m3819 Renesas Electronics Corporation., m3819 Datasheet - Page 191

• Manufacturer Part Number: m3819
• Description: Mitsubishi 8-bit Single-chip Microcomputer 740 Family / 38000 Series
• Manufacturer: Renesas Electronics Corporation.

3. APPENDIX
When a direction register is set to 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.
3.2.5 Setup for I/O ports
Setup I/O ports using hardware and software as follows:
<Hardware>
<Software>
3.2.6 Providing of watchdog timer function by
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 opera-
tion. 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 pro-
vided 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 process-
ing routine detects errors of the main routine.
This example assumes that interrupt processing is
repeated multiple times in a single main routine process-
ing.
<The 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.
Connect a resistor of 100
As for an input port, read data several times by a
As for an output port, since the output data may
Rewirte data to direction registers and pull-up control
Assigns a single byte of RAM to a software watchdog
Watches the operation of the interrupt processing routine by comparing the SWDT contents with counts of interrupt
Detects that the interrupt processing routine has failed and determines to branch to the program initialization routine
series.
program for checking whether input levels are equal
or not.
reverse because of noise, rewrite data to its data
registers (only the product having it) at fixed periods.
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
processing count after the initial value N has been set.
for recovery processing in the following cases:
register at fixed periods.
If the SWDT contents do not change after interrupt processing
If the changed SWDT contents are abnormal
(In Figure 3.2.9, the main routine determines that the interrupt processing routine has failed only if the SWDT
contents do not change).
software
(Counts of interrupt processing executed in each main routine)
or more to an I/O port in
3819 Group USER’S MANUAL
3.2 Countermeasures against noise
Fig. 3.2.8 Setup for I/O ports
Fig. 3.2.9 Watchdog timer by software
Data bus
Interrupt processing
N
Main processing
routine errors
(SWDT)
Direction register
Main routine
Data register
(SWDT)
= N?
CLI
= N
N
MITSUBISHI MICROCOMPUTER
Interrupt processing routine
(SWDT)
Interrupt processing
Main routine
3819 Group
(SWDT)
errors
I/O port
pins
0?
(SWDT)—1
0
Return
Noise
RTI
Noise
0
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