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    參數(shù)資料
    型號: M37542F4GP
    元件分類: 微控制器/微處理器
    英文描述: 8-BIT, FLASH, 8 MHz, MICROCONTROLLER, PQFP32
    封裝: 7 X 7 MM, 0.80 MM PITCH, PLASTIC, LQFP-32
    文件頁數(shù): 97/124頁
    文件大?。?/td> 1238K
    代理商: M37542F4GP
    7542 Group
    Rev.3.03
    Jul 11, 2008
    Page 72 of 117
    REJ03B0006-0303
    5. Setup for I/O ports
    Setup I/O ports using hardware and software as follows:
    <Hardware>
    Connect a resistor of 100 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, 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. 95 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.
    Fig. 96 Watchdog timer by software
    <The main routine>
    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 process-
    ing after the initial value N has been set.
    Detects that the interrupt processing routine has failed and de-
    termines 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 process-
    ing 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
    (SWDT)← N
    CLI
    Main processing
    (SWDT)
    Interrupt processing
    routine errors
    N
    Interrupt processing routine
    (SWDT) ← (SWDT)—1
    Interrupt processing
    (SWDT)
    Main routine
    errors
    >0
    ≤0
    RTI
    Return
    =N?
    ≤0?
    ≠N
    Direction register
    Port latch
    Data bus
    I/O port
    pins
    Noise
    N.G.
    O.K.
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