@Hippy,
"If 3V3 is too low, that can probably be jacked-up to 3V9 by putting a diode between regulator Common and 0V, pointy-end to 0V"
Have been thinking of how to set up a SuperCap/GoldCap to keep the PICAXE running long enough to shut things down gracefully and save temp data to eeprom when the main supply fails.
The issue is that GoldCaps below 10F value will only supply uA where the 10F and above will supply mA (different construction). The 10F mA types are much more expensive and a single cell has a working voltage 2.3V. So even if 2 were stacked the voltage rating would still be exceeded at 5V supply. A stack of 3, apart from having way too much capacity and being more expensive, also requires balancing to ensure 1 does not see too much voltage. The 5V supply is required as some devices need 4.5V and more, and will be supplied with less than 5V when fed from the GoldCap.
Having read your post I was wondering about using some similar technique to 'lift' the GoldCap 2.7V above ground.
The idea is to charge/discharge a single GoldCap via parallel reversed Schottky diodes. The diode's current/resistance reduces as the GoldCap approaches supply V and the GoldCap charges to the full supply voltage over time. When the supply disappears, the GoldCap will supply the PICAXE through the other Schottky but as the load is sufficient, the output will be around 0.3V less than normal and continue to drop as the charge is bled from the GoldCap.
CalibADC10 would be used in the main program loop to detect this change and shut down outputs, and save variables to eeprom. The program would then stay in a loop checking CalibADC10 until either power is restored, or the supply from the GoldCap runs out.
The problem is how to raise the "0V" of the GoldCap to 2.7V as the low leakage current through most semiconductors will allow the 0V side of the Gold cap to slowly approach PICAXE 0V over time and the GoldCap would then be subjected to excess voltage..
The other option is to use a boost circuit to raise the 2.3V to near 5V but this is not trivial either.
Any thoughts/suggestions appreciated.