samples: modem: hello_hl78xx: add auto-baud support and log current baud

Enable HL78XX modem auto-baudrate detection and switching in the sample
application. Set 921600 bps as the target baudrate with fallback options
(9600, 115200, 57600, 38400, 19200). Also read and log the current baudrate
from the modem during startup.

This helps verify and debug communication speed mismatches between the
modem and the device UART.

Signed-off-by: Zafer SEN <zafersn93@gmail.com>

Author: zafersn

samples/drivers/modem/hello_hl78xx/prj.conf

@@ -59,6 +59,26 @@ CONFIG_MODEM_HL78XX=y
 # Statistics
 CONFIG_MODEM_STATS=y
 
+# ============================================================================
+# Auto Baud Rate Detection and Switching
+# ============================================================================
+CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
+# CONFIG_MODEM_HL78XX_AUTOBAUD_AT_BOOT=y
+# CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=n
+# CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_9600=y
+# CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_115200=y
+CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
+CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="9600,115200,921600,57600,38400,19200"
+CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=4
+CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=3
+# After enabling auto-baud once WITH CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT,
+# the modem's baud rate is saved to NV-MEM with AT&W, so it persists across power cycles.
+# If you later disable CONFIG_MODEM_HL78XX_AUTO_BAUDRATE, the driver won't detect/switch,
+# but the modem is still at the target baud rate (e.g., 921600)
+# If your device tree UART is at 115200 but the modem is at 921600, communication will fail
+# CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT=n
+# CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=n
+
 # ============================================================================
 # APN Configuration (Disabled - using defaults)
 # ============================================================================

samples/drivers/modem/hello_hl78xx/src/main.c

@@ -299,6 +299,7 @@ int main(void)
 	enum hl78xx_cell_rat_mode tech;
 	enum cellular_registration_status status;
 	int16_t signal_strength = 0;
+	uint32_t current_baudrate = 0;
 	const char *newapn = "";
 	const char *sample_cmd = "AT";
 
@@ -343,6 +344,9 @@ int main(void)
 	/* Get the current network operator name */
 	hl78xx_get_modem_info(modem, HL78XX_MODEM_INFO_NETWORK_OPERATOR, (char *)operator,
 			      sizeof(operator));
+	/* Get the current baudrate */
+	hl78xx_get_modem_info(modem, HL78XX_MODEM_INFO_CURRENT_BAUD_RATE, &current_baudrate,
+			      sizeof(current_baudrate));
 
 	LOG_RAW("\n**********************************************************\n");
 	LOG_RAW("********* Hello HL78XX Modem Sample Application **********\n");
@@ -360,6 +364,7 @@ int main(void)
 	LOG_INF("RSRP : %d", signal_strength);
 #endif
 	LOG_INF("Operator: %s", (strlen(operator) > 0) ? operator : "\"\"");
+	LOG_INF("Current Baudrate: %dbps", current_baudrate);
 	LOG_RAW("**********************************************************\n\n");
 
 	LOG_INF("Setting new APN: %s", newapn);