why COP_ISR(void) can not work

[明超戴]

I use MC9S12XEP100 to test the function cop.

I have found a cop process function ,when the watchdog timer overflow ,the system can reset ,but it can not step into the

COP_ISR(LED can not turn on),Can you help me,thank you !

//******************************************************************************

// COP reset

//******************************************************************************

#pragma CODE_SEG NON_BANKED

interrupt 2 void COP_ISR(void)

{

    DDRA = 0xff;

    PORTA = 0x00;          //LEDs on - show we entered the COP_ISR    

    for(i=0; i<60000; i++)  //delay

    {

        asm nop;

    }

    PORTA = 0xff;

    asm jmp _Startup;      //jump to power-on reset vector

    // !!! RTI instruction cannot be executed because this is not interrupt !!!

}

[RadekS]

Hi &#26126;&#36229; &#25140;,

Please check capacitor at RESET pin.

The Too big time constant of RC circuit at RESET pin could interfere with COP/CM reset source detection ability.

In short: In the case of a reset, RESET pin will be hold down for 128 OSCCLK cycles. The voltage level at RESET pin will be detected after next 64 OSCCLK cycles. When the low-level voltage is detected, the External Reset source is assumed and POR reset vector is selected.

So, we have to ensure that voltage at RESET pin will achieve high voltage level prior these 64 OSCCLK cycles.

For example, if we take the case when we use 16MHz oscillator, 64 SYSCLK cycles presents 4us.

Therefore, if we have 2.2kOhm pull-up resistor at RESET pin, the capacitance at RESET pin should be below approximately 1.8nF. Otherwise, we detect POR / LVR / Illegal Address Reset / External Reset.

Note: Simply RC cell, RC constant=Tau=R*C represents the time when voltage achieves approximately 2/3 value. Detection level for high value at the pin is also approximately 2/3 value of VCC. So we can use simple calculation: C<64/(R*fOSC). In the calculation, we omitted capacitance and internal pull-up of RESET pin.

For more details please check:

https://community.freescale.com/docs/DOC-103737

Note: There is one additional option. We could connect Schottky diode between MCU reset pin and external RC circuit (capacitor and pull-up). In the case of system reset (caused by MCU), the external capacitor isn’t discharged and COP/CM detection works correctly even with 100nF capacitor. See attached circuit:

I hope it helps you.

Have a great day,
RadekS

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[明超戴]

Hi Radek Sestak

Thank you for your help ， I will tryand check  it 。

Have a great day

                                                       Dai Mingchao

[明超戴]

Hi Radek Sestak thank you for your help，I have tested it ，use appropriate capacitance the system can step into COP_ISR.

Best Regards

                                                                                                                                       Daimingchao