👨💻 Technical Sharing by Dayi
Unveiling SPI Interface: High-Speed Data Transmission Between MCU and PLC
Hello everyone, I am Dayi. Today I am bringing you a high-speed data transmission interface used in microcontrollers and PLCs—SPI. This interface is widely used in embedded systems and automation control, not only for its fast transmission rate but also for its convenience. Let’s learn together!
What is SPI?
SPI stands for Serial Peripheral Interface. As the name suggests, it is a protocol used for serial communication between devices. Unlike parallel communication, which requires a large number of IO ports, the SPI interface only requires four lines to transmit data, greatly saving hardware resources.
Components of the SPI Interface
A standard SPI interface consists of four lines:
- MOSI (Master Output Slave Input): The line for master device output and slave device input.
- MISO (Master Input Slave Output): The line for master device input and slave device output.
- SCK (Serial Clock): The clock line emitted by the master device to control data transmission timing.
- CS (Chip Select): The line used by the master device to select the slave device for communication.
During communication, the master device sends the data to be transmitted on the MOSI line, and the slave device receives data from MOSI. If the slave device has data to return, it sends it back to the master via the MISO line. The entire communication process is synchronized by the SCK clock line, while the CS line is used to enable the selected slave device.
Life Example 🌰: SPI is like an office, where the master device is the boss and the slave devices are the employees. When the boss wants to assign work, he presses the call button (CS line) to select an employee, then gives instructions through the microphone (MOSI), which the employee hears through the headset (MISO) and executes. If there is a response, the employee replies through the microphone (MISO), and the boss receives it through the headset (MOSI). The entire communication process is controlled by the boss’s rhythm (SCK clock).
Master-Slave Mode of SPI
The SPI interface operates in a master-slave mode, where one master device can connect to multiple slave devices, but only one slave device can communicate with the master at any time. This requires the use of the CS line; when the master pulls down a slave’s CS line, that slave is selected for data exchange. Therefore, multiple slave devices can be connected to an SPI bus, as long as each slave is assigned an independent CS line.
Circuit Example ⚡️: Below is a common circuit diagram connecting a microcontroller (master device) to an external EEPROM storage chip (slave device). You can see that the microcontroller completes data exchange with the EEPROM using MOSI/MISO/SCK, utilizing the CS line to select the EEPROM chip for communication.
<div style="text-align:center">
<img src="https://mysite.com/spi-circuit.png" alt="SPI Interface Circuit" style="max-width:80%">
<p>Microcontroller connects to external EEPROM storage chip via SPI</p>
</div>
SPI Timing and Speed
SPI data transmission occurs on the rising or falling edge of the SCK clock. The SPI interface parameters may vary between different devices, such as clock polarity and phase, and the maximum supported speed. Therefore, when connecting different SPI slave devices, the master device’s SPI parameters need to be adjusted according to the slave’s manual.
Modern microcontrollers and PLCs typically have SPI modules with transmission speeds reaching several megahertz, which is sufficient for most application scenarios. In high-speed situations, such as displays and storage devices, some high-end devices even support SPI transmission speeds of several tens of megahertz. In short, as long as the capabilities of both ends match, a suitable SPI frequency can be configured on the master device to improve system efficiency.
Exciting Applications: You may have used smart bracelets, which internally utilize microcontrollers communicating at high speed with flash memory chips and displays via SPI. Such compact and portable products rely on efficient interfaces like SPI.
Microcontroller SPI Programming
Now that we understand the principles of SPI, let’s take a look at how to program SPI on microcontrollers and PLCs!
Microcontroller Code Example
c
// SPI Master Mode Initialization
void SPI_MasterInit(void)
{
SPI_BaudRateSet(); // Set SPI clock frequency
SPI_MasterTransmit(0xFF); // Start transmission to generate clock
}
// Send a byte via SPI
uint8_t SPI_MasterTransmit(uint8_t byte)
{
SPI_TXDATA = byte; // Write to the transmit data register
while(!SPI_TX_COMPLETE); // Wait for transmission to complete
return SPI_RXDATA; // Return received byte
}
The above is a typical initialization and transmission function for the microcontroller’s SPI master mode. The SPI clock frequency must be set first, and then data transmission is initiated repeatedly to generate the SCK clock, thus synchronizing with the slave device. Sending and receiving are completed through the SPI data register.
It is worth noting that some microcontrollers automatically control the CS line during communication. For those that require manual control of CS, the CS must be pulled low before calling the above function for communication, and then pulled high after communication is complete.
PLC Ladder Diagram Example
In contrast, PLC SPI programming is much more intuitive. You can directly construct SPI read/write instructions on the ladder diagram, which is very convenient!
<div style="text-align: center;">
<img src="https://mysite.com/spi-plcladder.png" alt="PLC SPI Ladder Program" style="max-width:80%">
<p>PLC communicates with slave devices via SPI read/write instructions</p>
</div>
As shown in the figure, SPI data sending and receiving can be completed using the “SPI_WRITE” and “SPI_READ” function blocks. There is no need for extensive low-level programming; just set the relevant parameters like communication speed and data length according to the manual, and you can quickly integrate SPI communication functionality.
The PLC communication module has multiple SPI channels, each of which can be configured as master or slave mode. In master mode, the PLC can communicate with multiple slave devices simultaneously, which is very powerful.
SPI vs I2C
Finally, let’s briefly compare SPI with another common serial interface, I2C. The advantage of I2C is that it only requires two lines (SCL/SDA) and can connect multiple devices simultaneously. However, its downside is the lower speed, typically only a few hundred Kbps. In contrast, while SPI has more lines, its transmission speed is significantly higher, generally in the Mbps range.
Therefore, I2C is generally used in low-speed scenarios, like sensors and real-time clocks. For high-speed storage and displays, SPI is required. Of course, if there are insufficient system IOs or high power-saving requirements, I2C can also be considered.
Alternative Solutions: Some microcontrollers and PLCs also support other high-speed interfaces like UART and CAN. However, they have different functionalities; SPI focuses on data transmission between devices without the need for complex data packet encapsulation and parsing, resulting in lower overhead. Thus, SPI is widely adopted as a dedicated high-speed serial interface in embedded systems.
Practical Experience Sharing
Finally, I would like to share some practical experience using the SPI interface:
- ⚡ SPI is full-duplex, allowing the master to send and receive data simultaneously. Sometimes the slave may need to continuously feedback status information via MISO.
- ⚠️ To prevent conflicts during communication with multiple slaves, ensure that all MISO lines of unselected slaves are set to high impedance. Otherwise, bus confusion may occur.
- 💡 Unused SPI pins should ideally be set to input mode with pull-up or pull-down to prevent leakage current interference.
- 🔋 Many sensors and storage chips operate in low-power mode, and waking them often requires a virtual SPI write action. Therefore, it is advisable to attempt several write operations during initialization.
- 🔍 If you encounter abnormal SPI communication, capture the SPI timing using a logic analyzer or oscilloscope to check if the clock and data lines are functioning correctly. Identifying the root cause can expedite troubleshooting.
Hands-On Practice
Having understood so much, it’s time to practice! You can try:
- Implementing SPI master mode on a microcontroller to read and write communication with external EEPROM or other SPI slave devices;
- Constructing SPI read/write programs on a PLC to interact with temperature, humidity, and pressure sensors;
- For a more advanced challenge, build an SPI bus system and connect multiple SPI slave devices, practicing CS line control.
This concludes my sharing. If you have any questions, feel free to leave a message. The journey of learning is long and arduous, so let’s keep it up! I am Dayi, see you next time!