Integrating CobraNet
into Audio Products
(Advice from the Experts)
™
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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RELIMINARY
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Introduction
This applications note is intended to help hardware designers integrate the CobraNet
interfa ce into an audio system design. It covers some of the finer points involved in the
use of either the CS18101 de vice or CPB-18101-CM-2 (CM-2 module). F or more det ailed
electrical specifications , t iming di ag r ams , etc . p lease refer to the app ropriate Cirrus Logi c
data sheet. Additional information is available from Cirrus Logic at http://www.cirrus.com
and Cirrus Logic’s Professional Audio Division at http://www.peakaudio.com.
Whether your design requires the ease of use and upgradability provided by the
CobraNet CM-2 module or the cost-effective, on-board integration of the CS18101 and
support circuitry within your product design, CobraNet technology provides a pow erful,
easy-to-use solution for networked audio.
The CS18101 and CM-2 module provide up to eight audio channel s with input and out put
at 48kHz or 96kHz sampling and 32-bit resolution.
The 100Base-TX Ethernet interface is fully compliant with the IEEE802.3u standard.
Remote control, monitoring, and management is accomplished through an Ethernetbased Simple Network Management Protocol (SNMP) agent. This allows the use of
inexpensive category-5 cabling for new installations but the system can even be “piggybacked” onto an existing network.
IntroductionP
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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Using CobraNet Modules in Your System P
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 th at 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 requi res 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-po wered is acceptable. However , we recommend that if +5v
power is already available within your system, it should be connected.
RELIMINARY
Maintaining Signal Quality
Maintaining signal quality is essential for proper operation, meeting FCC/CE emission
requirements, and for over all 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 difficul t, but rather that many el ectrical
engineers that design audi o products are unfamil iar with high-speed digital logic (and the
analog effects of high-speed digital logic). What f oll ows is the "ac cumulated 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 canno t 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 connector s as
possible. Also connec t 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 boar d does no t 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.
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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RELIMINARY
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 hav e o nly a single load. This is true f or outputs as
well as inputs. In the case of an outpu t from the module having multiple loads, a buff er
should be used. If the signal is an input to the module then there shou ld 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.
Using CobraNet Modules in Your SystemP
The CM-2 has 25
improve si gnal 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 i t. W e recommend th at new des igns connec t to HADDR3 e v en if t he
design will never use the CM-2, since future CobraNet Modules will likely use HADDR3.
If pull-up or pull-down resi stors are placed on the SSI_DOUT signal s (in the e v ent 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 res istors on t he CobraNet modul e so any
additional resistors should be sufficiently weak to not interfere.
Unlike the CM-1, the HEN# sign al 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.
Ω source-termination resistors on the SSI_CLK and FS1 signals to
CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004
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