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I know that JTAG was specifically developed to support daisy-chaining of multiple devices, but I am still worried that my board will show up and I won't be able to talk to one of the devices because I did something wrong. Does anyone have any advice on what the common pitfalls are or what precautions should be taken when implementing more than 1 device on the JTAG bus?

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  • \$\begingroup\$ Look at voltage! Different voltages - like 3.3V and 5V - can not be chained. Some cores may have even lower voltage on JTAG lines. \$\endgroup\$
    – Turbo J
    Aug 6, 2011 at 9:15

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1) Don't put very many devices on one bus.

2) If there's something for which jtag is really critical, and you need jtag extensions for firmware download or debug, put it on it's own separate bus so electrical issues on a longer one don't force you to run at a painfully slow clock rate.

3) See (instead or as well) if you can design end-to-end functional tests that would detect assembly or component errors, thus including coverage of the parts that don't have jtag capability.

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