We are using the XBee3 LTE-M/NB-IoT device (XB3-C-A2-UT-001) as the communication module for a battery powered microcontroller board. We send messages periodically (most often every 10 minutes) to an MQTT service.
On every transmission, the cellular module wakes up (it takes about 10 seconds to get a valid IP address), sends the data (another approximately 15 secs until the server confirmation is received) and then is set to sleep – either by MicroPython or by asserting the SLEEP_RQ pin high. This is followed by approximately 20 secs (best case) until the ASSOC pin is low (so power can be switched off). The radio module is in PSM and this seems to work correctly.
Question: Is there a method to shorten this 20 sec interval? In Direct USB mode the radio module itself, for example, executes the power off command much faster (AT+ CPWROFF is followed by OK in approximately 3 secs).
No, there is not. Not to mention the time you are seeing by the CPWROFF commands OK is not accurate as to know when it is safe to power it off. There are other lines that need to be monitored as well. The Digi power off function does this. So it is actually better to leave the radio powered on and to use PSM or the Pin sleep functions.
I’ve noticed that this shutdown time can vary greatly depending on how the server is handling the closing of a socket, with the socket close being initiated by the modem. This was tested by 1) powering on the modem, 2) allowing association to complete, 3) sending a single Transmit IPv4 packet to the server with the transmit option set to “Terminate the TCP socket after transmission is complete”, and 4) sending the ATSD0 shutdown command in AT command mode along with an ATAI association indication command and waiting for the response showing a successful shutdown.
It seems that the ATSD# shutdown command is dependent on how the server is handling the socket closure. Is there anyway to configure or control the modem so that it can shut down sooner without having to wait for the server side?