7549 datasheet(62/86 Pages) RENESAS

62 / 86 page

Rev.2.02

Mar 31, 2009

Page 60 of 81

REJ03B0202-0202

7549 Group

5. 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 79. 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.

• 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 80. Watchdog timer by software

N.G.

O.K.

Noise

Data bus

Direction register

Port latch

I/O port

pins

≠N

≤0

Main routine

(SWDT)

← N

CLI

Main processing

(SWDT)

=N?

Interrupt processing

routine errors

Interrupt processing routine

(SWDT)

← (SWDT)−1

Interrupt processing

(SWDT)

≤0?

RTI

Return

Main routine

errors

7549 bảng dữ liệu(PDF) 62 Page - Renesas Technology Corp

7549 bảng dữ liệu(HTML) 62 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

bảng dữ liệu tải về

Link URL