Serial LTE Modem fails to build in ncs v1.8.0 without UART hardware flow control

[Michal Mühlpachr]

Hi,

we use UART with 
/delete-property/ hw-flow-control;
/delete-property/ rts-pin;
/delete-property/ cts-pin;
in UART DTS

Serial LTE Modem started to fail in build in ncs v1.8.0

Problematic code is in https://github.com/nrfconnect/sdk-nrf/blob/main/applications/serial_lte_modem/src/slm_at_host.c#L245-L264

How is CONFIG_UART_0_NRF_HW_ASYNC_TIMER related to UART hardware flow control please ?

Serial LTE Modem app should not require RTS pin defined in UART DTS.

Regards,
Michal

[hubpav]

Hello, I will follow up on Michal's ticket (as his colleague) with a possible solution but subject to discussion...

If we delete this definition in our custom-board DTS for UART0 node:

/delete-property/ hw-flow-control;
/delete-property/ rts-pin;
/delete-property/ cts-pin;

... and instead put this in there:

rts-pin = <0xffffffff>;
cts-pin = <0xffffffff>;

... the build finishes and the SLM application seems to work well. However, we are not sure whether this is a valid approach. I found this inspiration in the NCS tree:

https://github.com/nrfconnect/sdk-nrf/blob/3edafd5d3f05860962e17263a80d7928fec1bf15/samples/nrf9160/modem_shell/esp_8266_nrf9160ns.overlay#L27-L28

So the question is:

Is this approach invalid? If it is, what is the right solution - i.e. isn't this a configuration problem in the recently released SLM application?

Thank you.

[pirun lee]

This is actually configuration problem.

We assumed that UART HW flow control would be enabled default.

If UART HW flow control is not necessary, you can ignore the code section as workaround by changing...

#if defined(CONFIG_UART_0_NRF_HW_ASYNC_TIMER)

#if defined(CONFIG_UART_2_NRF_HW_ASYNC_TIMER)

to

#if false

We will fix this with DT node checking soon.

[hubpav]

Hi. Perfect. Thanks for the confirmation. We can help with testing before it gets to the release if you tell us the commit, so we can cherry-pick it.