Digital Signal Processing Reference
In-Depth Information
Different types of threads are given different priorities. Hardware interrupts
(HWIs) have the highest priorities, followed by software interrupts (SWI),
which include periodic functions (PRD).
3.
Real-time data exchange
(RTDX). This allows the exchange of data between
the host and the processor, via the onboard Joint Test Action Group (JTAG)
interface, while the processor is running. RTDX consists of both target and
host components. Data are transferred through “pipes” (for receiving and for
transmitting). If the CPU starts missing real-time data, one can find out from
the execution graph. Reprioritizing, if possible, could then solve this problem.
Figure 9.1
a
illustrates overloading the CPU with no-operation instructions (
NOP
s).
As the number of NOPs is increased, the effects on the output can be monitored.
Figure 9.1
a
indicates that the task of “audioSwi” has the highest priority and can
interrupt the lower priority task of “loadPrd.” In Figure 9.1
b
, “audioSwi” has a lower
priority and has to wait for the higher-priority tasks of “loadPrd” and “Prd_swi”.
This causes data to be missed. For example, with music as input and with the number
of
NOP
s increasing (up to a million), one can hear the gradual degradation of the
output signal as the CPU starts missing execution. The execution graph can show
when the CPU starts missing data.
Another consideration is the use of the LOG module LOG_printf( ) to monitor
a program in real time. The C function
printf( )
, supported by real-time library
support, takes too many cycles to be desirable for real-time monitoring (see
Example 1.3); the LOG module LOG_printf( ) takes considerably less time. The
LOG_printf( ) function can be used to record data in critical time, while the trans-
(
a
)
(
b
)
FIGURE 9.1.
CCS plots of execution graphs as the CPU is being overloaded with
NOP
s:
(
a
) output not degraded when setting
audioSwi
with the highest priority; (
b
) output
degraded when setting
audioSwi
with a lower priority.
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