If you have low power devices with basically no external input but a build in clock and a network interface that receives one packet every few minutes (often it even only sends data!), how do you create cryptographically secure random numbers for your encryption?
Noise.
Most - if not all - of these chips have analog to digital convertors because most of the applications involve monitoring some sort of signal. There's going to at least a bit or two of noise in the signal (or if not otherwise used, a floating input.) That's where I'd start if I was looking for entropy.
