HL78xx Auto Baud Rate - Quick Reference

Enable Feature

# In prj.conf
CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y  # Your target rate

Common Configurations

Preset 1: Smart Default (Recommended)

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="921600,9600,57600,38400,115200"
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=4
CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=3
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 when: General purpose, known and unknown baud rates
Boot time impact: ~50-100 ms (already at target), ~4-20 s (detection)
Why smart: Full detection only runs if initial communication fails

Preset 2: Ultra-Fast Boot (Known Rate)

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="921600"
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=1
CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=1
CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=y
CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=y

Use when: Modem is known to be at target rate
Boot time impact: ~50-100 ms
Warning: Fails if modem baud rate differs

Preset 3: High Speed

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="921600,460800,230400,115200"
CONFIG_MODEM_HL78XX_AUTOBAUD_TIMEOUT=3
CONFIG_MODEM_HL78XX_AUTOBAUD_RETRY_COUNT=3
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 when: High throughput required
Boot time impact: ~50 ms (best), ~3-12 s (worst)
Note: Ensure UART hardware supports high baud rates

Preset 4: Robust Production

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_DETECTION_BAUDRATES="115200,57600,38400,19200,9600,921600"
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 when: Field deployment, maximum reliability
Boot time impact: ~50 ms (best), ~40 s (worst)
Features: Diagnostic fallback, comprehensive detection

Preset 5: Testing / Development (Non-Persistent)

CONFIG_MODEM_HL78XX_AUTO_BAUDRATE=y
CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600=y
CONFIG_MODEM_HL78XX_AUTOBAUD_CHANGE_PERSISTENT=n
CONFIG_MODEM_HL78XX_AUTOBAUD_START_WITH_TARGET_BAUDRATE=y
CONFIG_MODEM_HL78XX_AUTOBAUD_ONLY_IF_COMMS_FAIL=y

Use when: Temporary testing
Note: Modem reverts baud rate on power cycle

Supported Baud Rates

Rate	Kconfig Option	Use Case
9600	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_9600`	Legacy
19200	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_19200`	Legacy
38400	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_38400`	Standard
57600	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_57600`	Standard
115200	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_115200`	Default (recommended)
230400	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_230400`	High speed
460800	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_460800`	High speed
921600	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_921600`	Very high speed
3000000	`CONFIG_MODEM_HL78XX_TARGET_BAUDRATE_3000000`	Maximum (HL7812 only)

State Flow

Power ON
    ↓
[START_WITH_TARGET=y?] → Try target first → [AUTOBAUD_AT_BOOT=y?]
    ↓                                        ↓
Wait KSUP → [ONLY_IF_COMMS_FAIL=y?] → Try Target
    ↓                                        ↓
Post-Restart → Communication Success? → Skip Detection
    ↓                     ↓
    |              Communication Fails
    |                     ↓
    +----------→ Detect Baud Rate
                          ↓
                  Rate Detected?
                          ↓
                 Switch to Target (AT+IPR)
                          ↓
              [CHANGE_PERSISTENT=y?] → AT&W
                          ↓
                  Initialize Modem

Log Messages

Success - Smart Default

[hl78xx] Checking if modem is ready at target baud rate 921600...
[hl78xx] Modem ready at target rate, skipping detection
[hl78xx] Baud rate switch not needed

Detection Triggered

[hl78xx] Modem not responding at target rate 921600, starting detection...
[hl78xx] Trying baud rate: 9600
[hl78xx] Modem responded at 9600 baud
[hl78xx] Saving persistent baud rate with AT&W...

Non-Persistent Mode

[hl78xx] Switching baud rate from 9600 to 921600
[hl78xx] Non-persistent mode: skipping AT&W

Failure

[hl78xx] Failed to detect modem baud rate
[hl78xx] Retrying baud rate detection (attempt 2/3)

Troubleshooting

Symptom	Likely Cause	Solution
Failed to detect modem baud rate	Detection list missing rate	Add rates to detection list
Times out on each rate	Modem not ready	Increase timeout
Hangs on AT&W	UART mismatch	Fixed: UART reconfigured first
Rate not persistent	Persistence disabled	Enable `CHANGE_PERSISTENT`

AT Commands Reference

AT+IPR?
AT+IPR=115200
AT&W

API Functions (Internal)

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

Performance Tips

Enable smart flags for fastest boot
Order detection list by likelihood
Use shorter timeouts in known environments
Enable persistence to avoid re-detection
Use AUTOBAUD_AT_BOOT only if timing is controlled

Compatibility

HL7812 / HL7800
Zephyr 4.4+
All supported boards
Smart detection reduces boot from ~20 s to ~50-100 ms

License

SPDX-License-Identifier: Apache-2.0

Last Updated: 2025-12-06