arch/avr/avrdx: provide tickless OS option

This patch adds tickless OS support for AVR architecture in alarm mode.
Currently, only devices from AVR DA/DB family are supported.

Documentation/platforms/avr/avrdx/index.rst

@@ -18,6 +18,13 @@ System timer is provided by RTC peripheral and takes over it. (In theory,
 the Periodic Interrupt - PIT - component of it can be still used
 but they share single clock.)
 
+Tickless OS mode is supported in alarm mode. Configure it
+in :menuselection:`RTOS Features --> Clocks and Timers --> Support tick-less OS`
+:menuselection:`Tickless alarm` must be set, timer mode is not supported.
+:menuselection:`System timer tick period (microseconds)` also needs to be
+changed to value of at least 300. Higher value is recommended though,
+300us is not going to be precise at all.
+
 Supported Boards
 ================
 

arch/avr/Kconfig

@@ -27,6 +27,7 @@ config ARCH_CHIP_AVRDX
 	bool "AVR 32/64/128 DA/DB core"
 	select ARCH_FAMILY_AVR
 	select MM_SMALL
+	select ARCH_HAVE_TICKLESS
 	---help---
 		Atmel/Microchip AVR 32/64/128 DA/DB core family.
 

arch/avr/src/avrdx/Make.defs

@@ -53,6 +53,8 @@ endif
 CHIP_ASRCS = avrdx_exceptions.S
 CHIP_CSRCS = avrdx_lowconsole.c avrdx_lowinit.c avrdx_serial.c
 
-ifneq ($(CONFIG_SCHED_TICKLESS),y)
+ifeq ($(CONFIG_SCHED_TICKLESS),y)
+CHIP_CSRCS += avrdx_timerisr_tickless_alarm.c
+else
 CHIP_CSRCS += avrdx_timerisr.c
 endif