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Eight questions concerning j-tag

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Oct 24, 2005
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some questions on j-tag

I would like to understand the jtag. Maybe if I design a board with embedded jtag controller in the future?
So, I have some questions on that:

1.) How can a BS controller recognize the devices in a chain? Automatic recogition. Is it possible? Should I know the IDCODEs to recognize? Or BSDL? Are all idcodes 32bit?

2.) How can the BS controller select one IC from the many ICs in the chain, to give a commad to that? and only for that one. After that, what do the others? For what time or until what event?

3.) If a test is running in one IC, why do not recognizes the other ICs in the chain the datastream (some pattern) as commands?

4.) What is after system-reset? and after that? Who gets the first command?

5.) how can I accees to the n. device and after that, the n+1. ? or n-1. ?

6.) what is the difference between a command, and some data? how can I select, what is what the IC gets in the TDI? How can I say that: this is a command.

7.) Is there any standard, on command/bscan register lengths?

8.) Can all jtag devices operate in a chain? or some only in standalone tested device? The boundary scan controllers? Can all scan a chain with more than one jtag compliant device? I red through some SN74LVT8980-like datasheets, but it was not clear for me.

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