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Altera_Forum
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14 years ago

jtag bypass: why isn't SCAN-IN to bypassed interfaces controlled?

In devkits, like the Arria II GX, there are jumpers for bypassing chips or interfaces (MAX II, HSMA, HSMB, etc) in the JTAG chain. These mux bypass the TDO so that the 'bypassed' interface's T...
Altera_Forum's avatar
Altera_Forum
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14 years ago

When operating a JTAG chain, the master is required to shift no-op instructions, usually BYPASS, to all devices he doesn't want to address. As far as I'm aware of, the Quartus software sends a '1' sequence in front of an instruction, so "hidden" devices behind the addressed device will be deactivated. This should be always the case, when the first chain devive (the FPGA) is addressed.

But if you are operating a JTAG devive behind the MAX II with MAX II bypassed, there's a risk of unintented actions, I think.