VSC8201DL
MAC Transmit and Receive Interface Pins
These signals on these pins have rise/fall times of about 0.7 to 1 ns. To adequately accommodate these signals on a PCB, it is recom-
mended that the traces be designed as either a microstrip or stripline transmission lines with a characteristic impedance of 50Ω . It is
also important that an unbroken ground plane exist above and/or below these signals.
For the MAC transmit interface, if the output resistance of the corresponding pins on the MAC side is less than 50Ω , additional series
termination resistor should be placed close to the MAC device.
For the VSC8201 MAC receive interface, each pin is self-calibrating to an output resistance of 50Ω . Thus, external series termination
resistors are not required as long as the characteristic impedance of the trace is 50Ω .
As shown in the previous section, for a rise time of 0.7 ns, the minimum length of a microstrip for it to be considered a transmission line
is 0.827 inches (for standard FR4 material with dielectric constant of 4.4). Hence above mentioned series termination considerations are
important only if the microstrip length is greater than 0.827 inches.
For most cases the MAC can be placed close to the PHY’s MAC interface pins.
Twisted-Pair Interface pins
As the interface to external CAT-5 cable, these pins are organized in four differential pairs for each port.These are labelled “TXIP_x” and
“TXIN_x”, where ‘x’ is the particular pair within a single cable. When routing these pairs on a PCB, they must be routed using transmis-
sion lines with a characteristic impedance of 50Ω , and the traces for each differential pair must be routed symmetrically as close to each
other as possible. By keeping the differential pairs together on the board, the result is the required 100-ohm differential impedance for
each twisted pair.
Other Impedance Controlled pins
The following signals have 50 ohm integrated series termination resistors and therefore should be routed using 50Ω transmission lines:
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ADDR(0)/LINK10
ADDR(1)/LINK100
ADDR(2)/LINK1000
ADDR(3)/DUPLEX
ADDR(4)/ACTIVITY
MODE10
MODE100
MODE1000
TDO
OSC_EN/CLK125
REG_EN/QUALITY
MDIO
Other Board Layout Considerations
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Place the RJ-45, transformer and the PHY device as close as possible to each other.
All traces that run from the transformer to the RJ-45 should be matched in length and be as short as possible.
Keep a continuous ground plane underneath the high speed MAC Tx/Rx data signal traces. These traces should be of equal length
and be as short as possible.
Rev. 1.6.3 - 9/29/04
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