ULPNode is much more power efficient in powerDown
I did a quick prototype yesterday to check how low I could go even with the current HW. Idea was to use listen mode as WDT without any interactivity benefits. Every 2 minutes I send a wakeup burst, all nodes send their replies, sequenced based on time stamp in the wakeup package. Then every node selects its listen idle period such that the next RX period overlaps with the next wakeup burst. After each wakeup I recalibrate the RC oscillator in the radio.
This works pretty well - but there is an obvious downside: if a node misses one wakeup burst for whatever reason you either need to send a 1 minute wakeup burst to collect it again, or you have to wait an hour or so until the RX period and the wakeup burst period overlap again. That said it's run all night without hickup here.
Total sleep power consumption:
1200 nA - radio in idle mode
100 nA - 328p in powerDown mode
90 nA - radio in RX periods
450 nA - radio receiving wakeup packet
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1840 nA
That's a saving of around 1700 nA over interactive listen mode.
Don't know yet if I'll use that in production - but I though it was an interesting experiment.
Joe