drivers: DTR/RI virtual UART implementation

Terms:
Data Terminal Equipment (DTE), AKA. host.
Data Communication Equipment (DCE), AKA. Serial Modem.
Data Terminal Ready (DTR)
Ring Indicate (RI)

DTR operation:
When DCE notices DTR asserted fom DTE, it resumes UART, enables
RX and sends any possibly pending TX data.

When DCE notices DTR deasserted from DTE, it aborts possible TX,
disables RX and suspends UART.

RI operation:
When DCE observes DTR deasserted and it needs to send data, it sets
RI up to notify DTE.

Application integration:
* uart_rx_enable is used as indicator that application is ready.
* UART_RX_BUF_REQUEST is sent by virtual UART to application to
  request a buffer.
* Logic level 1 is asserted, use DT to change 0/1 level of that.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>
Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>

dtr

dtr

Author: MarkusLassila

CMakeLists.txt

@@ -6,4 +6,5 @@
 
 zephyr_include_directories(include)
 
+add_subdirectory(drivers)
 add_subdirectory(lib)

Kconfig

@@ -5,4 +5,5 @@
 # as the module Kconfig entry point (see zephyr/module.yml). You can browse
 # module options by going to Zephyr -> Modules in Kconfig.
 
+rsource "drivers/Kconfig"
 rsource "lib/Kconfig"

drivers/CMakeLists.txt

@@ -0,0 +1,5 @@
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+add_subdirectory_ifdef(CONFIG_DTR_UART dtr_uart)

drivers/Kconfig

@@ -0,0 +1,8 @@
+#
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+menu "Drivers"
+rsource "dtr_uart/Kconfig"
+endmenu

drivers/dtr_uart/CMakeLists.txt

@@ -0,0 +1,8 @@
+#
+# Copyright (c) 2025 Nordic Semiconductor
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+
+zephyr_library()
+zephyr_library_sources(dtr_uart.c)

drivers/dtr_uart/Kconfig

@@ -0,0 +1,31 @@
+#
+# Copyright (c) 2025 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+
+config DTR_UART
+	bool "DTR UART implementation - DCE side"
+	default y
+	depends on DT_HAS_NORDIC_DTR_UART_ENABLED
+	select GPIO
+	select UART_ASYNC_API
+	select RING_BUFFER
+	select PM_DEVICE
+	help
+	  Data Terminal Ready (DTR) UART implements UART API and extends UART communication
+	  with DTR and Ring Indicator (RI) signals.
+
+	  Data Communication Equipment (DCE) is controlled by DTE (Data Terminal Equipment)
+	  using DTR signal. When DTR is deasserted, DCE will stop the UART communication and
+	  power down the UART peripheral. When DTR is asserted, DCE will power up the UART
+	  peripheral and start the UART communication.
+
+	  RI signal is used by DCE to notify DTE that data is available to read.
+
+if DTR_UART
+
+module = DTR_UART
+module-str = dtr uart
+source "subsys/logging/Kconfig.template.log_config"
+
+endif # DTR_UART