Bottom line, it's clear to me at least that any 328p circuit using a series cap coupled auto-reset feature should have the reset diode installed for best reliablity.
The documented minimum voltage to enter HV programming mode is +11.5vdc on the reset pin
QuoteThe documented minimum voltage to enter HV programming mode is +11.5vdc on the reset pinThats most likely the minimum voltage to reliably enter HV programming mode.It may be possible that some chips or certain other conditions may cause it to enter HV programming mode with a lower voltage.Bottom line is that the voltage on any pin should not exceed 5.5v for reliable behavior.
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