Understanding H Radio Code: A Deep Dive into LoRa Transmitter Code for STM32WL

The STM32WL microcontroller, with its integrated LoRa radio, offers a powerful platform for developing long-range, low-power wireless applications. However, implementing LoRa transmitter and receiver functionality can be challenging for newcomers to the STM32 ecosystem. This article delves into a specific LoRa transmitter code example for the STM32WL, providing a detailed explanation and suggestions for modification. We’ll focus on understanding the code structure and how to adjust key parameters like transmission duration.

Deconstructing the LoRa Transmitter Code

The provided code snippet utilizes the Ping-Pong application from the STM32WL SDK as a foundation for building a custom LoRa transmitter. Let’s break down the code into its key components:

Initialization (SubghzApp_Init)

This function sets up the necessary peripherals and configurations for LoRa transmission:

• Timer Initialization: A timer (timerLed) is initialized to control the blinking of LEDs, providing visual feedback. This is not directly related to LoRa functionality but is useful for debugging and monitoring.
• Radio Initialization: The Radio.Init function initializes the LoRa radio with specified event callbacks (RadioEvents). These callbacks define the actions to be taken upon specific radio events like successful transmission (OnTxDone) or transmission timeout (OnTxTimeout).
• Transmission Configuration: Radio.SetTxConfig configures the LoRa radio parameters, including output power, frequency, bandwidth, spreading factor, coding rate, and preamble length. These parameters significantly influence the communication range, data rate, and power consumption of the LoRa device. Understanding these settings is crucial for optimizing LoRa performance.
• Payload and Channel Configuration: Radio.SetMaxPayloadLength sets the maximum size of the data packet that can be transmitted. Radio.SetChannel sets the operating frequency for the LoRa communication.
• Transmission Initiation: Finally, Radio.Send initiates the transmission of the data stored in the Buffer with the specified BufferSize.

Event Callbacks

These functions handle specific radio events:

• OnTxDone: This callback function is executed when a transmission is successfully completed. In this example, it puts the radio to sleep (Radio.Sleep) and schedules a task using UTIL_SEQ_SetTask.
• OnTxTimeout: This callback function is triggered if a transmission fails to complete within the specified timeout period. Similar to OnTxDone, it puts the radio to sleep and sets the state to TX_TIMEOUT before scheduling a task.

Modifying Transmission Duration

The original question asks how to modify the transmission duration. While the provided code doesn’t directly control the duration of each individual message, it does send a fixed-length payload defined by BufferSize and LORA_FIX_LENGTH_PAYLOAD_ON. To modify the airtime:

• Change Payload Size: Adjust the BufferSize variable to alter the amount of data being transmitted. A larger payload will result in a longer transmission time. Remember to also update Radio.SetMaxPayloadLength accordingly. Note that this will only change the duration if LORA_FIX_LENGTH_PAYLOAD_ON is enabled, which forces the transmission of the full buffer size even if the actual data is smaller. If this setting is disabled, the transmission duration will be determined by the actual data size.
• Modify LoRa Parameters: Adjusting parameters like bandwidth and spreading factor impacts the data rate and, consequently, the transmission time for a given payload size. Lower data rates (e.g., higher spreading factor) lead to longer airtime but increased sensitivity and range. This requires careful consideration based on the application requirements.

Conclusion

Understanding the intricacies of LoRa transmitter code is essential for developing robust and efficient LoRa applications on the STM32WL platform. This article has dissected the core components of a sample transmitter code, focusing on initialization, event handling, and techniques for modifying transmission duration. By carefully configuring LoRa parameters and understanding the interplay between payload size and data rate, developers can optimize their applications for specific performance requirements.