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UART shows strange value

Hi Devzone!

I'm using uart to send AT commands to LTE modem.

but when I send AT commands the response is quite strange, and I think the command is sent twice. I don't know why.

The below image is about a right response when I send AT command with PUTTY to modem directly.

and this image is about s strange response when I send AT command at the nRF52840.

The whole code is below

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/**
* Copyright (c) 2014 - 2019, Nordic Semiconductor ASA
*
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without modification,
* are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form, except as embedded into a Nordic
* Semiconductor ASA integrated circuit in a product or a software update for
* such product, must reproduce the above copyright notice, this list of
* conditions and the following disclaimer in the documentation and/or other
* materials provided with the distribution.
*
* 3. Neither the name of Nordic Semiconductor ASA nor the names of its
* contributors may be used to endorse or promote products derived from this
* software without specific prior written permission.
*
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

What is the problem, and how can I fix it?

Best regards,

Baek