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[SOLVED] JTAG serial TAP connection

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Jun 1, 2011
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I have a question related to the JTAG (1149.1) protocol, related to a serial connection of TAPs. It is stated in the Standard that in the Capture-IR state the lowest two bits of IR are changed to 01. So, if I load an instruction to the first TAP, and then load a second instruction to the same TAP - hoping that the first one will now affect the second TAP in scan chain - how can it be that this bit alternation will not ruin the instruction code?

For example, the first instruction has code 0b1100, after Capture-IR it is 0b1101. When I load the second instruction to the first TAP - 0b1101 is what i get in the second TAP IR, right?


You can assume, that all devices except the active one are loaded with a BYPASS instruction on UPDATE_IR.

Yes, but it seems that I did not explain well what my real problem is.

I'll try to explain now :)

Let's say we have two TAPs connected in series. I want to load instruction X to the second TAP. How do I do it? I thought that the right way is to load X to the first TAP, then BYPASS -> this way I should get X in TAP2 and BYPASS in TAP1 so I can send data related to X directly to TAP2...

Yes, but it seems that I did not explain well what my real problem is.
I think, the problem is, that you didn't fully understand JTAG operation.

The BYPASS instruction is only affecting the data register selection. The IR operations always work on the chained instruction registers of all connected devices. That means, you have to load instruction for all devices prior to each UPDATEIR operation.
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Thanks, FvM!
I was pretty sure that this was how it is supposed to be, but the IP I have is not behaving like that - it always pulls up TMS after an instruction so my slave FSM goes to Update-IR then Capture-IR instead of staying in Shift-IR until all instructions pass through. The question was if I had to change the slave or the master should behave differently (at least should be driven differently to achieve this).

The JTAG don't need to know about the chain structure, but the master must. The total IR chain length corresponds to the sum of individual IRs, and the bypass registers have to be considered when accessing the DR.

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