AMD Elan SC520, User Manual, Page: 412 / 444

AMDebug™ Technology
26-6 Élan™SC520 Microcontroller User’s Manual
26.4 OPERATION
The AMD software debugging strategy enables a range of debugging tool solutions offered
by tool providers. The AMDebug port provides for commands to be sent to the ÉlanSC520
microcontroller for processing by microcode. The AMDebug communication and data reg-
isters are used to exchange information between the target (ÉlanSC520 microcontroller)
and a host system used to control the target.
The low-cost communication path, which meets the requirements of most software devel-
opers, uses the serial connection based on the enhanced JTAG protocol. This option re-
quires very few signal pins to the processor and enables a 12-pin connector to be placed
on each board design. A PC-port-to-AMDebug-port converter can be acquired inexpen-
sively (see AMD FusionE86 partners documentation on p. iii under Third-Party Support).
The high-performance communication path, which is made available only to tool developers
such as in-circuit emulator manufacturers, uses a parallel port connection that provides
command and data exchange between the AMDebug port and the host. The higher pin
count (25 instead of 8) requires that the extra signal pins be provided on a bond-out package.
The die for both connection methods is the same. A standard parallel-interface format
greatly simplifies the task of supporting high-speed data exchange with the target processor.
The parallel access also enables execution trace data to be provided on the bond-out
parallel access pins. This is the same data that is gathered in the on-chip trace cache,
described in Section 26.4.1, except that now trace depth is limited by the external hardware
rather than the depth of the on-chip trace cache provided by the non bond-out processor.
26.4.1 On-Chip Trace Cache
An on-chip instruction cache makes it difficult to fully trace a program's execution path by
merely observing the external pins. Software engineers need to know a program's address
flow without turning off the cache or in any way intrusively monitoring the processor's
operation. The use of clock scaling and high internal clock speed make it difficult to provide
trace information to the outside world without the use of on-chip trace cache.
The AMD software debug strategy provides for a small on-chip trace cache that stores only
critical information, such as the outcome of a branch decision. The compression techniques
employed enable much of the execution path to be retained in the on-chip trace cache. The
cache can also log other information, such as operating system activity or performance-
critical parameters. On the bond-out package, the trace information can be continually
provided off-chip at system bus speeds rather than the higher internal clock speeds. This
is an advantage to the in-circuit emulator developer.
The trace cache is also useful when a multitasking operating system is employed. It is
possible to unobtrusively trace the execution of a single task, thus extending the debugging
capability beyond what is normally offered by debuggers incorporated with operating sys-
tems. This method overcomes the typically poor integration between operating systems
and external trace capture hardware, such as a logic analyzer or in-circuit emulator.