Page 199 - Embedded Microprocessor Systems Real World Design

common than it used to be. Most monitor programs have some basic capabilities
in common:

The ability to examine and alter memory.
The ability to read and write 1/0 ports, if any.
The ability to set breakpoints in the code.
The ability to download code from a host PC.
The ability to interrupt code execution from the keyboard.
The ability to communicate via an FS-232 serial port, Ethernet port, or other
interface.

The monitor program generally gets control on power-up. In the early days, the
user interface was a dumb terminal, but a dedicated PC is used universally today.
The user generally will download code into the target system RAM and execute it
there. The reason the code must run out of RAM is so that breakpoints can be set.
A breakpoint is a branch instruction (usually a one-byte or one-word instruction)
that replaces the instruction that normally would appear at a particular location.
The branch returns control to the monitor program.
To use a monitor program, the user puts a breakpoint at some useful place in
the code, runs the program, and then examines memory and registers when the
breakpoint occurs. Breakpoints can be placed in error routines to determine
whether the error occurs.
As I mentioned earlier, some modern processors have hardware debug onchip.
In those parts, you can use a debugger that runs out of ROM or flash memory and
still have limited breakpoint capability.

Logic Analyzer Breakpoints

Since embedded systems are connected to the real world, you often need to know
what the processor is doing when specific real-world events occur. This may be
because the event represents an error condition or just because you need to know
how the software handles it (not how it is supposed to handle it, but how it really
does). Such events may occur when a sensor changes state, when a waveform passes
through a particular voltage, or when a rotating shaft reaches a particular position.
You may be able to capture the event on a logic analyzer or DSO, but how do you
take a “snapshot” of what the software is doing at that time? In many cases, you can
take advantage of the trigger output port of your logic analyzer or DSO. This tup
ically is a BNC connector with a logic level signal that goes high or low, or pulses,
when the analyzer or DSO triggers it.

180 Embedded Macrqtn-ocessm Systems