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Design Flow of the Virtual JTAG Megafunction

Designing with the Virtual JTAG megafunction includes the following processes:

• Configuring the Virtual JTAG megafunction with the desired Instruction Register length and instantiating

the megafunction.

• Building the glue logic for interfacing with your application.

• Communicating with the Virtual JTAG instance during runtime.

In addition to the JTAG datapath and control signals, the Virtual JTAG megafunction encompasses the VIR.

The Instruction Register size is configured in the MegaWizard Plug-In Manager. The Instruction Register

port on the Virtual JTAG megafunction is the parallel output of the VIR. Any updated VIR information can

be read from this port after the virtual_state_uir signal is asserted.

After instantiating the megafunction, you must create the VDR chains that interface with your application.

To do this, you use the virtual instruction output to determine which VDR chain is the active datapath, and

then create the following:

• Decode logic for the VIR

• VDR chains to which each VIR maps

• Interface logic between your VDR chains and your application logic

Your glue logic uses the decoded one-hot outputs from the megafunction to determine when to shift and

when to update the VDR. Your application logic interfaces with the VDR chains during any one of the non-

shift virtual JTAG states.

For example, your application logic can parallel read an updated value that was shifted in from the JTAG

port after the virtual_state_uir signal is asserted. If you load a value to be shifted out of the JTAG port,

you would do so when the virtual_state_cdr signal is asserted. Finally, if you enable the shift register to

clock out information to TDO, you would do so during the assertion of virtual_state_sdr.

Maintaining TDI-to-TDO connectivity is important. Ensure that all possible instruction codes map to an active

Note that TCK (a maximum 10-MHz clock, if using an Altera programming cable) provides the clock for the

entire SLD infrastructure. Be sure to follow best practices for proper clock domain crossing between the

JTAG clock domain and the rest of your application logic to avoid metastability issues. The decoded virtual

JTAG state signals can help determine a stable output in the VIR and VDR chains.

After compiling and downloading your design into the device, you can perform shift operations directly to

the VIR and VDR chains using the Tcl commands from the quartus_stp executable and an Altera

programming cable (for example, a USB-Blaster

™

, a MasterBlaster

™

, or a ByteBlaster

™

II cable). The

quartus_stp executable is a command-line executable that contains Tcl commands for all on-chip debug

features available in the Quartus II software.

The figure below shows the components of a design containing one instance of the Virtual JTAG

megafunction. The TDI-to-TDO datapath for the virtual JTAG chain, shown in red, consists of a bank of DR

registers. Input to the application logic is the parallel output of the VDR chains. Decoded state signals are

used to signal start and stop of shift transactions and signals when the VDR output is ready.

The IR_out port, not shown, is an optional input port you can use to parallel load the VIR from the FPGA

core logic.

Virtual JTAG Megafunction (sld_virtual_jtag)

Altera Corporation

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Design Flow of the Virtual JTAG Megafunction

2014.03.19

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