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I assume you're asking about the reverse diode from Vout to Vin?
You shouldn't need it for ICSP. That diode is mainly to protect the 7805 when it has a large Vout cap and the input may be shorted, like when an appliance has a DC input that goes into the 7805 Vin terminal.
I've done ICSP powered +5v on lots of projects and just leave the 7805 connected with no Vin and the +5v on it's Vout.
No, not a diode from Vout to Vin. ICSP wouldn't work this way as all the current VDD pin of a programmer supplies would go to ground through this diode. I was talking about a protecting diode in series with Vout.
But ok, so you're saying it is safe to have +5V on Vout while Vin has no voltage applied? I know that output should never have a higher potential than input, but if input is left unconnected, it is safe to apply 5V to output, right? As I understand now, damage would only be done if for some reason input of the regulator is shorted to ground and then 5V is supplied to Vout during ICSP.
You could add a diode in series with the common of the 7805 (anode to the common terminal /cathode to ground) so that you offset the output voltage by +0.7V.
Then adding a diode in series with the output, to protect the 7805, will drop the output back to 5 Volts on its cathode. Remember that both diodes will need to carry the load current. !n4002 (or higher) should do the job.
It will lower the VCC the PIC gets bellow it's limits though if you're using VCC from the other side of the diode on other parts of the circuit, you'll get current flowing through the clamp diodes in the PIC (if it has them) unless every PIC lead has a modest amount of resistance on it.. If the entire circuit draws it's power after the diode, it's not a problem.