Serial port programming is the first software interface many engineers encounter, especially those who started with 51 MCUs. As hardware integration continues to rise and chip resources become more limited, engineers have turned to an alternative method called "serial download," which requires only a single data line.
With the rapid development of the semiconductor industry, chip integration has increased significantly. Today’s PCBA boards can be as small as a coin in size without compromising functionality. How is this possible? It's a challenge for engineers to reduce hardware resources while maintaining full performance. The key lies in ensuring that programming can still be done effectively after resource reduction.
There are several programming interfaces available, such as UART, I2C, SPI, and SWD. These are well-known to engineers, but most require between two to six lines for programming. However, one interface uses just a single data line, making it ideal for projects where hardware minimization is crucial.
Figure 1: Coin-sized PCBA
PCBAs the size of a coin are now common. To fit fully functional systems into such a tiny space, hardware resource multiplexing or compression is essential. Using minimal interfaces for programming helps achieve this goal.
Single-line serial port is a communication method that uses a single wire for data transmission. Unlike traditional serial ports that use two lines (TXD and RXD), the single-line version uses one line for both sending and receiving, operating in half-duplex mode. This reduces cost and simplifies design, making it perfect for compact and resource-constrained applications.
Figure 2: Serial data frame
A single-line serial port, also known as a "single-wire asynchronous serial communication interface," differs from the standard serial port by using only one data line for bidirectional communication. This makes it highly efficient for reducing hardware complexity.
Figure 3: Schematic diagram of single-line serial cable
As shown in the figure, only four lines—MOD, /RESET, VCC, and GND—are needed to program the device. Among these, only one is used for data transmission. The MOD/IO pin on the chip can also be used to switch between different working modes. For example, during power-on reset, the timing waveform on the MOD pin determines the mode entered. If the system enters the single-line programming mode, the MOD pin becomes the data communication line, further reducing the number of required pins.
By connecting these four lines with a tool like P800isp, engineers can easily perform programming tasks with minimal hardware. This approach not only saves space but also simplifies the overall design, making it a popular choice in modern embedded systems.
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