# JTAG DV UVM Agent

JTAG UVM Agent is extended from the DV library agent classes, which provide common knobs and verification logic that help ease the effort of writing interface agents.

## jtag_if

This interface is used to drive and monitor the JTAG transactions. It is the physical layer of the testbench, which directly connects to the JTAG signals of the DUT.

## jtag_dtm_reg_block

This RAL model represents the JTAG DTM register space. This region is defined in the RISC-V debug specification 0.13.2, chapter 6. It mirrors the registers in the design, which is useful to predict what we thought we wrote to these registers and compare what we expect the design to return on reads. This RAL model is hand-written, given that it is only a few set of registers. It can be used to drive reads and writes using our standard CSR routines. It can also be used in higher level testbench components to develop the predicted verification logic.

This object extends from the standard uvm_reg_adapter class, to implement the underlying mechanics of accessing the modeled JTAG DTM registers in jtag_dtm_reg_block using the JTAG sequencer. An instance of this class is created in the jtag_agent, and hooked up to the default_map of jtag_reg_block to facilitate the CSR accesses via the RAL model.

## jtag_agent_cfg

This agent configuration class provides a handle to the jtag_if instance. It also creates the JTAG DTM RAL model as a member. An instance of this class is created externally and passed on to jtag_agent via uvm_config_db. The jtag_agent then sets the cfg handle in all sub-components, so that they all have access to it.

## jtag_item

This represents the JTAG transaction class. It holds the IR as well as DR information. It is used by the driver as well as the monitor. The ir_len and dr_len fields indicate whether to drive or sample an IR update, or DR update or both types of transactions. If each of these lengths is 0, then that type of transaction is not driven / monitored.

The dout field indicates the captured DR value.

## jtag_driver

This component receives transactions from sequences via the sequencer using the standard UVM TLM mechanism. If the ir_len and dr_len in the received transaction item are both non-zero, it drives the IR update first, followed by the DR update. If any of these lengths is 0, then that type of transaction is not driven. It throws an error if both of these lengths are 0.

When driving the DR update, it also captures the DR by sampling tdi into the transaction item’s dout field, which is sent back to the sequence as response.

Currently, the driver is only implemented in host mode. The device mode implementation will be addressed in future.

## jtag_monitor

The JTAG monitor implements an FSM to detect an IR update cycle or a DR update cycle. It discretely captures the IR update and DR update separately and writes the transaction to the analysis_port. Transactions coming from this monitor on the analysis_port will only have either the ir_len or the dr_len set to a non-zero value. The higher level testbench components must check these lengths to determine whether the IR was updated or the DR.