Cirrus Logic AN Integrating CobraNet into Audio Products - Manual (Page 5)

Using CobraNet Modules in Your System P RELIMINARY
3
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
w w w . c i r r u s . c o m
Signals to and from the module should be kept as short as possible. High-speed traces
on the host board longer than about 4 inches should be avoided or use buffers. Keep in
mind that "high speed" in this case refers to signals with short rise and fall times, not just
the frequency of the signal.
Loading and Termination
Signals to and from the module should have only a single load. This is true for outputs as
well as inputs. In the case of an output from the module having multiple loads, a buffer
should be used. If the signal is an input to the module then there should be no additional
loads on that signal on the host PCB. Obviously there are situations where this rule can
be relaxed, but more often than not this is the case.
Pay close attention to signal termination on the MCLK_OUT (FS512_OUT), FS1, and
SSI_CLK signals. If these signals are used as outputs from the module to your host PCB
then the board must include some sort of R-C termination at the end of the trace. On the
other hand, if these signals are inputs to the module from the host PCB then we
recommend using series termination at the driver.
The CM-2 has 25
Ω
source-termination resistors on the SSI_CLK and FS1 signals to
improve signal quality. On some systems, this may conflict with some end termination
schemes.
Special Signals
The CM-1 (and CM-1-FW) pin RSVD0 is used as HADDR3 on the CM-2 and there is a
pull-down resistor on it. We recommend that new designs connect to HADDR3 even if the
design will never use the CM-2, since future CobraNet Modules will likely use HADDR3.
If pull-up or pull-down resistors are placed on the SSI_DOUT signals (in the event that the
CobraNet module is not installed), these resistors should be very weak (greater than
50k
Ω
). There are already pull-up and pull-down resistors on the CobraNet module so any
additional resistors should be sufficiently weak to not interfere.
Unlike the CM-1, the HEN# signal is used on the CM-2. CM-1 documentation has
recommended that the HEN# be tied low on the CM-1. This recommendation is
compatible with the CM-2.