samples: boards: stm32: add backup SRAM sample

The sample illustrates how to define and use a variable in the backup
SRAM.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

samples/boards/stm32/backup_sram/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(stm32_backup_sram)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/boards/stm32/backup_sram/README.rst

@@ -0,0 +1,41 @@
+.. _stm32_backup_sram:
+
+STM32 Backup SRAM
+#################
+
+Overview
+********
+
+Multiple STM32 microcontrollers have a small backup SRAM that can be used as a
+NVM when VBAT pin is supplied with a voltage source, e.g. a coin button cell.
+
+This example shows how to define a variable on the Backup SRAM. Each time the
+application runs the current value is displayed and then incremented by one. If
+VBAT is preserved, the incremented value will be shown on the next power-cycle.
+
+.. note::
+
+    On most boards VBAT is typically connected to VDD thanks to a jumper.
+    To excercise this sample with an independent VBAT source, you will need to
+    remove the jumper.
+
+Building and Running
+********************
+
+In order to run this sample, make sure to enable ``backup_sram`` node in your
+board DT file.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/memc/stm32_backup_sram
+   :board: nucleo_h743zi
+   :goals: build
+   :compact:
+
+Sample Output
+=============
+
+.. code-block:: console
+
+    Current value in backup SRAM: 11
+    Next reported value should be: 12
+    Keep VBAT power source and reset the board now!

samples/boards/stm32/backup_sram/prj.conf

@@ -0,0 +1 @@
+CONFIG_STM32_BACKUP_SRAM=y

samples/boards/stm32/backup_sram/sample.yaml

@@ -0,0 +1,8 @@
+sample:
+  description: Usage of backup SRAM on STM32
+  name: stm32_backup_sram
+tests:
+  sample.boards.stm32.backup_sram:
+    depends_on: backup_sram
+    build_only: true
+    filter: dt_compat_enabled("st,stm32-backup-sram")

samples/boards/stm32/backup_sram/src/main.c

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2021 Teslabs Engineering S.L.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include <sys/printk.h>
+
+/** Value stored in backup SRAM. */
+__stm32_backup_sram_section uint32_t backup_value;
+
+void main(void)
+{
+	printk("Current value in backup SRAM: %d\n", backup_value);
+
+	backup_value++;
+#if __DCACHE_PRESENT
+	SCB_CleanDCache_by_Addr(&backup_value, sizeof(backup_value));
+#endif
+
+	printk("Next reported value should be: %d\n", backup_value);
+	printk("Keep VBAT power source and reset the board now!\n");
+}