Dear Mr. Waclawek,

Thank you for your fast reply.

According to its datasheet, RST pin on S8253 can be as input or output. As input if it receive external reset signal from R-C circuit or supervisory circuit IC. As output if its watchdog timer has been activated but bit-3 on WDTCON register (DISRTO bit) must be set to '0' first. RST pin will exclude a '1' pulse after watchdog times out to reset entire system. Therefore, a BAT51 diode must be added on RC ciruit or supervisory circuit IC output to avoid reset signal collision.

And I apologized that my problem has just been solved. There was my fault that caused by myself. I was not thorough. Sorry if I'm bothering you.

Thank you for your great attention.

Best regards,
-EKA-