hi,

You place the jumper. Press reset (pulling it high for a moment, then releasing to low again) You're in ISP mode. You can remove the jumper (release the hardware conditions) or leave it there, doesn't matter, you're in ISP and until next reset/power cycle the system doesnt't care what you do with PSEN.

Hmm, it looks very strange for me (read below). But you talk about ADuC812 and maybe it is okay for this chip. Atmel does not allow such way and now I try explain why.

Indeed, if at reset falling edge the PSEN pin is tied to ground then MCU goes to ISP mode. But Atmel (for example, AT89C51RD2) does not provide a special hardware mode for ISP. It just maps in program memory frame the extra built-in memory which contains bootloader software and then does jump to it. After it, this software executes like ordinary one; by other words MCU works in normal operating mode. It is one side of question. Another side is that in normal operating mode PSEN is used as external program memory read strobe. Hardware realizes PSEN output with both strong pull-up and strong pull-down. Passive state of PSEN (when there are no fetches from external program memory) is high level driven with strong pull-up. Now imagine what happens when PSEN sources high level but is connected to ground externally. It is overloaded and may be damaged.
Atmel has special note about it somewhere in datasheets:

As PSEN is an output port in normal operatiing mode after reset, user application should take care to release PSEN after falling edge of reset signal.

There is no matter you are in user application or in bootloader ISP - you must delete that jumper immediatelly after reset.

But there are two parameters I cannot find in datasheets: PSEN Setup Time and PSEN Hold Time. It is why I asked you about your knowledge (you said 24 clocks).

Regards,
Oleg