7
Using Ethernet with OSD335x-
AM335x System in Package
Rev.1 2/18/2019
Octavo Systems LLC
Copyright 2019
3.2
Layout
When laying out an Ethernet PHY, there are two different sets of connections: MAC to
PHY and PHY to RJ45 connector (i.e. the Ethernet port). The MAC to PHY connections
for the OSD335x Family of devices uses MII, RMII or RGMII. These have a bus speed
of 25 MHz, 50 MHz, 125 MHz, respectively. The PHY to RJ45 connections use
differential Ethernet signaling in the 10s to 100s of MHz depending on the Ethernet
speed required for the link.
The following are some general recommendations for Ethernet PHY layout design:
1. Use only 45
o
/135
o
angles in traces to avoid signal reflections.
2. Trace widths should be constant to maintain uniform impedance.
3. Length match TX_CLK, TXDx and RX_CLK, RXDx signals between AM335x and
Ethernet PHY. Depending on the interface type, the tolerance for length
matching becomes tighter due to the higher bus speeds
4. Length match TX+,TX- and RX+, RX- traces between Ethernet PHY and RJ45
connector/magnetics module. We have found that RJ45 connectors with an
integrated magnetic module are easier to route.
5. Keep the overall trace length between AM335x and RJ45 connector < 75mm.
6. If the RJ45 connector (with thru-hole pins) is placed on the top layer, route traces
to its pins on the bottom layer to avoid signal stubs caused by the thru-hole pins.
7. Avoid vias and layer changes for Ethernet traces.
