Page 198 - Embedded Microprocessor Systems Real World Design

Table 6.2
Short Trace Buffer.

Address (offset into
start of table) Trace Value Meaning
0005 01 Timer interrupt entry
0006 02 Timer interrupt exit
0007 04 Start of output processing
0008 05 Output processing done
0009 09 Command received from host
OOOA 07 Operator pressed key
OOOB OA Time rollover
oooc 04 Start of output processing
OOOD 01 Xmer interrupt entry
OOOE 02 Timer interrupt exit
OOOF 05 Output processing done
0000 08 RS-232 byte received
0001 01 Timer interrupt entry
0002 02 Timer interrupt exit
0003 01 Timer interrupt entry
0004 FF Table terminator

This technique is useful if the only access to the outside world is via a serial port,
which is not fast enough to handle the data rate of sending the action codes in real
time. One caution though: If a regular time interrupt is used in the system, beware
of adding it to the trace buffer. If an error occurs and the timer keeps running, it
can quickly fill the buffer, and the information you need will be lost.
A last note about trace buffers: Some software debuggers provide an event
capture function that can be called from the application code. This function typi-
cally is passed information that is time-tagged and stored in a trace buffer. These
debuggers, in effect, provide the circular buffer with very little extra work. Of
course, the debugger software must be resident for this to work. In the final
product, that can mean additional license fees.

Monitor Programs

A software monitor/debugger, mentioned in Chapter 1, is a program that usually
resides in PROM and allows the user to examine memory, registers, and 1/0 ports.
Many software engineers write their own monitor programs, although this is less

Adding Debug Hardware and Software 179