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

[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]

6.a) Question is - Do we need to have  similar hardware circuitry as present in nRF9160 DK ( usb connector and debugger circuitry) in our product in order to code flashing in both Modem Processor and Application Processor.

[helsing]

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]

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. 

 

[helsing]

2.a) Using the GPIO is the recommended way to go. If saving one pin is important, you may leave the peripheral running while the application is sleeping. This would add an extra ~0.5 mA. 

[Ilanchezhian T]

Thanks for the clarification on this