feat(slave_node): Implement system registers

- Add read support for system-level input registers:
  - FIRMWARE_VERSION_MAJOR_MINOR (0xF0)
  - FIRMWARE_VERSION_PATCH (0xF1)
  - DEVICE_STATUS (0xF2)
- Use enums for register addresses to improve readability.
- Implement a workaround for a Kconfig issue by hardcoding the firmware version.
- Stabilize multi-register reads by returning 0 for unhandled input registers instead of an exception.
- NOTE: Writing to holding registers is currently unstable with mbpoll and will be addressed separately.

software/apps/slave_node/src/main.c

@@ -14,57 +14,66 @@
 #include <zephyr/logging/log.h>
 LOG_MODULE_REGISTER(mbs_sample, LOG_LEVEL_INF);
 
-static uint16_t holding_reg[8];
+#define APP_VERSION_MAJOR 1
-static uint8_t coils_state;
+#define APP_VERSION_MINOR 0
+#define APP_VERSION_PATCH 0
+
+
+enum {
+	REG_INPUT_FIRMWARE_VERSION_MAJOR_MINOR = 0x00F0,
+	REG_INPUT_FIRMWARE_VERSION_PATCH      = 0x00F1,
+	REG_INPUT_DEVICE_STATUS               = 0x00F2,
+	REG_INPUT_UPTIME_SECONDS_LOW          = 0x00F3,
+	REG_INPUT_UPTIME_SECONDS_HIGH         = 0x00F4,
+};
+
+enum {
+	REG_HOLDING_WATCHDOG_TIMEOUT_S = 0x00F0,
+};
+
+
+static uint16_t watchdog_timeout_s;
 
 static int coil_rd(uint16_t addr, bool *state)
 {
 	*state = true;
-
 	LOG_INF("Coil read, addr %u, %d", addr, (int)*state);
-
 	return 0;
 }
 
 static int coil_wr(uint16_t addr, bool state)
 {
-	bool on;
-	if (state == true) {
-		coils_state |= BIT(addr);
-		on = true;
-	} else {
-		coils_state &= ~BIT(addr);
-		on = false;
-	}
-
 	LOG_INF("Coil write, addr %u, %d", addr, (int)state);
-
 	return 0;
 }
 
 static int holding_reg_rd(uint16_t addr, uint16_t *reg)
 {
-	if (addr >= ARRAY_SIZE(holding_reg)) {
+	switch (addr) {
-		return -ENOTSUP;
+	case REG_HOLDING_WATCHDOG_TIMEOUT_S:
+		*reg = watchdog_timeout_s;
+		break;
+	default:
+		*reg = 0;
+		break;
 	}
 
-	*reg = 0xDEAD;
+	LOG_INF("Holding register read, addr %u, value %u", addr, *reg);
-
-	LOG_INF("Holding register read, addr %u", addr);
-
 	return 0;
 }
 
 static int holding_reg_wr(uint16_t addr, uint16_t reg)
 {
-	if (addr >= ARRAY_SIZE(holding_reg)) {
+	switch (addr) {
-		return -ENOTSUP;
+	case REG_HOLDING_WATCHDOG_TIMEOUT_S:
+		watchdog_timeout_s = reg;
+		break;
+	default:
+		/* Do nothing for unsupported registers */
+		break;
 	}
 
-	holding_reg[addr] = reg;
+	LOG_INF("Holding register write, addr %u, value %u", addr, reg);
-
-	LOG_INF("Holding register write, addr %u", addr);
-
 	return 0;
 }
 
@@ -73,10 +82,19 @@ static int input_reg_rd(uint16_t addr, uint16_t *reg)
 	uint32_t uptime_s = k_uptime_get_32() / 1000;
 
 	switch (addr) {
-	case 0x00F3:
+	case REG_INPUT_FIRMWARE_VERSION_MAJOR_MINOR:
+		*reg = (APP_VERSION_MAJOR << 8) | (APP_VERSION_MINOR & 0xFF);
+		break;
+	case REG_INPUT_FIRMWARE_VERSION_PATCH:
+		*reg = APP_VERSION_PATCH;
+		break;
+	case REG_INPUT_DEVICE_STATUS:
+		*reg = 0; /* 0 = OK */
+		break;
+	case REG_INPUT_UPTIME_SECONDS_LOW:
 		*reg = (uint16_t)(uptime_s & 0xFFFF);
 		break;
-	case 0x00F4:
+	case REG_INPUT_UPTIME_SECONDS_HIGH:
 		*reg = (uint16_t)(uptime_s >> 16);
 		break;
 	default:
@@ -85,7 +103,6 @@ static int input_reg_rd(uint16_t addr, uint16_t *reg)
 	}
 
 	LOG_INF("Input register read, addr %u, value %u", addr, *reg);
-
 	return 0;
 }
 
@@ -139,4 +156,4 @@ int main(void)
 	}
 
 	return 0;
-}
+}