
Debug Interface
7-2
Copyright ARM Limited 1997, 1998, 2000. All rights reserved.
ARM DDI 0192A
7.1
About the debug interface
In this chapter ARM7TDM refers to the ARM7TDMI core excluding the
EmbeddedICE Logic. The ARM7TDM debug interface is based on IEEE Std.
1149.1-1990,
Standard Test Access Port and Boundary-Scan Architecture
. Refer to this
standard for an explanation of the terms used in this chapter and for a description of the
TAP controller states.
7.1.1
Debug extensions
ARM7TDM contains hardware extensions for advanced debugging features. These are
intended to ease the development of application software, operating systems, and the
hardware itself.
The debug extensions allow you to stop the core either on a given instruction fetch
(breakpoint) or data access (watchpoint), or asynchronously by a debug-request. When
this happens, ARM7TDM is said to be in
debug state
. At this point, the internal state of
the core and the external state of the system can be examined. Once examination is
complete, the core and system state can be restored and program execution resumed.
Debug state
ARM7TDM is forced into debug state either by a request on one of the external debug
interface signals, or by an internal functional unit known as EmbeddedICE Logic. Once
in debug state, the core isolates itself from the memory system. The core can then be
examined while all other system activity continues as normal.
Internal state
The internal state of the ARM7TDM is examined through a JTAG-style serial interface,
that allows instructions to be serially inserted into the pipeline of the core without using
the external data bus. When in debug state, a
STore Multiple
(
STM
) can be inserted into
the instruction pipeline and this dumps the contents of the ARM7TDM registers. This
data can be serially shifted out without affecting the rest of the system.
7.1.2
Pullup resistors
The IEEE 1149.1 standard effectively requires that
XTDI
,
XnTRST
, and
XTMS
have
internal pullup resistors. In order to minimize static current draw, these resistors are
not
fitted to ARM7TDM. Accordingly, the four inputs to the test interface (the above three
signals plus
XTCK
) must all be driven to good logic levels to achieve normal circuit
operation.