Wolfgang wrote:
> I am developing a elektronic for my bike with an pic 16C84. It gets its
> power from the bicycle generator. The voltage rises and falls very slow.
> With an 4 Mhz oscillator i need the browout circuit from the microchip
> manual with the 3 resistors, the z-diode and the transistor. But with an
> 32768 Hz crystal resonator it seem to work without the brownout. It boots
> always, i have tested it very often with my regulated power supply. I can
> dip the power below 2 volt or to 0 volt and rise again to 5 volt and i works
> always.
> Why?

Because it takes the 32kHz crystal a long time to start up, your
voltage is sufficient for reliable operation by the time you start
executing instructions.

Your 4 Mhz xtal starts up much more quickly, and you may not
have sufficient voltage to get reliable execution.
Have you turned on the power up delay timer (assuming the 84
has one)?

You may also be dealing with internal capacitance issues. A slow
clock allows full signal propagation at marginal voltages while
chip capacitance will result in different loadings at higher clock
rates, all other things being equal.