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7542 Group
5. Setup for I/O ports
Setup I/O ports using hardware and software as follows:
<Hardware>
• Connect a resistor of 100 Ω or more to an I/O port in series.
<Software>
• As for an input port, read data several times by a program for
checking whether input levels are equal or not.
• As for an output port, since the output data may reverse because
of noise, rewrite data to its port latch at fixed periods.
• Rewrite data to direction registers and pull-up control registers at
fixed periods.
Data bus
Direction register
Port latch
Fig. 92 Setup for I/O ports
6. Providing of watchdog timer function by software
If a microcomputer runs away because of noise or others, it can
be detected by a software watchdog timer and the microcomputer
can be reset to normal operation. This is equal to or more effective
than program runaway detection by a hardware watchdog timer.
The following shows an example of a watchdog timer provided by
software.
In the following example, to reset a microcomputer to normal op-
eration, the main routine detects errors of the interrupt processing
routine and the interrupt processing routine detects errors of the
main routine.
This example assumes that interrupt processing is repeated mul-
tiple times in a single main routine processing.
Rev.3.02
Oct 31, 2006
REJ03B0006-0302
Noise
O.K.
Noise
N.G.
I/O port
pins
Page 71 of 134
<The main routine>
• Assigns a single byte of RAM to a software watchdog timer
(SWDT) and writes the initial value N in the SWDT once at each
execution of the main routine. The initial value N should satisfy
the following condition:
N+1 ≥ (Counts of interrupt processing executed in each main
routine)
As the main routine execution cycle may change because of an
interrupt processing or others, the initial value N should have a
margin.
• Watches the operation of the interrupt processing routine by
comparing the SWDT contents with counts of interrupt process-
ing after the initial value N has been set.
• Detects that the interrupt processing routine has failed and de-
termines to branch to the program initialization routine for
recovery processing in the following case:
If the SWDT contents do not change after interrupt processing.
<The interrupt processing routine>
• Decrements the SWDT contents by 1 at each interrupt process-
ing.
• Determines that the main routine operates normally when the
SWDT contents are reset to the initial value N at almost fixed
cycles (at the fixed interrupt processing count).
• Detects that the main routine has failed and determines to
branch to the program initialization routine for recovery process-
ing in the following case:
If the SWDT contents are not initialized to the initial value N but
continued to decrement and if they reach 0 or less.
Main routine
(SWDT)← N
CLI
Main processing
≠N
(SWDT)
=N?
N
Interrupt processing
routine errors
Fig. 93 Watchdog timer by software
Interrupt processing routine
(SWDT) ← (SWDT)—1
Interrupt processing
>0
(SWDT)
≤0?
RTI
≤0
Return
Main routine
errors
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