1. incomplete startup reset, resulting in undefined register state (most importantly perhaps, the program counter)
2. all sort of nasty things which might and do happen when VCC gets too low for normal operation of micro
3. this is slightly more of black magic, but it shall also protect against power-glitch induced faults and consequent problems; I personally think this gets more and more important with switcher supplies.

The micro itself does not get corrupted by these. In fact, you are protecting the peripherals (including any FLASH around).


It provides a valid reset signal of appropriate length whenever VCC is low.

Richard, this is analoguous to the power supply. Once the manufacturer states VCC min and max, you use an elaborate circuit to ensure that (including things like decoupling). Consider the complexity of the simplest power supply (but you don't complain) plus the "art" of making quiet GND/VCC - while you could claim that in MANY CASES the '51 runs perfectly from a resistor-diode-cap combo, nest-wired directly from mains.

So, if you consider it normal to use a complex circuitry just to ensure VCC of some given parameters, you should also consider it normal to use a reset signal of parameters required by the manufacturer (i.e., high all the time VCC is lower than VCCmin plus some time after that).


I don't know what is ratiometric input, but I am quite convinced you simply described what's inside the reset IC. I doubt you can produce the same in discrete form for less $.



You should give at least some credit to the manufacturers. In CMOS, it is not impossible to make things fail graciously at arbitrarily low supply voltage, IF there is a valid "low voltage" signal (a.k.a. RESET). (It is also not impossible to produce the "low voltage signal" in the same technology on the same chip - don't ask me why they don't do so in '51s, in other micros this is the state of the art currently).

Jan