SLM: Refactor DTR+RI UART handler

Work in progress: Not ready.

* Refactor to use just Zephyr GPIO API
* Use Devicetree to define pins
* Refactor DTR UP/DOWN events to ASSERT/DEASSERT
* Logic level 1 is asserted, use DT to change 0/1 level of that.
* Remove support of DTE device, that is separate driver

TODO:
* sw_dtr should not keep RX buffers. Instead when RX is enabled, feed
  buffer to UART from application call.
* When buffer is needed, make a callback to app to request a buffer.
* When UART is powered down because of DTR, block RX_STOP and RX_DISABLE
  events going to application, so app thinks the UART keeps on going.


Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo

applications/serial_lte_modem/overlay-zephyr-modem-nrf9160dk-nrf52840.conf

@@ -5,4 +5,30 @@
 #
 
 # nRF52 <=> nRF91 interface pin 4 (see https://docs.nordicsemi.com/bundle/ug_nrf91_dk/page/UG/nrf91_DK/board_controller.html)
-CONFIG_SLM_POWER_PIN=22
+CONFIG_SLM_POWER_PIN=-1
+
+CONFIG_USE_SEGGER_RTT=n
+# Where console messages (printk) are output.
+# By itself, SLM does not output any.
+CONFIG_RTT_CONSOLE=n
+CONFIG_UART_CONSOLE=y
+# Where SLM logs are output.
+CONFIG_LOG_BACKEND_RTT=n
+CONFIG_LOG_BACKEND_UART=y
+
+CONFIG_SHELL=y
+CONFIG_GPIO=y
+CONFIG_GPIO_SHELL=y
+CONFIG_PM_DEVICE_SHELL=y
+
+CONFIG_UART_1_INTERRUPT_DRIVEN=n
+CONFIG_UART_1_ASYNC=y
+CONFIG_UART_USE_RUNTIME_CONFIGURE=y
+CONFIG_UART_ASYNC_API=y
+
+CONFIG_SLM_SMS=n
+CONFIG_SLM_GNSS=n
+CONFIG_SLM_NRF_CLOUD=n
+CONFIG_SLM_GPIO=n
+
+CONFIG_SIZE_OPTIMIZATIONS_AGGRESSIVE=y

applications/serial_lte_modem/overlay-zephyr-modem-nrf9160dk-nrf52840.overlay

@@ -8,11 +8,18 @@
 
 / {
 	chosen {
-		ncs,slm-uart = &uart1;
+		ncs,slm-uart = &dtr_uart;
 	};
 };
 
 &uart1 {
         current-speed = <115200>;
         hw-flow-control;
+	status = "okay";
+	dtr_uart: nrf-sw-dtr-uart {
+		compatible = "nordic,nrf-sw-dtr-uart";
+		dtr-gpios = <&interface_to_nrf52840 4 (GPIO_PULL_UP | GPIO_ACTIVE_LOW)>;
+		ri-gpios = <&interface_to_nrf52840 5 (GPIO_ACTIVE_LOW)>;
+		status = "okay";
+	};
 };