7545 datasheet(48/62 Pages) RENESAS

48 / 62 page

Rev.1.06

Mar 07, 2008

Page 48 of 59

REJ03B0140-0106

7545 Group

4. Setup for I/O ports

Setup I/O ports using hardware and software as follows:

• Connect a resistor of 100

Ω or more to an I/O port in series.

• 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.

Fig. 56 Setup for I/O ports

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 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.

• 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

processing after the initial value N has been set.

• Detects that the interrupt processing routine has failed and

determines to branch to the program initialization routine for

recovery processing in the following case:

If the SWDT contents do not change after interrupt processing.

• Decrements the SWDT contents by 1 at each interrupt

processing.

• 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.

Fig. 57 Watchdog timer by software

Direction register

Port latch

Data bus

I/O port pins

Noise

N.G.

O.K.

Main routine

(SWDT)

← N

CLI

Main processing

(SWDT)

Interrupt processing

routine errors

Interrupt processing routine

(SWDT)

← (SWDT)1

Interrupt processing

(SWDT)

Main routine

errors

≤0

RTI

Return

=N?

≤0?

≠N

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