Integrating CobraNet ™ into Audio Products (Advice from the Experts) CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004 www.cirrus.
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P RELIMINARY Introduction Introduction This applications note is intended to help hardware designers integrate the CobraNet interface into an audio system design. It covers some of the finer points involved in the use of either the CS18101 device or CPB-18101-CM-2 (CM-2 module). For more detailed electrical specifications, timing diagrams, etc. please refer to the appropriate Cirrus Logic data sheet. Additional information is available from Cirrus Logic at http://www.cirrus.
Using CobraNet Modules in Your System P RELIMINARY 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.
P RELIMINARY Using CobraNet Modules in Your System 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.
Integrating CobraNet Circuitry into PCBs P RELIMINARY Integrating CobraNet Circuitry into PCBs There are several advantages to integrating the CobraNet circuitry into your own PCB rather than using one of the CobraNet modules. Signal quality, higher integration, and lower cost are just some of them. Integrating the CM-1 Circuit The CM-1 is a challenging circuit to integrate. We do not recommend integrating this circuit unless it is absolutely necessary.
P RELIMINARY Integrating CobraNet Circuitry into PCBs on the same side as the ICs, individual caps work better since it allows for a more even distribution around the ICs. The CM-2 module was designed using a 6-layer PCB and 6-mil traces. This is due to the amount of circuitry packed into the small PCB. If the PCB were larger, or the second Ethernet connector removed, a 4-layer PCB would have been adequate. However, we do not recommend using a 2-layer PCB. CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004 www.
Thermal P RELIMINARY Thermal While the CobraNet modules and the CS18101 are relatively low-power devices, heat dissipation can not be ignored. If the system will be in a sealed case with no ventilation and/or your host board is dissipating some heat then it is necessary to plan for system cooling. Thermal analysis of a complete system is complex and difficult. Like EMI, the theory behind it is well understood and documented but the practical application of it is more of an art than a science.
P RELIMINARY Thermal The Practice Theory is good, but it falls well short in practice. For starters, the results are only as good as the data and the data is often missing or questionable. Tja, Tjc and Tj(max) are often missing from the data sheets altogether. And when they are listed Tja and Tjc often have a wide error margin. To make matters worse, Pd is sometimes no better than a guess based on some ad-hoc test dreamed up by the semiconductor maker.
Thermal P RELIMINARY Tja (max) = 33.6°C/Watt. Tja (typ)= 33.6°C/Watt. Therefore: Ta(max) = Tj(max) - (Tja(max) * Pd(typ)) = 100 - (33.6 * 1.0) = 66.4°C According to the math we are not above the 70°C spec on the CM-1. That is because the math above does not take into account the heat-sink effect of the PCB. Under the FPGA are four copper planes and 49 vias that transport the heat to those copper planes. There are four more signal layers that also aid in heat conduction away from the FPGA.
P RELIMINARY Thermal There are not many other parts on the CM-2, and all of them are low-power enough to not worry about. How To Test There are so many factors that were not, and could not be, considered in this thermal analysis that testing should be considered. The first question to answer is, "Do I need to test at all?" It seems that if the ambient temperature is below approximately 55°C then you probably do not need to extensive tests. (The ambient temperature should be measured about 0.
Thermal P RELIMINARY Conclusion The practical results of all this analysis is that the CM-1 and CM-2 modules should operate just fine in an environment of 70°C ambient still air, but you need to do proper testing to verify that everything works properly. 10 CS18101 (CM-2) AppNote1 - rev 1.1 Jan, 2004 www.cirrus.
P RELIMINARY EMI EMI Like thermal issues, the theory of EMI is well known but the practice is something else entirely. There are so many factors that can effect EMI that dealing with EMI can be akin to black magic. In fact, the title of a good book on EMI reflects that (and we highly recommend that book): High Speed Digital Design: A Handbook of Black Magic Howard W. Johnson & Martin Graham Prentice-Hall, Inc.
FAQ P RELIMINARY FAQ The following are some frequently asked questions about the CM-2. Q: Does the CM-2 make the CM-1 obsolete? A: No! The CM-2 does not offer the number of audio channels that the CM-1 does. For this reason (and others) the CM-1 will remain in production. Q: Can a host system be made to use either the CM-1 or the CM-2? A: Yes. The electrical interfaces are almost identical, and with a little software design effort either can be made to work.
P RELIMINARY FAQ Q: Should I use 3.3v or 5v logic when interfacing to the CM-2 (or CM-1)? A: The CM-1 and CM-2 interface with 3.3v logic levels, but all the inputs are 5v tolerant. That being said, every effort should be made to phase out 5v circuits. Today the only major reason to use 5 volts is when interfacing to some audio converters, but all other logic is available at 3.3 volts. By using 3.3 volts you lower power consumption, lower EMI, and can use higher-speed logic.
FAQ P RELIMINARY Contacting Cirrus Logic Support For all product questions and inquiries contact a Cirrus Logic Sales Representative. To find the one nearest to you go to www.cirrus.com IMPORTANT NOTICE "Preliminary" product information describes products that are in production, but for which full characterization data is not yet available. Cirrus Logic, Inc. and its subsidiaries ("Cirrus") believe that the information contained in this document is accurate and reliable.
