how to interface external MCU host viaUART interface with nrf9160, with only 3 pins from UART (RD,TX,GND)

Question

We would like to use nRF9160 has MODEM only feature, such that MCU in our product can talk to nRF9160 via UART interface. 
 We have found an sample application [Serial LTE Modem] that comes with nRFConnect SDK , we would need to use this application with following constraint 
 We have UART TX , UART RX and GND ( 3 signals ) and do not have provision for CTS/RTS or any other additional GPIO for interface MODEM(nRF9160) and our UART I/O signal TTL level is 3.3V 
 Is this possible to accomplish the same with the sample application [Serial LTE Modem]. 
 External Com port ( 115200 bps, 8N1) , 
 Pin connection between nrf9160 and 3.3v ttl-uart-usb cable connected to pc 
 
 1. ( P0.11(RX) to usb to TTL cable (TX)) 
 2. (P0.10(TX) to usb to TTL cable (RX)) 
 3. Common ground 
 In serial lte modem app .proj file modified to enable Uart2 based macros and disabled the UART 0 based macro. 
 Note: I/O switch is kept at 3.3v side 
 Still couldn't get response for AT commands. 
 
 Please let me know is there is a steps to configure UART 2 with no flow control and no additional gpio . 
 I am using segger embedded studio. 
 
 With Best Regards, 
 Ilanchezhian T

helsing · Accepted Answer

6.a) Thank you for the clarification. In order to program the nRF9160 SiP you only need the programming interface mentioned as a reply to 6).

helsing · Answer

Ilanchezhian T said: 2) What are the necessary UART pins required ( As per the application note we have seen that 5 pins (RTS,CTS,RX,TX,GPIO) are mentioned, I think GND pin is also required as the uart signals are not differential signals) 
 2) Normally, only the three connections RX, TX and ground are needed for UART to work without flow control. I have not yet been able to confirm whether someone has tested disabling this with serial_lte_modem, specifically. 
 Ilanchezhian T said: 2.a) If all the pins are not required ( then RX,TX,GND) is ok to establish connection between modem and our host microcontroller 
 2.a) The serial_lte_modem has a config option, CONFIG_SLM_INTERFACE_PIN - Interface GPIO to wake up or exit idle mode , for setting up an additional GPIO between nRF9160 and the external MCU. This GPIO seems to be used for exiting nRF9160 from IDLE or sleep. If you have another way to wake up nRF9160, then only RX, TX and GND are needed between the two devices.

how to interface external MCU host viaUART interface with nrf9160, with only 3 pins from UART (RD,TX,GND)

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