Implementing Software-Level Flow Control

In this lesson, you will learn how to prevent data loss in serial communication by implementing a software-based signaling mechanism to pause incoming data when your system’s internal buffers are nearly full.

The Problem: Buffer Overrun

In asynchronous communication (like UART), the sender is often unaware of what the receiver is doing. If your device receives data faster than your processor can handle it via an Interrupt Service Routine (ISR), your circular buffer will eventually fill up. If new data arrives while the buffer is full, you face a "buffer overrun," leading to data corruption and lost information.

The Solution: Software-Level Signaling

Software flow control uses special control characters within the data stream to tell the sender to "pause" or "resume" transmission. The most common standard is XON/XOFF.

When your circular buffer reaches a high-water mark (e.g., 80% capacity), your receiver sends an XOFF character (ASCII 0x13) to the sender. Upon receiving this, the sender pauses its transmission. When your system processes enough data and the buffer drops below a low-water mark (e.g., 20% capacity), you send an XON character (ASCII 0x11) to tell the sender it is safe to continue.

Implementing the Logic

Inside your receive ISR or data processing loop, you must constantly monitor the buffer occupancy:

// Inside the data processing loop
if (buffer_count() > HIGH_WATER_MARK && !xoff_sent) {
    transmit_byte(XOFF); // Signal sender to stop
    xoff_sent = true;
} 

if (buffer_count() < LOW_WATER_MARK && xoff_sent) {
    transmit_byte(XON);  // Signal sender to resume
    xoff_sent = false;
}

The logic relies on two distinct watermarks to prevent "chatter"—where the system rapidly toggles the stop/start signal if the buffer size fluctuates right at the threshold. By ensuring the buffer clears significantly before requesting more data, you create a stable, reliable flow of information that respects the processing constraints of your embedded system.

Key Takeaway: Software-level flow control preserves data integrity by using in-band signaling characters to dynamically synchronize the sender's transmission rate with the receiver's available buffer capacity.