veeceeoh
April 13, 2018, 10:44pm
758
Here is the probable explanation:
As you probably know, the Xiaomi sensors are very sleepy. Most sleepy end devices that we support will wake up every 6 seconds to 2 minutes to check for messages and to make sure its parent knows it is still there. The Xiaomi sensors do this closer to once an hour. That is zigbee compliant behavior so everything is fine so far.
The zigbee specification includes a section called End Device Aging. It states that a parent must forget about a child that doesn’t check in for a certain amount of time. In other words if the parent (like the hub or another router on the network) hasn’t heard from a device within a certain amount of time it must tell the device to leave and rejoin the network. This isn’t an uncommon thing to happens on a zigbee network and most devices will handle this just fine. When the device does it right you’ll never even notice it was gone for a brief period.
By using a zigbee sniffer I can see all the communication between the sensor and the hub. The sensor is quiet for about an hour and then sends a checkin message to the hub. The timeout for end devices has elapsed so the hub’s zigbee radio doesn’t recognize the sensor anymore. Therefore per the specification it sends the sensor a leave and rejoin request. The sensor replies with a message saying it is going to leave and rejoin. Then it does leave but it does not attempt to rejoin. Instead it appears to factory reset itself. This is why it drops offline and I believe is non-compliant behavior.
There isn’t any way to “wake up” or get Xiaomi sensors to rejoin in SmartThings IDE or the mobile app because they are no longer on the Zigbee mesh network.
Xiaomi devices were designed to work a Xiaomi Gateway hub, not SmartThings, so unfortunately they don’t always behave like other Zigbee HA (home automation) devices.
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