Cirrus Logic AN Integrating CobraNet into Audio Products - Manual (Page 4)

Using CobraNet Modules in Your System P RELIMINARY
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CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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Using CobraNet Modules in Your System
Cirrus Logic, Inc. manufacturers several CobraNet modules that can be integrated into
your products. This is the easiest and quickest way to get CobraNet into your products.
Currently, the modules that are available are the CM-1, CM-1-FW and CM-2.
The modules consist of a 3.5 x 3.5 inch PCB. One or more connectors or connection
points are used to connect to a standard Ethernet network. A pair of high density
connections are used to interface the module to the host system. An optional metal
faceplate is used to facilitate mounting the module to the chassis. Details for these
modules can be found in the appropriate module’s data sheet.
Note: This applications note is intended to supplement the data sheets. It is not intended to replace or
supersede the data sheets. Also, these are guidelines only and do not apply to all situations.
Power
The CM-1 and CM-1-FW use both +3.3v and +5.0v power supplies. The CM-2 requires a
single +3.3v supply. We do not anticipate any future CobraNet modules will require the
+5v power supply. If you do not believe your design will ever use the CM-1 or CM-1-FW,
leaving the +5v pins un-powered is acceptable. However, we recommend that if +5v
power is already available within your system, it should be connected.
Maintaining Signal Quality
Maintaining signal quality is essential for proper operation, meeting FCC/CE emission
requirements, and for overall reliability. Experience has shown that the vast majority of
CobraNet hardware problems have come from lack of signal and power quality. This isn't
because implementing and using CobraNet is difficult, but rather that many electrical
engineers that design audio products are unfamiliar with high-speed digital logic (and the
analog effects of high-speed digital logic). What follows is the "accumulated wisdom" from
years of designing and debugging CobraNet products.
De-coupling capacitors (sometimes called AC signal return caps or bypass caps) are
critical for maintaining signal and power quality. Since the CobraNet modules run at high
speeds and the digital signals have very short rise/fall times, de-coupling caps cannot be
ignored.
On the host PCB, connect six 0.1uF caps between +3.3v and Ground. Distribute these
caps across the length of the connectors, keeping them as close to the connectors as
possible. Also connect a single 0.1uF cap between +5v and Ground at the +5v pins on the
connector. Even if you are using only the CM-2, which does not require +5v, the cap on
+5v must still be present. And even if your host board does not have a +5v power rail
available, connect the cap between the +5v pins and Ground (while leaving the +5v pins
un-powered).
These caps reduce the AC signal return path, thus reducing EMI and improving signal
quality. The pinout of the module host connector is optimized for signal quality and
assumes that these caps will be used on the host PCB.