Add IRQ-driven LED-blinky lesson

Lesson 02 - Blinky IRQ/Makefile

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+PROJECT:=blinky_irq
+
+# shared directory between lessons
+COMMON:=../Common
+
+# toolchain settings
+CROSS:=arm-none-eabi-
+CC:=$(CROSS)gcc
+OBJDUMP:=$(CROSS)objdump
+OBJCOPY:=$(CROSS)objcopy
+
+# gcc & ld compiler flags
+OPT:=-Og
+FLAGS:=-mcpu=cortex-m4 -mthumb
+CFLAGS:=$(FLAGS) $(OPT) -I$(COMMON)/Include -I$(COMMON)/Include/CMSIS5 -fno-common -g3 -Wall -Werror -Wextra
+LDFLAGS:=$(FLAGS) -T$(COMMON)/Linker/minimal.ld -nostartfiles -nostdlib -lnosys
+
+.PHONY: all
+
+all: $(PROJECT).asm $(PROJECT).bin
+
+%.bin: %.elf
+	$(OBJCOPY) $< -O binary $@
+
+%.asm: %.elf
+	$(OBJDUMP) -S $< > $@
+
+%.elf: %.o
+	$(CC) $(LDFLAGS) -o $@ $<
+
+clean:
+	rm -f *.elf *.bin *.asm *.hex

Lesson 02 - Blinky IRQ/blinky_irq.c

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+/**************************************************************************//**
+ * @file     blinky_irq.c
+ * @brief    Demonstrate IRQ-driven program, blinking LED2
+ * @date     12. August 2018
+ ******************************************************************************/
+/*
+ * Copyright (c) 2018 Milosch Meriac <milosch@meriac.com>. All rights reserved.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Licensed under the Apache License, Version 2.0 (the License); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an AS IS BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include <common.h>
+
+/* Lesson 02 gets LED2 */
+#define LED_PIN 18
+
+static void OnIrqUnused(void)
+{
+    /* Spin and do nothing, useful for detecting
+     * spurious interrupts while debugging */
+    while(true);
+}
+
+static void OnSysTick(void)
+{
+    /* Alternate on every SysTick between LED 'on' or LED 'off'
+     * in an interrupt-safe way. Please explain what this code does
+     * and why this is an improvement over Lesson 01 */
+    if(NRF_P0->OUT & (1UL << LED_PIN))
+        NRF_P0->OUTCLR = 1UL << LED_PIN;
+    else
+        NRF_P0->OUTSET = 1UL << LED_PIN;
+}
+
+/* reset function called by CPU upon start */
+void OnReset(void)
+{
+    /* Initialize LED pin as output */
+    NRF_P0->PIN_CNF[LED_PIN] =
+          (GPIO_PIN_CNF_DIR_Output << GPIO_PIN_CNF_DIR_Pos)
+        | (GPIO_PIN_CNF_INPUT_Disconnect << GPIO_PIN_CNF_PULL_Pos);
+
+    /* Configure periodic SysTick timer interrupt next.
+     * Fire every 2^24 CPU cyles, at the maximum counter value */
+    SysTick->LOAD = SysTick_LOAD_RELOAD_Msk;
+    /* Start counting at zero */
+    SysTick->VAL  = 0;
+    /* Configure Systick behaviour */
+    SysTick->CTRL = SysTick_CTRL_CLKSOURCE_Msk | /* Use CPU clock */
+                    SysTick_CTRL_TICKINT_Msk |   /* Trigger Interrupt */
+                    SysTick_CTRL_ENABLE_Msk;     /* Run SysTick timer */
+    /* Infinitely wait for interrupts to happen,
+     * explaining in Lesson 03 for why this is a bad idea */
+    while(true);
+}
+
+/* stack declared in blinky.ld */
+extern const uint32_t StackTop;
+
+/* declare vector table */
+__attribute__ ((section(".vtor")))
+const void* VectorTable[] = {
+    &StackTop,   /* CPU will automatically       *
+                  * set stack its pointer        *
+                  * to this value                */
+
+    OnReset,     /* -15: Reset_IRQn              */
+    OnIrqUnused, /* -14: NonMaskableInt_IRQn     */
+    OnIrqUnused, /* -13: HardFault_IRQn          */
+    OnIrqUnused, /* -12: MemoryManagement_IRQn   */
+    OnIrqUnused, /* -11: BusFault_IRQn           */
+    OnIrqUnused, /* -10: UsageFault_IRQn         */
+    OnIrqUnused, /*  -9:                         */
+    OnIrqUnused, /*  -8:                         */
+    OnIrqUnused, /*  -7:                         */
+    OnIrqUnused, /*  -6:                         */
+    OnIrqUnused, /*  -5: SVCall_IRQn             */
+    OnIrqUnused, /*  -4: DebugMonitor_IRQn       */
+    OnIrqUnused, /*  -3:                         */
+    OnIrqUnused, /*  -2: PendSV_IRQn             */
+    OnSysTick,   /*  -1: SysTick_IRQn            */
+};