The **UART** (Universal Asynchronous Receiver/Transmitter) is a fundamental component in serial communication, acting as an asynchronous transceiver that converts parallel data into serial form and vice versa. It's commonly integrated into microcontrollers or used as a standalone chip, often compliant with the RS-232C standard. To interface with external devices, it is typically paired with signal level conversion chips like the MAX232 from Maxim.
UARTs are widely used in embedded systems for bidirectional, full-duplex communication between a host and peripheral devices—such as between a car audio system and an external AP, or between a microcontroller and a PC for debugging or EEPROM communication.
One of the main challenges when using UART is determining when a complete frame of data has been received. Unlike synchronous communication, where a clock signal defines timing, UART relies on start and stop bits to frame data. However, this introduces ambiguity about when a frame ends. So how do we know when the current frame is complete?
There are several approaches:
**Method One:**
In this approach, the driver handles protocol parsing directly within the receive interrupt. When a byte is received, it’s stored in a buffer, and once the frame header is detected, the frame length is determined. This method is efficient but requires exposing the interrupt handler to the user layer, which can complicate modular design.
**Method Two:**
This method involves including a byte count at the beginning of the frame. The receiver uses this value to determine how many bytes to expect, making it easier to detect the end of a frame. However, it depends on all parties following the same protocol, which limits its flexibility.
**Method Three:**
Here, a timer is used to detect idle time. For example, if no new data arrives within 20ms after the last byte, the frame is considered complete. This is simple to implement but can reduce overall throughput due to the waiting time. It also requires careful management of transmission intervals to avoid false frame endings.
**Method Four:**
This is my preferred approach: using a circular buffer. Data is continuously written to the buffer by the UART interrupt, and the application layer reads from it as needed. This method avoids relying on timers or fixed frame lengths, but it requires proper synchronization to prevent race conditions.
I’ve been working on packaging the UART driver into a more modular and reusable form. I defined several interfaces, such as initialization, configuration, sending, and receiving functions. These allow users to configure the UART and handle data without needing to manage low-level details.
To improve decoupling, I introduced a queue-based structure. I defined a `byte_queue_t` data structure with basic operations like `enqueue`, `dequeue`, and checking if the queue is empty. This allows the UART driver to store incoming data in a buffer, and the application can read from it at its own pace.
However, one critical issue remains: thread safety. Since the buffer is accessed both in the interrupt service routine and in the main application, there's a risk of data corruption unless proper synchronization mechanisms are in place. I've started implementing critical section locks to ensure atomic access to shared variables like `hwLength`.
This process has helped me better understand how to design robust, modular communication systems. While there are still challenges, especially around synchronization and performance, the use of circular buffers and abstraction layers makes the code cleaner and more maintainable.
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