"Serial port download is the first programming method that many engineers, especially those working with 51 MCUs, have ever encountered. As hardware integration continues to increase and chip resources become more limited, engineers are exploring alternative methods of "serial download" that require only a single data line.
With the rapid development of the semiconductor industry, chip integration has reached new heights. Today’s PCBA designs can be as small as a coin in size without compromising functionality. How is this possible? It's a challenge for every engineer to minimize hardware resources while maintaining full functionality. The key lies in ensuring that programming can still be performed efficiently after resource reduction.
There are various programming interfaces available, such as UART, I2C, SPI, and SWD, all of which are familiar to engineers. Most of these interfaces typically require between two to six lines for programming. However, one interface stands out by requiring only a single data line, making it ideal for engineers looking to reduce hardware complexity.
Figure 1: Coin-sized PCBA
Today, PCBA boards the size of a coin are not uncommon. To fit powerful functions into such a small space, hardware resource multiplexing or compression becomes essential. By using as few interfaces as possible for programming, engineers can achieve significant hardware resource savings.
Single Line Serial Port
A single-wire serial port is a communication method used in serial interfaces. In serial communication, data is transmitted bit by bit, one at a time. This method is known for its simplicity, requiring only one pair of transmission lines for two-way communication—making it cost-effective and suitable for long-distance communication, although it is slower compared to parallel communication.
The communication method where each piece of information is sent sequentially, bit by bit, is called serial communication. Its main characteristic is that it can be completed with at least one transmission line.
Figure 2: Serial Data Frame
A single-line serial port, also known as a "single-line asynchronous serial communication interface," is referred to as "single-line serial port" in this article. A standard serial port, on the other hand, is known as a "standard asynchronous serial communication interface."
Both use the same communication protocol, but the difference lies in the number of data lines. While a standard serial port uses two lines (TXD and RXD), a single-line serial port uses just one data line for both sending and receiving. Therefore, it operates in half-duplex mode.
Figure 3: Schematic Diagram of Single-Line Serial Cable
As shown in the figure, only four lines are needed to program the device: MOD, /RESET, VCC, and GND. Among these, only one line is used for data transmission and reception.
The communication port labeled MOD/IO on the chip can also be used to select different operating modes. For example, during power-on reset, the timing waveform on the MOD pin is detected. Based on this waveform, the system determines which mode to enter—such as the single-line serial programming mode.
Once in this mode, the MOD pin is repurposed for data communication, allowing engineers to minimize the number of pins used for programming and further compress hardware resources. Understanding this mechanism, once P800isp connects these four lines, programming becomes straightforward and efficient.
Figure 4: Programming Interface Connection"
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