4552 Datasheet, PDF (63/145 Pages) Renesas Technology Corp – SINGLE-CHIP 4-BIT CMOS MICROCOMPUTER

4552 Group
4. 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 or an I/O port, since the output data may re-
verse because of noise, rewrite data to its port latch at fixed
periods.
• Rewrite data to pull-up control registers at fixed periods.
5. 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 fol-
lowing 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.
<The main routine>
• Assigns a single word 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 rou-
tine)
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 com-
paring the SWDT contents with counts of interrupt processing
after the initial value N has been set.
• Detects that the interrupt processing routine has failed and deter-
mines 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 processing 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
Interrupt processing routine
(SWDT)← N
EI
(SWDT) ← (SWDT)—1
Interrupt processing
Main processing
≠N _(SWDT)
=N?
N
(SWDT) ^>0
≤0?
≤0 ^RTI
Return
Interrupt processing
routine errors
Main routine
errors
Fig. 61 Watchdog timer by software
Rev.3.02 Dec 22, 2006 page 63 of 142
REJ03B0023-0302