arch/arm/ameba: add shared RTC driver for RTL8721Dx / RTL8720F / RTL8721F

Documentation/platforms/arm/rtl8720f/boards/rtl8720f_evb/index.rst

@@ -43,6 +43,8 @@ Supported in this NuttX port:
   the SDK fwlib timer register layer
 * ADC channels exposed as an ``/dev/adc0`` character device, driven directly
   on the SDK fwlib register layer
+* On-chip RTC exposed as a ``/dev/rtc0`` date/time character device with
+  alarm support, driven directly on the SDK fwlib register layer
 
 Buttons and LEDs
 ================
@@ -161,6 +163,20 @@ example::
 
     nsh> adc -n 1                        # one sweep of /dev/adc0
 
+rtc
+---
+
+Minimal NSH with the on-chip RTC driver and the ``alarm`` example enabled
+(no Wi-Fi). The RTC is registered at ``/dev/rtc0`` from the board bring-up
+(``boards/arm/rtl8720f/rtl8720f_evb/src/rtl8720f_rtc.c``); it has no board
+wiring (it is an internal clock). The hardware stores year + day-of-year, so
+the shared driver bridges to a full calendar. Read and set the clock with the
+NSH ``date`` command, and arm a one-shot wakeup with the example::
+
+    nsh> date                            # read /dev/rtc0
+    nsh> date -s "Jun 16 12:00:00 2026"  # set the RTC
+    nsh> alarm 10                        # fire an alarm in 10 seconds
+
 nsh
 ---
 

Documentation/platforms/arm/rtl8721dx/boards/pke8721daf/index.rst

@@ -45,6 +45,8 @@ Supported in this NuttX port:
   the SDK fwlib timer register layer
 * ADC channels exposed as an ``/dev/adc0`` character device, driven directly
   on the SDK fwlib register layer
+* On-chip RTC exposed as a ``/dev/rtc0`` date/time character device with
+  alarm support, driven directly on the SDK fwlib register layer
 
 Buttons and LEDs
 ================
@@ -170,6 +172,20 @@ example::
 
     nsh> adc -n 1                        # one sweep of /dev/adc0
 
+rtc
+---
+
+Minimal NSH with the on-chip RTC driver and the ``alarm`` example enabled
+(no Wi-Fi). The RTC is registered at ``/dev/rtc0`` from the board bring-up
+(``boards/arm/rtl8721dx/pke8721daf/src/rtl8721dx_rtc.c``); it has no board
+wiring (it is an internal clock). The hardware stores year + day-of-year, so
+the shared driver bridges to a full calendar. Read and set the clock with the
+NSH ``date`` command, and arm a one-shot wakeup with the example::
+
+    nsh> date                            # read /dev/rtc0
+    nsh> date -s "Jun 16 12:00:00 2026"  # set the RTC
+    nsh> alarm 10                        # fire an alarm in 10 seconds
+
 Wi-Fi
 =====
 

Documentation/platforms/arm/rtl8721f/boards/rtl8721f_evb/index.rst

@@ -44,6 +44,8 @@ Supported in this NuttX port:
   the SDK fwlib timer register layer
 * ADC channels exposed as an ``/dev/adc0`` character device, driven directly
   on the SDK fwlib register layer
+* On-chip RTC exposed as a ``/dev/rtc0`` date/time character device with
+  alarm support, driven directly on the SDK fwlib register layer
 
 Buttons and LEDs
 ================
@@ -164,6 +166,20 @@ trigger. Read the channels with the example::
 
     nsh> adc -n 1                        # one sweep of /dev/adc0
 
+rtc
+---
+
+Minimal NSH with the on-chip RTC driver and the ``alarm`` example enabled
+(no Wi-Fi). The RTC is registered at ``/dev/rtc0`` from the board bring-up
+(``boards/arm/rtl8721f/rtl8721f_evb/src/rtl8721f_rtc.c``); it has no board
+wiring (it is an internal clock). The hardware stores year + day-of-year, so
+the shared driver bridges to a full calendar. Read and set the clock with the
+NSH ``date`` command, and arm a one-shot wakeup with the example::
+
+    nsh> date                            # read /dev/rtc0
+    nsh> date -s "Jun 16 12:00:00 2026"  # set the RTC
+    nsh> alarm 10                        # fire an alarm in 10 seconds
+
 nsh
 ---
 

arch/arm/src/common/ameba/Kconfig

@@ -117,4 +117,21 @@ config AMEBA_ADC
 		polling in task context (the chip does not enable a hardware
 		software-trigger path).
 
+config AMEBA_RTC
+	bool "RTC"
+	default n
+	select RTC
+	select RTC_DRIVER
+	select RTC_DATETIME
+	---help---
+		Expose the Ameba on-chip real-time clock as a NuttX date/time RTC
+		at /dev/rtc0 (rdtime/settime).  Enable RTC_ALARM as well to get a
+		single one-shot alarm whose expiry fires the RTC interrupt and the
+		registered upper-half callback.
+
+		The driver (arch/arm/src/common/ameba/ameba_rtc.c) sits on the SDK
+		fwlib RTC register layer.  The hardware keeps a year plus a
+		day-of-year (no month/day register); the driver bridges that to the
+		NuttX month/day calendar with the libc UTC calendar routines.
+
 endmenu # Ameba Peripheral Support