3802 GROUP USER’S MANUAL
3-31
APPENDIX
3.4 Countermeasures against noise
3.4.5 Setup for I/O ports
Setup I/O ports using hardware and software as follows:
<Hardware>
G
Connect a resistor of 100
or more to an I/O port
inseries.
<Software>
G
As for an input port, read data several times by a
program for checking whether input levels are
equal or not.
G
As for an output port, since the output data may
reverse because of noise, rewrite data to its port
latch at fixed periods.
G
Rewirte data to direction registers and pull-up
control registers (only the product having it) at fixed
periods.
When a direction register is set for input port again at fixed periods, a several-nanosecond short pulse may be
output from this port. If this is undesirable, connect a capacitor to this port to remove the noise pulse.
Fig. 3.4.8 Setup for I/O ports
3.4.6 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 operation, 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 multiple times in a single main routine
processing.
<The main routine>
G
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)
Fig. 3.4.9 Watchdog timer by software
As the main routine execution cycle may change because of an interrupt processing or others, the initial value N
should have a margin.
G
Watches the operation of the interrupt processing routine by comparing the SWDT contents with counts of
interrupt processing count after the initial value N has been set.
G
Detects that the interrupt processing routine has failed and determines to branch to the program initialization
routine for recovery processing in the following cases:
If the SWDT contents do not change after interrupt processing
Direction register
Port latch
Data bus
I/O port
pins
Noise
Noise
N.G.
O.K.
Main routine
(SWDT)
←
N
CLI
Main processing
(SWDT)
=N
Interrupt processing
routine errors
≠
N
=N
Interrupt processing routine
(SWDT)
←
(SWDT)—1
Interrupt processing
(SWDT)
≤
0
Main routine
errors
>
0
≤
0
RTI
Return