BroadMobi BM818 modem seems to sometimes stop responding to AT commands

[Elias Rudberg]

Hello,

I have a Librem 5 phone with a BroadMobi BM818-E1 modem. The operating 
system is PureOS 10 with ModemManager 1.14.12. Making phonecalls works 
most of the time, but sometimes it does not work and I have been trying 
to figure out why.

Having added the ModemManager --debug option and compared logs for cases 
when making phonecalls work and not, now I think it looks like the 
problem is that the modem sometimes stops responding to AT commands.

For a successful case, part of the log related to [modem0/ttyUSB1/at] 
looks like this:

<debug> [1640639307.692391] [modem0/ttyUSB1/at] --> 'AT+CCWA=1<CR>'
<debug> [1640639307.703945] [modem0/ttyUSB1/at] <-- '<CR><LF>OK<CR><LF>'
<debug> [1640639307.704293] [modem0/ttyUSB1/at] device open count is 1 
(close)
<debug> [1640639313.155180] [modem0/ttyUSB1/at] device open count is 2 
(open)
<debug> [1640639313.155742] [modem0/ttyUSB1/at] --> 'AT+CCLK?<CR>'
<debug> [1640639313.168020] [modem0/ttyUSB1/at] <-- '<CR><LF>+CCLK: 
"21/12/27,21:08:32+04"<CR><LF><CR><LF>OK<CR><LF>'
<debug> [1640639313.168825] [modem0/ttyUSB1/at] device open count is 1 
(close)
<debug> [1640639333.490220] [modem0/ttyUSB1/at] device open count is 2 
(open)
<debug> [1640639333.490682] [modem0/ttyUSB1/at] device open count is 3 
(open)
<debug> [1640639333.490880] [modem0/ttyUSB1/at] --> 'ATD90510;<CR>'
<debug> [1640639333.534638] [modem0/ttyUSB1/at] <-- '<CR><LF>OK<CR><LF>'

If I understand correctly, each "-->" line indicates an AT command sent 
to the modem, and each time there is a corresponding "<--" line that 
shows the response from the modem. That looks fine above, where each 
"-->" line is followed by a "<--" line.

Below is the corresponding part of the log for a case when I could not 
make phonecalls:

<debug> [1640636606.219473] [modem0/ttyUSB1/at] --> 'AT+CCWA=1<CR>'
<debug> [1640636606.230312] [modem0/ttyUSB1/at] <-- '<CR><LF>OK<CR><LF>'
<debug> [1640636606.230674] [modem0/ttyUSB1/at] device open count is 1 
(close)
<debug> [1640636611.155941] [modem0/ttyUSB1/at] device open count is 2 
(open)
<debug> [1640636611.156211] [modem0/ttyUSB1/at] --> 'AT+CCLK?<CR>'
<debug> [1640636614.159204] [modem0/ttyUSB1/at] device open count is 1 
(close)
<debug> [1640636660.623905] [modem0/ttyUSB1/at] device open count is 2 
(open)
<debug> [1640636660.624328] [modem0/ttyUSB1/at] device open count is 3 
(open)
<debug> [1640636660.624529] [modem0/ttyUSB1/at] --> 'ATD90510;<CR>'
<debug> [1640636662.656478] [modem0/ttyUSB1/at] device open count is 4 
(open)

In that case the modem responds to the "AT+CCWA" command, but for the 
"AT+CCLK" and the "ATD" commands there is no response from the modem.

There are also messages about timeouts, like this:

<debug> [1640636614.158768] [modem0] couldn't load network timezone: 
Serial command timed out

<warn>  [1640636750.659447] [modem0/call0] couldn't start call: Serial 
command timed out

Why would the modem stop responding to AT commands?

Any ideas about possible solutions or workarounds, or ways to 
investigate further?

/ Elias