Device Tree, proj.conf and strange uart behaviour

[Jas39]

What is going on here?

I'm trying to get the central_uart example to run on the xiao ble sense nrf52840 board but uart_init() fails when trying to set the callback:

uart_callback_set(uart, uart_cb, NULL);  // This fails since no set function

If I select to build for ntf52840 DK nrf52840, same code but different Device Tree, uart_init works.

I've tried to compare the two Trees by diffing the zephyr.dts files but find them same for the uart0 setup:

uart0: xiao_serial: uart@40002000 {
compatible = "nordic,nrf-uarte";
reg = < 0x40002000 0x1000 >;
interrupts = < 0x2 0x1 >;
status = "okay";
current-speed = < 0x1c200 >;
pinctrl-0 = < &uart0_default >;
pinctrl-1 = < &uart0_sleep >;
pinctrl-names = "default", "sleep";
};

uart0_default: uart0_default {
phandle = < 0x2 >;
group1 {
psels = < 0x6 >, < 0x20005 >;
};
group2 {
psels = < 0x10008 >, < 0x30007 >;
bias-pull-up;
};
};
uart0_sleep: uart0_sleep {
phandle = < 0x3 >;
group1 {
psels = < 0x6 >, < 0x10008 >, < 0x20005 >, < 0x30007 >;
low-power-enable;
};
};

The proj.conf file sets

CONFIG_UART_NRFX=y

CONFIG_UART_NRFX_UARTE_LEGACY_SHIM=y

CONFIG_UART_0_ASYNC=y

CONFIG_UART_0_ENHANCED_POLL_OUT=y

When I compare the two different .config files I find that the xiao boards has

CONFIG_UART_INTERRUPT_DRIVEN=y

CONFIG_UART_CONSOLE=y

CONFIG_UART_EXCLUSIVE_API_CALLBACKS=y

CONFIG_UART_0_INTERRUPT_DRIVEN=y

Whereas the nrf52840 DK has

CONFIG_UART_0_ASYNC=y

Is there anyone with a better understanding of the build system that can advice where/why this difference arise?

[Vidar Berg]

Hello,

The xiao ble sense board is using USB CDC for console, logging, etc, and the USB stack is not compatible with the Asynchronous API, only the Interrupt-driven API. However, it should be possible to disable  enable the ASYNC api for the uart0 node by setting CONFIG_UART_0_INTERRUPT_DRIVEN=n and CONFIG_UART_0_ASYNC=y in your prj.conf file.

Best regards,

Vidar

Edit: Replaced the word 'disable' with 'enable'. Attached below is also a modified version of the central UART sample I created to support the USB interface if you prefer that over using UART. (Note: it requires SDK v2.5.2 to build, as it has not been ported to v2.6.0).

central_uart_w_usb_ncs_2_5_2.zip

[Jas39]

Thanks Vidar.

Worth noting is that order seems to be important, this sequence works:

# Expilict async on uart0 to override USB CDC

CONFIG_UART_0_INTERRUPT_DRIVEN=n

CONFIG_UART_0_ASYNC=y

[Vidar Berg]

Hm, the order should not matter. I recommend checking the generated .config file in build/zephyr if you want to verify if the symbol is set correctly. You should also see Kconfig warnings in the build log if assignments from your prj.conf are ignored due to missing dependencies or conflicting configurations.

[Jas39]

When set in the other order the buildsystem warned that CONFIG_UART_0_INTERRUPT_DRIVEN=y when setting CONFIG_UART_0_ASYNC=y

I switched order and did not investigate further. Your statement is inline with my expectations on build that order should not matter.