Introduction Embedded systems are application-centric, computer technology-based, and customizable hardware and software systems suitable for applications with stringent requirements regarding functionality, reliability, cost, size, and power consumption. They generally consist of four parts: an embedded microprocessor, peripheral hardware devices, an embedded operating system, and user applications, used to control, monitor, or manage other devices. Embedded control technology can be seen in everything from scientific research equipment to household microwave ovens, and it has been successfully applied in various fields, increasingly permeating people's lives. In control circuit design, the data input/output ports are crucial for the controller to complete data output and reception functions; therefore, the quality of this circuit design directly affects the controller's proper functioning. 1. Digital Input Port Logic Design Analysis Centered on the controller, and according to the data flow, controller ports are divided into digital input terminals and digital output terminals, with digital input being the simplest I/O form. The following discussion focuses on the design of the simplest digital input terminal, exploring the practical problems and solutions encountered in input port design. Because controllers have a limited number of data input/output pins and require time-division multiplexing during use, buffers or latches are typically used to connect the controller and external devices. As shown in Figure 1, the 74HC244 buffer is placed between the processor and the external device. When the processor needs to read signals from a device connected to the external interface, it activates the 74HC244 output pin via the READ pin. This allows data from the external device to be transmitted through the A0-A3 and B0-B3 pins of the 74HC244 to its buffer, and then sent to the data bus, enabling the microcontroller to read the device data. Figure 1 illustrates an interface suitable for situations with few inputs. However, for modern SoC designs targeting portable devices, high performance, small size, and low power consumption are crucial. Generally, SoCs have very low static power consumption but high dynamic power consumption due to charging and discharging load capacitors. If many functional devices are connected to the bus, the capacitive load on the bus will be very large. If the bus connects to external devices, the controller must also drive long external connections and the external devices themselves. If the system design has many digital input terminals, then using the 74HC244 input scheme will have some problems. This is because the minimum capacitance value of the 74HC244's tri-state output is 20 pF, which is three orders of magnitude higher than the 0.05 pF capacitance load of each node within the SoC. Too many 74HC244 connections will result in a large capacitance load on the processor's data bus, making it unable to receive data. To reduce the impact of capacitance on data input, the scheme shown in Figure 1 can be improved to the scheme shown in Figure 2, using a data selector to replace the 74HC244, such as the 74HC257. The maximum capacitance value of the 74HC257's output is 15 pF, slightly smaller than that of the 74HC244's output. As can be seen from Figure 2, using the 74HC257 allows one data bus of the controller to connect to two input terminals, which is equivalent to the input capacitance value of only 7.5 pF for one data bus. Of course, 8-to-1 digital logic circuits, such as the 74LS138 or 74HC151, can also be used. However, they lack tri-state functionality and therefore need to be used in conjunction with the 74HC244 to provide digital input functionality. This reduces the input capacitance of each data bus of the processor to 1/8 of that when using only the 74HC244. Figure 2 shows that if the system design does not require simultaneous sampling of more digital input terminals than the number of data buses, the above 74HC244 and 74HC257 solutions are perfectly suitable. If the system design requires simultaneous sampling of a large number of digital input terminals, latches must be used in the circuit design to latch the data. The 74HC374 and 74HC574 are commonly used latches in circuit design, and these two latches have equivalent functions. Because the input and output pins of the 74HC574 are located on opposite sides of the integrated circuit, this arrangement simplifies the wiring during PCB fabrication. Furthermore, the 74HC574's output capacitance is 15 pF, almost identical to that of the 74HC244. Therefore, the 74HC574 is generally chosen in designs, as shown in Figure 3. Latches can be used to sample a large number of data inputs simultaneously. Data selectors can reduce the capacitance load on each bus, but cannot sample data inputs simultaneously. Using data latches increases the capacitive load on the data bus, requiring a balance between the number of sampled data lines and the number of data selectors. Figure 4 shows a better solution. In this circuit, shift registers 74HC597 are cascaded and connected to the controller bus, providing the processor with a large number of digital input pins while minimizing the capacitive load on each bus. The 74HC597 is a shift register with eight flip-flops connected to its