX is enabled).
The transmit output signal from the BCM5221 must be connected to the isolation transformer first, followed by the common
mode choke (as depicted in Figure 5). If any common mode choke is placed between the transmit output pins and the iso-
lation transformer, potentially severe signal distortion results while operating in 10Base-T and auto-negotiation modes.
The following list includes recommended Single and Quad channel magnetics components from various vendors for use with
the BCM5221 (and BCM5220). The use of these magnetics is recommended regardless of whether Auto-MDI/MDIX is en-
abled.
Bel. S558-5999-W2 (Single) S558-5999-Q9 (Quad - for use with single height or stacked RJ connectors)
Pulse Engineering. H1102 (single) H1164 (Quad)
Halo. TG110-S050N2 (single)
Figure 5 on page 10 illustrates specific magnetics interconnect and differential cable termination requirements for applica-
tions that use Auto-MDI/MDIX.
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