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JTAG is defined as a serial communication protocol and a state machine accessible via a TAP. But how the data is captured in capture_IR or capture_DR state it is the main issue and the data is shifted out through mux on TDO but what are the control signals at that time.
The control of JTAG is defined by the TAP state machine. Read the spec, it is well documented. The TMS level to exit each state is shown in every TAP controller FSM that you'll find.
I have read the spec, and I am not getting it correctly that in case of if I am passing an instruction then any data register is accessible according to that instruction but how the values are captured in data register.
And I also willing to know about at which phase of DFT ,JTAG comes, i.e. before production or after.
On the CAPTURE DR state the data register is loaded and the data register shifts data (TDI-DR-TDO) as long as it stays in the SHIFT_DR state with TMS = 0. If the DR is 16-bits then you keep TMS low for 16 TCK. The entire FSM is controlled by TMS.
It's used during production for checking connectivity of parts on a PCB
Both during production or after for programming devices, for debugging, and as interfaces to FPGAs, CPLDs, uP, etc.
e.g. We use JTAG to program Microsemi FPGAs in our production floor, JTAG to program prototype Xilinx boards in the lab, and debug Xilinx boards after production runs when problems are found and need to be debugged.
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