LP2387 ENET_REF_CLK specs

Discussion created by lpcware Employee on Jun 15, 2016
Latest reply on Jun 15, 2016 by lpcware
Content originally posted in LPCWare by eeckenrode on Thu Feb 25 09:04:56 MST 2016
We have a two board design where we connect a LAN8720A daughter board to a third party LPC2387 main board that we purchase.  We have noticed some radiated emission issues to the point we cannot pass emission certifications for our product.  The ENET_REF_CLK passes through a connect between the boards.  This is the source of our emissions issue.  We have been able to reduce emissions significantly by slowing the edges on the ENET_REF_CLK with a capacitor.  We are looking for LPC2387 timing requirements for the MAC interface (in particular the ENET_REF_CLK) because we need to determine if we are violating any timing requirements.  We are most curious if rounding the ENET_REF_CLK signal is a major concern?

Placed a 47pF cap on ENET_REF_CLK.  This essentially makes the clock signal mostly sinusoidal. 
Trace lengths on Daughter Card - 0.25 inches (impedance matched)
Trace lengths on Main Board - 0.375 inches
Connector between boards
As tested with additional capacitor, signal integrity seems good and data transmission seems OK in the lab.