Dear All,

I have been reading all communications regarding the "Reset" Problem being faced on a 8051 Family Micro-Controller.

I would like to add a few tips for your kind considerations.

1] First & most important, accept the fact that for a 8051 family microcontroller, the Reset is a Positive +5 Volts Logic Level Signal and the Processor is in Reset Mode as long as this pin is held high. The Reset Action is completed provided the minimum reset timing is satisfied, after the Reset returns low for a normal run mode operation.

2] If you want to depend on an internal pull-down resistor on the reset input pin, inside the microcontroller, then note that value and choose the correct value of the external capacitor between the reset pin and the Vcc. A lower than the necessary value will not allow a complete & proper reset, but a slightly larger than required value will be on the safer side.

3] During the resetting period, it is expected that the Crystal Oscillator starts-up and become statble by the time the reset action is completed. Low-Q value Crystals can fail to stabilize during the reset period

4] Many users add a resistor from reset pin to ground in order not to depend on the internal "R". But do not forget that the external R, in parallel with an internal "R", reduces the effective value of the "R" for reset action, reducing the duration of the reset. So, choose the higher value of "C" because the Effective "R" x External "C" decides the reset time constant.

5] A Good Reset action assumes your C was fully discharged and will charge from zero state at power-up to get proper reset duration. In case you do rapid power switch on and off, the "C" may not get fully discharged at power off. A partially charged "C" will reduce the reset duration and may not complete the action properly. Therefore it is always desirable to add a 1N4148 type diode in parallel to the internal or external "R" to provide a low impedance path for the capacitor to discharge at every power-down so that at power-up it gets a chance to charge from zero.
The diode polarity is such that the Anode is connected to the Gnd and the the Cathode to the Reset Pin.

Pl. check if you have taken care of all of the above.

6] There were no problems faced at any time with above scheme, and no need of a supervisor IC was faced at any time.

7] Supervisor IC is useful where you high high probability of the uC misbehavior due to electrical noise and you want to get it working again if it goes hay-wire.

Proper understanding of the datasheet and implentations make reliable circuits operation.

Thanks,

- Jay. Puranik.