I’m curious about the limitiations of power management on the imp. For example, if I have a sensor that returns a few bytes of data every 60 seconds, what are the implications of this on power consumption? I was under the impression that 802.11 has a fairly sizeable overhead for connection setup and tear-down and that this would then affect power consumption negatively. Any information someone can provide on how the imp manages the connection and power would be greatly appreciated. Thanks.
802.11 has a significant penalty, yes - compared to other more lightweight protocols - but the imp can generally boot, associate with wifi, DHCP, make a secure connection to the server, sample, and sleep in around a second.
Your mileage will vary depending on how fast your DHCP and DNS servers are (DNS will be taken out of this path shortly), but that’s the performance we see on our office network - the servers still being on amazon’s network. The critical items for good battery life are the sleep current (we’re down at 6uA) and the time spent awake.
Our nora reference design can run for around 3 years on 2xAA’s when sampling temperature/humidity every 15 minutes. The next big impOS release adds wifi-less wakes, which would allow you to wake to sample once a minute but only transmit readings (say) once an hour, which could give you well over 5 years battery life despite a much faster sampling interval. In initial tests we’ve been able to wake, collect data & store it in nvram, then sleep in well under 100ms.
With the faster sampling, you can obviously decide to transmit buffered data immediately if some programmable threshold has been crossed, allowing you to not sacrifice latency for important events.
Thanks Hugo, this is extremely helpful. Just to be clear, my question wasn’t an indictment of WiFi – but rather an effort to understand how you’re able to get such excellent power consumption numbers. I was pleasantly surprised to learn that the handshaking and data comms could occur in under a second. Keep up the great work, I’m looking forward to powering mine up this weekend.