Hi for all !

When using PIC16C505 in external RC oscillator mode (RB4/OSC2/CLKOUT
configured to CLKOUT), frequency stability is very poor.
Observing CLKOUT pin with a scope we see a "frequency noise" having a
magnitude of about 1% of CLKOUT frequency.
Changing state of other I/O ports configured as inputs also changes
CLKOUT frequency with the same magnitude.
Testing the same software using PIC16C54/56 or PIC16C711 (also in
external RC oscillator mode) results with a very good oscillator stability
(no problem).
Testing again with 16C505 in internal RC mode (RB4/OSC2/CLKOUT
configured to CLKOUT) results also without any problem (frequency is
stable).

Application conditions:
Vdd = 5volt. Room's temperature. CLKOUT frequency = 700 - 1000 khz.

Can you explain why we have such a problem and how to avoid it ?

Thanks advance.

Mark.

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