Altera Virtual JTAG IP Core - Manual (Page 8)

Figure 5: Extension of the JTAG Protocol for PLD Applications
IR Shift Registers
IR Update Registers
DR Shift Register 1
DR Update Register 1
USER 0 Data Registers
USER 1 Data Registers
TDI TDO
TAP
Controller
Output
TAP
Controller
Output
Altera PLD JTAG Extension
Altera PLD JTAG Extension
Node 1
Node N
USER0 / USER1 and
SLD_HUB Control Signals
TDI TDO
VIR
VDR 1
VDR N
VIR
VIR 1
VIR N
SLD Hub Finite State Machine
The SLD hub decodes TMS independently from the hard JTAG TAP controller state machine and implements
an equivalent state machine (called the “SLD hub finite state machine”) for the internal JTAG path. The SLD
hub performs a similar function for the VIR and VDR chains that the TAP controller performs for the JTAG
IR and DR chains. It enables an SLD node as the active path for the
TDI
pin, selects the TDI data between
the VIR and VDR registers, controls the start and stop of any shift transactions, and controls the data flow
between the parallel hold registers and the parallel shift registers of the VIR and VDR.
Because all shifts to VIR and VDR are encapsulated within a DR shift transaction, an additional control
signal is necessary to select between the VIR and VDR data paths. The SLD hub uses the
USER1
command
to select the VIR data path and the USER0 command to select the VDR data path.
Virtual JTAG Megafunction (sld_virtual_jtag) Altera Corporation
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SLD Hub Finite State Machine 8
2014.03.19