Page 193 - Embedded Microprocessor Systems Real World Design

Table 6.1
Action Code Output Values for a Typical Embedded System.
Value (hex) Meaning

01 Throughput too high-ramping down
02 Throughput too low-ramping up
03 Document detected at sensor 1
04 Document detected at sensor 2
40 Document spacing error detected
80 Timer interrupt entry
81 Timer interrupt exit
82 Command interrupt entry
83 Command interrupt exit

an unused 1/0 decode to the clock of the analyzer and captured the 8-bit 8031
data bus as illustrated in Figure 6.1. The firmware was modified to write data to the
1/0 port when certain events occurred. Using the logic analyzer, the sequence of
events and the time between them could be examined. A typical table of output
values for a document processing subsystem like the 8031 system might look like
Table 6.1.
This is just a sample of the sort of thing I collected for that problem. The actual
table of output data was longer, had more interrupt points, and some of the actual
debug definitions would make little sense here without a lengthy explanation of
how the code worked. The significant events are indicated by an output to the data
port. Note that not every instruction or even every subroutine call is captured, just
major events.
Interrupt entry and exit are captured so the time spent in the interrupt routine
can be measured. In Table 6.1, the interrupt routines use data values of 80h and
above. This is because most logic analyzers can be programmed to ignore data using
a binary mask. If no mask is used, all the action codes are captured. If you know the
interrupts are not involved in the problem, you can configure the analyzer to ignore
any action codes where bit D7 is set. The interrupt entry and exit codes are not
stored, so there is more room in the buffer to capture all the other codes. In the
actual situation, I did exactly this-I captured everything until I could verify that
the interrupts were not a factor, then I ignored the interrupt trace values after that.
One of the tasks this 8031 performed was to control the speed of a motor. I
wanted to monitor certain parameters about the motor-speed control algorithms
as well as capture the debug data. Unfortunately, the data could be any value, so I
could not mix it with the trace debug data. I would have had no way to tell the dif-
ference between data and action codes.

174 Embedded Microprocessor System