Message

message/Makefile

@@ -0,0 +1,12 @@
+CC ?= gcc
+CFLAGS ?= -Wall -O2
+
+TARGET = main.elf
+SRC = message.c main_demo.c
+
+$(TARGET): $(SRC)
+	@echo CC $@
+	@$(CC) $(CFLAGS) -o $@ $^ $(LDFLAGS)
+
+clean:
+	rm -f $(TARGET)

message/main_demo.c

@@ -0,0 +1,14 @@
+#include "message.h"
+#include <stdio.h>
+#include <assert.h>
+
+int main(void) {
+    Message m;
+    message_create(&m, UI_BATTERY, (MessageData){75});
+
+    printf("%04X\n", m.crc);
+    int16_t crc =  compute_crc16((const uint8_t *)&m, offsetof(Message, crc));
+    printf("%04X\n", crc);
+    assert(message_validate(&m));
+    return 0;
+}

message/message.c

@@ -0,0 +1,52 @@
+#include "message.h"
+#include <assert.h>
+
+uint16_t compute_crc16(const uint8_t *data, uint16_t len);
+bool     is_type_valid(MessageType t);
+
+/**
+ * @brief Create a message
+ *
+ *      Use as: create_message(&m1, UI_VOLTAGE, (MessageData){.voltage = 6.3});
+ *
+ * @param m A pointer to the message struct
+ * @param type The enum type of the message you want to send
+ * @param data The data corresponding to the message type
+ */
+void message_create(Message *m, MessageType type, MessageData data) {
+    m->type = type;
+    m->data = data;
+    m->crc = compute_crc16((const uint8_t *)m, offsetof(Message, crc));
+}
+
+bool message_validate(const Message *m) {
+    return is_type_valid(m->type) && m->crc == compute_crc16((const uint8_t *)m, offsetof(Message, crc));
+}
+
+// Standard CRC-16-CCITT (poly 0x1021, init 0xFFFF)
+uint16_t compute_crc16(const uint8_t *data, uint16_t len) {
+    uint16_t crc = 0xFFFF;
+    for (uint16_t i = 0; i < len; i++) {
+        crc ^= (uint16_t)data[i] << 8;
+        for (int j = 0; j < 8; j++) {
+            if (crc & 0x8000)
+                crc = (crc << 1) ^ 0x1021;
+            else
+                crc <<= 1;
+        }
+    }
+    return crc;
+}
+
+bool is_type_valid(MessageType t) {
+    return t >= UI_SPEED_MS && t <= UI_VOLTAGE;
+}
+
+int message_tests(void) {
+    Message m;
+    message_create(&m, UI_BATTERY, (MessageData){75});
+
+    assert(message_validate(&m));
+
+    return 0;
+}

message/message.h

@@ -0,0 +1,31 @@
+#ifndef MESSAGE_H
+#define MESSAGE_H
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+typedef enum {
+    UI_SPEED_MS,
+    UI_BATTERY,
+    UI_VOLTAGE,
+} MessageType;
+
+typedef union {
+    int   battery;
+    int   speed_ms;
+    float voltage;
+} MessageData;
+
+typedef struct {
+    MessageType type;
+    MessageData data;
+    int16_t     crc;
+} Message;
+
+void message_create(Message *m, MessageType type, MessageData data);
+bool message_validate(const Message *m);
+int  message_tests(void);
+uint16_t compute_crc16(const uint8_t *data, uint16_t len);
+
+#endif