68HC908MR16 NXP controller : Reset timing

[divakarnath_sin]

Dear Sir,

In our one of circuit we are using 68HC908MR16 controller.

We want to know if we require additional delay circuit ( Low to High time at reset pin) at reset pin to make external crystal oscillator stabilised.

In datasheet of 68HC908MR16 there is internal Power on Reset system. Please see below snapshot of datasheet.

 

Please arrange for the clarification and our discussion with the application Engineer for NXP in India.

Regards,

Divakar

[divakarnath_sin]

Mr. Fang Li,

Thanks for your valuable reply.

But what I wanted to know that , do I realy need external hardware for Reset to add some delay at power up or internal POR is enough to take care of Crystal stabilisation required time.

Thanks & Regards

Divakar Nath Singh

[miduo]

Hello,

From the datasheet, after POR or LVI the SIM module takes 4096 periodes of the input clock source. During this period the internal circuit forces the RST pin low. After this period the internal reset (IRST) signal takes next 64 + 3 cycles and then the MCU starts run user software by reading the reset vector from address $FFFE - $FFFF. This whole procedure takes 4163 clock impulses.

The minimal duration of the external RST low impulse is 100ns. This RST impulse can start reset process which takes 67 clock cycles.