Thank you Tom and I was afraid of that; it seems my lack of understanding in the datasheet and my inability to understand a good deal of your explanation means that this may be more than a non-EE can handle. So an emphasis on a 1ohm resistance means I should use Ohm's law to work out how much current can be passed at a given voltage? If so 3.3VDC/1ohm = 3.3amp which seems much higher than I would have expected this teeny chip to be able to switch on/off.
You would have to be careful about power (IxV) first. But, the way to think about 1ohm is that if you're using an RFM69 transmitting with a nominal current of 65mA, you would have 65mV of ground bounce on the VSS side of your entire design when the transmission takes place, and then a much lower drop when just the 328P is operating. All of this voltage change would appear as signal shifts at the control pins between the AB1815 and the processor. It might be ok, but, with high frequency spikes exceeding this voltage, it probably won't be.
Regarding ground planes and RF, I'm 100% out of my depth there. I hear you saying that Vss mode is ill advised if my circuit includes a radio which it will. In looking at FET datasheets it isn't at all clear to me how much current they leak when they are off. I trust that the FET that Felix used on the TPL5110 breakout board is uber low leakage but it seems most FETs leak more current than a sleeping mote so one has to be careful in their selection.
'Most' FETs are power FETs with large area which will leak a lot more than simple signal FETs used in the processors and such. The leakage is generally correlated with voltage across the FET, negative bias on the gates, and temperature. Complex relationship to be sure.
I suppose the main thrust of my questioning is whether the AB1815 + a low leakage FET will use less current than the TPL5110 breakout board. 14nA WDT current on the AB1815 + the leakage on the FET seems like it could well be lower than the WDT current of 35nA on the TPL5110.
I suspect that other than the 'academic' side of your concern, I think you will find that the power difference between an AB1815 and TPL5110 implementation is so far into the noise that neither one materially affect the battery life of your project. It will be the 'awake' operating current and duty cycle, however small, that will overwhelmingly dominate the net power requirements.
The AB1815 is useful when you want to set specific times for wake up, especially if these vary. TPL5110 is small, easy, and very accurately repetitive, but only occurs at ONE fixed rate - that is determined by the programming resistor. This will get you pretty close to a specific interval, but the exact time will be determined by when the device was last powered on.
By the way, the AB1815 is a very precise timer, but you have to calibrate it to compensate for any error in its oscillator frequency. Abracon purposefully shifts the frequency to slightly higher than 32.768KHz so that you can program 'dither' calibration coefficients to get the average time to exactly 32.768KHz +/- 2PPM.
Tom