seems I forgot another thing: some vendors (for example, Atmel) implement the ONCE(tm) MODE. It is especialy designed for "on-circuit emulation". While the device is in this mode the port 0 goes into a float state and other ports and ALE,PSEN signals are weakly pulled high. The oscillator circuit remains active. The ONCE mode is involked:
- while MCU is in reset state, pull ALE low and PSEN high;
- hold ALE low as RST is deactivated.
Read hardware manual for details.
Good days!