UART AT Command hang when make a phone call

We are using RC7620 to make a phone call using ATD…
When the callee answers the RC7620 does not return the CONNECT response via UART interface. We need to send another command AT to the module to get back the CONNECT response.
However, when we disable the CMUX mode using the command AT!MUXMODE=0, the module responses intermediately. But disable the CMUX mode makes the module can not response to any all protocol commands at all.
In our hardware design we leave the USB_D+/- open. If we connect the module with USB so that the USB_D+/- is connected, then the module responses.
So we do NOT understand what happened to the UART protocol interface.
Any advise is very appreciated.

I don’t see problem in UART port:

Manufacturer: Sierra Wireless, Incorporated
Model: RC7620
Revision: SWI9X07H_00.08.07.00 0ce4c1 jenkins 2021/03/17 02:18:20
IMEI: 353634110000980
FSN: 7Q9472000710AA

+CREG: 0,1



Hi Jyijyi,
Thank you for your response. Could you share me the hardware that you used? In my case, I use the development kit. The problem occurred only when the kit is powered by battery. If the kit is connected with a PC, the problem did not occur. I guested that when USB_D+,D- are connected, there is no issue.
With best regards,

I used HL dev kit …

Do you connect the Kit with the PC via USB? We used Mango Yellow.

No, i do not connect usb

Can you provide me the model of your HL Dev kit?

It looks like this board

Hi jyijyi
I used kit Mangoh Yellow, powered by battery from CN1000 and not pluged USB, there was same problem. and when I pluged usb connector, problem was sloved. From Mangoh Yellow schematic, I see the only difference is that USB_D+, USB_D- are connected to the PC.
Please help.

Does the module go to sleep mode if no usb is connected?

yes. I used AT+KSLEEP=2 before ATD…

does problem happen if setting AT+KSLEEP=1 ?

The problem still happens with AT+KSLEEP=1,

for AT+KSLEEP=2 or AT+KSLEEP=1, you can workaround the issue by keep sending AT after “ATD1234567;”
Is it correct?

Another way you can try is to use AT+KSLEEP=0, after "ATD1234567; ", you can pull down the DTR and pull up the DTR again.

As i see, HL dev kit which you used just using for HL branch. Please check.

but I found that I can use it to communicate with RC module via USB and UART

Do you have schematic of your kit? I can’t find it, please share it with me

No, I don’t have, but I think I can see same issue.
After I dial the call by ATD, if the receiver side does not answer too quickly, I can see same issue.

that is why I suggest you to keep typing AT and see if you can still see this issue.

Yep, when it stuck, you can ping “AT” or any AT command to get respond from RC but We can not ping AT continuously, we don’t know when it has problem. And we find out that stuck can occur in many other case. So we need correct this problem from root cause. Thank you.

how about this one?

you can try is to use AT+KSLEEP=0, after "ATD1234567; ", you can pull down the DTR and pull up the DTR again and see if the port is stuck.