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I wiil DIY a develop board like MCB2130 ,I don't kown what it is the function of ETM.Is it must need in this board? thanks
Click on search at the top of this page and type in ETM. It looks like there are several pages to read that should answer your question.
"I don't kown what it is the function of ETM. Is it must need in this board?" It's part of the chip - whether you think you "need" it or not! You need to study the Datasheet carefully before designing-in any chip! http://www.keil.com/dd/chip/3648.htm
ETM is considered the second level of debug interface. Look at the ARM specs for the purpose and operation of ETM. At present the Keil tools DO NOT support ETM. Some ARM tools support ETM with an optional purchased module plus their ADS and Multi-Ice. The 2138 board has the physical and electrical break out but the connector is not populated. If you have space on your DIY board, I suggest that you include the ETM interconnect for possible future use just as they implemented the 2138 eval board.
ETM stands for "Embedded Trace Macrocell", and is a module that provides enhanced software debugging capability that some chip manufacturers might include in their product. The ETM lets you set up breakpoints to trap on writes to particular addresses, for example. Used in conjunction with fancy JTAG probes, the probes can store large amounts of execution history and even try to single step "in reverse". You can read all about the component at arm.com.