Added example demonstrating reading and writing from and to flash memory

examples/stm32/f3/stm32f3-discovery/flash/Makefile

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+##
+## This file is part of the libopencm3 project.
+##
+## Copyright (C) 2019 Roel Gerrits <[email protected]>
+##
+## This library is free software: you can redistribute it and/or modify
+## it under the terms of the GNU Lesser General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This library is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public License
+## along with this library.  If not, see <http://www.gnu.org/licenses/>.
+##
+
+BINARY = flash
+
+# Note: in this example we use a custom linker script!
+LDSCRIPT = stm32f3-discovery.ld
+
+include ../../Makefile.include
+

examples/stm32/f3/stm32f3-discovery/flash/README.md

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+# README
+
+Example code that demonstrates the reading and writing of flash 
+at runtime.
+
+For this we add a custom section in the linker file that defines
+which section of flash we will use to write to.
+
+The linker file will provide the C code with a variable that is used
+to get the address of our flash section.
+
+The example will read the variable from flash, write it to the LED's, 
+then it will increase the counter and write it back to flash.
+
+This will cause the LED pattern to change at every reset or power cycle.
+
+NOTE: The flash memory in stm32 chips has a limited durability of
+about 10.000 erase/write cycles, after which it may become unreliable.
+Keep this in mind when designing your application.
+

examples/stm32/f3/stm32f3-discovery/flash/flash.c

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+/*
+ * This file is part of the libopencm3 project.
+ *
+ * Copyright (C) 2019 Roel Gerrits <[email protected]>
+ *
+ * This library is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <libopencm3/stm32/rcc.h>
+#include <libopencm3/stm32/gpio.h>
+#include <libopencm3/stm32/flash.h>
+
+static void gpio_setup(void)
+{
+	/* Enable GPIOE clock. */
+	rcc_periph_clock_enable(RCC_GPIOE);
+
+	/* Set GPIO12 (in GPIO port E) to 'output push-pull'. */
+	gpio_mode_setup(GPIOE, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE,
+		GPIO8 | GPIO9 | GPIO10 | GPIO11 |
+		GPIO12 | GPIO13 | GPIO14 | GPIO15);
+}
+
+
+/*
+ * This variable is made available by the linker script
+ * it's address is the address where the my_flash section starts
+ */
+extern uint8_t _my_flash_start;
+
+
+int main(void)
+{
+	uint16_t counter;
+
+	gpio_setup();
+
+    /* Read value from flash, for reading we can just read from
+	 * the address as with any other variable. Writing requires a
+	 * little more effort as we will see later on.
+	 */
+	counter = _my_flash_start;
+
+    /* Write the counter's binary value to the LED's on port E */
+	gpio_set(GPIOE, (counter & 0xff) << 8);
+
+	/* In order to write to flash we have to unlock it first */
+	flash_unlock();
+
+	/* Flash memory can only be written if it's been erased first
+	 * so let's do that now */
+	uint32_t my_flash_addr = (uint32_t) &_my_flash_start;
+	flash_erase_page(my_flash_addr);
+
+    /* Increment counter and write it to flash */
+	counter++;
+	flash_program_half_word(my_flash_addr, counter);
+
+
+	while (1) {
+		/* do absolutely nothing */
+	}
+
+	return 0;
+}
+

examples/stm32/f3/stm32f3-discovery/flash/stm32f3-discovery.ld

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+/*
+ * This file is part of the libopencm3 project.
+ *
+ * Copyright (C) 2019 Roel Gerrits <[email protected]>
+ *
+ * This library is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Lesser General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public License
+ * along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/* Linker script for ST STM32F3DISCOVERY (STM32F303VC, 256K flash,
+ * 48K RAM (Of which 8kB are CCM RAM and 40KB are SRAM). */
+
+/* Define memory regions. */
+MEMORY
+{
+	/* NOTE: we decrease the length of the rom section with 2K
+	 * as to make space for our custom flash section! */
+	rom (rx) : ORIGIN = 0x08000000, LENGTH = 254K 
+	ram (rwx) : ORIGIN = 0x20000000, LENGTH = 40K
+
+    /* reserve the last page of memory for permanent storage functionality */
+    my_flash (rw) : ORIGIN = 0x08000000 + 254K, LENGTH = 2K
+}
+
+
+
+/*
+ * Define a custom flash section.
+ * This is done for the single purpose of obtaining a
+ * variable (_my_flash_start) that is located at the start of
+ * the my_flash section. This variable can then be used in code
+ * to learn the the start address of that flash section,
+ *
+ * &_my_flash_start will be equal to 0x803f800 in this case.
+*/
+SECTIONS
+{
+    .my_flash :
+    {
+        _my_flash_start = .;
+    } > my_flash
+}
+
+
+/* Include the common ld script. */
+INCLUDE cortex-m-generic.ld
+