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

[Ilanchezhian T]

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

[Ilanchezhian T]

Before connecting any other host MCU, is there a way to connect the external USB to UART TTL cable to test the function of modem nrf9160 using AT commands. 

If so please let me know the steps to do that  in nRF9160 development kit https://www.nordicsemi.com/Products/Development-hardware/nrf9160-dk

With Regards

Ilanchezhian 

[helsing]

Hi

Take a look at this case and let me know how it goes.

Disable HWFC in serial modem application

Best regards,

Håkon

[Ilanchezhian T]

Thanks for the response. 

1. I could see the different information ( mentioned in nRF9160 DK HW manual and application description in developer guide) regarding the UART2 pins of the nrF9160. Which one is correct I am really confused. 

2.regarding the settings to be touched I have taken a screen shot, could you provide your feedback for the same. 

3. As I am new to embedded linux based development, I do not really understand how feature configuration and pin configuration works in the form of .dts, .dtsoverlay, menuconfig and prj.conf. Please let me know the proper steps to configure and use application processors UART2 of nRF9160 in 2 pin mode. My end objective is to interface my external MCU host via UART interface to communicate via UART I/F with nrF9160 modem via AT commands.

Thanks 

Ilanchezhian T

[Ilanchezhian T]

Hi ,

Let me put down what I am trying to Evaluate.

Our objective is to use the nrF9160 as a module which can act as AT modem and can be
interfaced with our 16bit microcontroller via UART I/F.

Setup for Evaluation:

Hardware used - nRF9160-DK

Software used -

1. Modem Firmware - mfw_nrf9160_1.3.0

2. SDK - version 1.6.1

3. Application Software - LTE Serial Modem ( present in the installed sdk under location : C:\ESLI\nrfIDE\v1.6.1\nrf\applications\serial_lte_modem)
- Please suggest is this a right application we have choosen for our use case

4. With the USB cable connected and emulated via VCOM port, we are able to communicate via NRF link Manager software using AT commands

Queries:

Please note as we do not know how the "LTE Serial Modem" software is written we have certain queries which are listed below,
also for initial evaluation we are planning to use the nrF9160-DK, before going with custom hardware

1) For our initial validation to check external UART is possible, I am planning to connect the pins of USB-232-3V3 cable
to the UART pins present in the nrF9160-DK and provide AT command via any PC serial terminal software like Docklite for example.
1.a) Could you please let us know this appoach is fine. This is what we are trying and could not do as of now.

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.a) If all the pins are not required ( then RX,TX,GND) is ok to establish connection between modem and our host microcontroller

3) What are pins present in nRF9160-DK can be used to have external UART interface with host microcontroller
3.a) Does any modifications in "LTE Serial Modem" software [OR] some configuration that needs to be done to have "LTE Serial Modem" with respect to UART interface

4) Our microcontroller I/O line is 3.3V does the nRF9160 supports 3.3V on its I/O line

5) We might need to develop the custom hardware that gets plugged into our product, How easy is to customize the "LTE Serial Modem" software available as part of nrf9160 SDK
5.a) If it is very simple only in terms of configurations please provide us some steps to perform this

6) In our custom hardware , to flash the nrf9160 modem software and its MCU for applications, What are the additional circuitry required other than the uart I/F connections, VCC and GND ?
6.a) I think we need usb and jlink circuitry, please provide your input in this regard

[helsing]

1.a) This approach should be fine. Just make sure the voltages are correct. It could be a good idea to measure the voltages from the USB-232-3V3 cable to see if they infact are 3V3. Have you checked this communication with a scope? Also remember to set up nRF9160 DK for 3V3 communication. You can set the VDD voltage to 3V0 with SW9, however, you probably need to go to 3V3. This can be done by customizing the DK's voltage level. If you apply 3V3 to VIO_REF(P20) then the GPIO voltages should be adjusted automatically. 

Ilanchezhian T said:

3) What are pins present in nRF9160-DK can be used to have external UART interface with host microcontroller

 3) You may choose any pin that is not already in use. I could get back to you with a few examples. 

Answers to the rest of your questions will follow later today. 

Best regards,

Håkon