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Notes
79RC32438 User Reference Manual
20 - 1
November 4, 2002
Chapter 20
EJTAG System
Introduction
This chapter describes the behavior and organization of on-chip EJTAG hardware resources on the
RC32438 device. EJTAG is a hardware/software subsystem that provides comprehensive debugging and
performance tuning capabilities to system-on-a-chip components that include a MIPS CPU core. It exploits
the infrastructure provided by the IEEE 1149.1 JTAG Test Access Port (TAP) standard to provide an
external interface, and it extends the MIPS instruction set and privileged resource architectures to provide a
standard software architecture for integrated system debugging.
Functional Description
EJTAG provides a standard debug I/O interface, enabling the use of traditional MIPS debug facilities on
system-on-a-chip components. In addition, EJTAG provides the following new capabilities for software and
system debug:
Off-board EJTAG Memory
EJTAG allows a MIPS processor in Debug Mode to reference instructions or data that are not resi-
dent on the system under test. This EJTAG memory is mapped to the processor as if it were physi-
cal memory, and references to it are converted into transactions on the TAP interface. Both
instructions and data can be accessed in EJTAG memory, which allows debugging of systems with-
out requiring the presence of a ROM monitor or debugger scratchpad RAM. It also provides a com-
munications channel between debug software executing on the processor and an external
debugging agent.
Hardware Breakpoints
EJTAG introduces two types of hardware breakpoints, which can be configured to cause a debug
exception on:
–
an instruction fetch from a specific virtual address
–
a memory reference from a specific virtual address, which additionally can be qualified by a data
value.
These breakpoints can be used to implement watchpoints and breakpoints in programs executing
out of ROM or RAM.
Single-Step Execution
EJTAG provides support for single-step execution of programs and operating systems, without
requiring that the code reside in RAM.
System Access via the EJTAG TAP
EJTAG allows an external debugging agent connected to the EJTAG TAP to obtain information
about the configuration and state of the processor under test and to force processor entry into
Debug Mode. Debug software can then provide further system access via EJTAG memory.
Debug Breakpoint Instruction
EJTAG introduces a new breakpoint instruction, SDBBP, which differs from the MIPS32 and
MIPS64 BREAK instruction in that the resulting exception, like the single-step and hardware break-
point debug exceptions described above, places the processor in Debug Mode and can fetch its
associated handler code from EJTAG memory.
EJTAG Components
EJTAG hardware support consists of several distinct components: extensions to the MIPS processor
core, the EJTAG Test Access Port, the Debug Control Register, and the Hardware Breakpoint Unit. Figure
20.1 shows the relationship between these components on the RC32438 device.