HL78xx Auto Baud Rate Switching

Overview

This feature enables automatic baud rate detection and switching for the Sierra Wireless HL78xx modem driver. The driver can automatically detect the modem’s current baud rate and switch to a configured target baud rate using the AT+IPR command.

Features

  • Auto-detection: Automatically detects the modem’s current baud rate by trying a list of common baud rates

  • Dynamic switching: Changes the modem’s baud rate to the configured target using the AT+IPR command

  • Configurable: Supports multiple baud rates from 9600 to 921600 bps

  • Retry mechanism: Configurable retry count for robust detection

  • State machine integration: Seamlessly integrated into the modem initialization sequence

Configuration

Enable the feature in your project’s prj.conf:

# Enable auto baud rate detection and switching
CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y

# Set target baud rate (default is 115200)
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y

# Configure detection baud rates (comma-separated list)
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="115200,9600,57600,38400,19200"

# Set timeout for each detection attempt (in seconds)
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=4

# Set number of retries
CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=3

# Save baud rate change to modem NVMEM (default: y, recommended)
CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT=y

# Try target baud rate first before detection list (default: y, faster)
CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=y

# Only perform auto-baud if initial communication fails (default: y, efficient)
CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=y

# Perform auto-baud immediately at boot, skip KSUP wait (default: n)
CONFIG_MODEM_HL78XX_AUTOBAUD_AT_BOOT=n

Supported Baud Rates

The following baud rates are supported:

  • 9600 bps

  • 19200 bps

  • 38400 bps

  • 57600 bps

  • 115200 bps (default)

  • 230400 bps

  • 460800 bps

  • 921600 bps

  • 3000000 bps (HL7812 only)

How It Works

Detection Phase (MODEM_HL78XX_STATE_SET_BAUDRATE)

When the modem enters the SET_BAUDRATE state after power-on:

  1. The driver tries each baud rate from the detection list

  2. For each rate, it:

    • Configures the UART to that baud rate

    • Sends an AT command

    • Waits for an OK response

  3. Once a response is received, the current baud rate is identified

Switching Phase

If the detected baud rate differs from the target:

  1. Send AT+IPR=<target_baudrate> at the current baud rate

  2. Wait for OK response

  3. Wait 2.5 seconds for the modem to apply the change

  4. Reconfigure the host UART to the new baud rate

  5. Wait 50 ms for UART stabilization

  6. Verify communication by sending an AT test command

  7. Send AT&W to save configuration to NVMEM

Note

After baud rate switching completes, the state machine runs the post-restart script to wait for the KUSP URC message before proceeding to initialization.

State Transitions

AWAIT_POWER_ON --> SET_BAUDRATE --> RUN_INIT_SCRIPT
                        |
                        +--> [On Failure] --> RUN_INIT_FAIL_DIAGNOSTIC_SCRIPT

Use Cases

Use Case 1: Unknown Modem Baud Rate

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="115200,9600,57600,38400,19200"

Use Case 2: High-Speed Communication

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y

Use Case 3: Fast Boot with Known Baud Rate

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=y
CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=y
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=2

Result: Boot time ~50–100 ms if modem is already at target rate.

Use Case 4: Production / Field Deployment

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="115200,57600,38400,19200,9600"
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=8
CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=5
CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT=y
CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=y
CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=y

Use Case 5: Temporary Baud Rate Change

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT=n

Note

The modem will revert to the previous baud rate after a power cycle or AT+CFUN reset.

AT Commands Used

Query Current Baud Rate

AT+IPR?

Response:

+IPR: <rate>
+IPR: 0

Set Fixed Baud Rate

AT+IPR=<rate>

Where <rate> can be:

  • 9600

  • 19200

  • 38400

  • 57600

  • 115200

  • 230400

  • 460800

  • 921600

  • 3000000 (HL7812 only)

Important

The new baud rate takes effect after ~2 seconds. Use AT&W to persist the configuration across power cycles.

Troubleshooting

Detection Fails

Symptoms: Modem enters diagnostic state after multiple retries

Solutions:

  1. Verify detection list includes the modem’s current baud rate

  2. Increase timeout

  3. Check UART wiring and signal quality

  4. Verify modem is powered and responsive

Switching Fails

Symptoms: Detection succeeds but switching fails

Solutions:

  1. Verify target baud rate is supported by modem and UART

  2. Check UART clock limitations

  3. Try a lower baud rate

  4. Verify firmware supports AT+IPR

Communication Lost After Switch

Root Cause: Host UART not reconfigured before sending AT&W

Solutions:

  1. Reconfigure host UART before AT&W

  2. Wait 2.5 seconds after AT+IPR

  3. Wait for KUSP URC

  4. Check UART buffering and flow control

Disabling Auto-Baud Causes Failure

Root Cause: Modem remains at saved baud rate while UART defaults differ.

Solutions:

  • Keep auto-baud enabled (recommended)

  • Update device tree UART speed:

&uart_modem {
    current-speed = <921600>;
};

  • Manually reset modem baud rate before disabling auto-baud

Implementation Details

Key Functions

static int hl78xx_try_baudrate(struct hl78xx_data *data, uint32_t baudrate);
static int hl78xx_detect_current_baudrate(struct hl78xx_data *data);
static int hl78xx_switch_baudrate(struct hl78xx_data *data, uint32_t target_baudrate);

State Handler

static int hl78xx_on_set_baudrate_state_enter(struct hl78xx_data *data);
static void hl78xx_set_baudrate_event_handler(struct hl78xx_data *data,
                                              enum hl78xx_event evt);

Data Structure

struct uart_status {
#ifdef CONFIG_MODEM_HL78XX_AUTO_BAUDRATE
    uint32_t current_baudrate;
    uint32_t target_baudrate;
    uint8_t baudrate_detection_retry;
#endif
};

License

Copyright (c) 2025 Netfeasa Ltd.

SPDX-License-Identifier: Apache-2.0

Last Updated: 2025-12-06