input pins. These flip-flops are edge-triggered input latches. Additionally, the 74HC597 has another eight edge-triggered latches connected in series to form the shift register. In Figure 4, when a rising edge signal from the glue logic is sent to RCLK, the signal on the data input pin is simultaneously sampled. The processor then sends a signal to SRLOAD via the glue logic, shifting the sampled data from the input latches into the shift register. Within the shift register, the processor uses SRCLK to shift the data by one bit per clock cycle. When the data is read from the READ pin, it is sent to the data bus via the D0 pin. This circuit can also be modified by connecting the QH signal pin of the 74HC597 to each data bus via a multiplexer, such as the 74HC244. This improvement reduces the time spent processing serial data and allows for one-time reading. 2. Data Input Port Protection Design Analysis We have already discussed various methods for addressing the interface between the microprocessor data bus and external devices. Below, we will discuss methods to avoid external interference from a practical perspective. In circuit design, leaving the input terminals of CMOS devices floating is a bad design practice. Because CMOS devices are voltage-controlled, unconnected input terminals tend to approach the CMOS threshold voltage, causing unnecessary switching of the internal transistors. This increases noise interference and wastes system power. Generally, pull-up or pull-down resistors are used to connect unconnected input pins to the power supply or ground, giving them a defined voltage value. The maximum input current of CMOS input pins is very small, only about 1 μA, so 1 MΩ is chosen as the pull-up or pull-down resistor. In many embedded systems, the effective voltage of the input pins is generally above 5 V or negative (to ground). In this case, using a few resistors can prevent overvoltage of the input pins. As shown in Figure 5, the two diodes inside the CMOS integrated circuit can clamp the voltage at the CMOS device input voltage value. These two diodes are part of the electrostatic discharge (ESD) protection measures for high-speed CMOS devices (74HC series). As shown in Figure 6, connecting two Schottky diodes at the input provides safer protection for the input port, but at a higher cost. The front-end voltage drops to one-third of the conduction voltage of the two internal diodes, preventing them from conducting and allowing all current to flow through the forward-biased Schottky diodes at the front end. This voltage protection circuit is essential in some applications. In general designs, no input pins require such additional protection because external voltage protection necessitates sophisticated printed circuit boards, and mounting these components on the board incurs manufacturing costs that are not negligible for designs using only passive components. Figures 3, 4, 5, 6, and 7 illustrate another protection method with two functions: first, a capacitor and resistor form a low-pass filter to reduce input signal spikes while allowing low-frequency signals to pass; second, the low-pass filter also provides electrostatic discharge (ESD) protection. This design will be discussed below. For an ideal capacitor, a 0.1 μF capacitor in series with a 22 kΩ resistor can provide ESD protection. However, actual devices do not operate under ideal conditions. In circuits, there are equivalent series resistances and equivalent series inductances, as shown in Figure 8. Figures 7 and 8 are also provided by capacitor manufacturers to describe the typical ESR (equivalent series resistance) and ESL (equivalent series inductance) of their capacitors, facilitating designers in building appropriate circuit models and analyzing circuit operation. However, building such models is still difficult because the parameters of some components in the circuit model are hard to determine. The only way to overcome this difficulty is through experimental verification, which increases the cost of experimental equipment. Another electrostatic discharge (ESD) protection device in circuits is the transient voltage suppressor (TVS), a high-efficiency protection device in the form of a diode. When the two terminals of a TVS diode are subjected to a reverse transient high-energy impact, it can change its high impedance to low impedance within a timeframe of 10⁻¹² seconds, absorbing surge power up to several kW and clamping the voltage between its terminals to a predetermined value, effectively protecting the precision components in electronic circuits from damage by various surge pulses. Due to its advantages such as fast response time, high transient power, low leakage current, small breakdown voltage deviation, easily controllable clamping voltage, no damage limit, and small size, it has been widely used in the protection circuits of precision electronic devices such as computers and digital cameras. An input pin that is well protected in practice, where L1 is an inductor coil used to reduce radio frequency interference. While it may seem overly complex for most designs, the circuit shown in Figure 9 is the best choice for precision designs or designs with stringent requirements, without the use of opto-isolators. In such designs, component price and the arrangement of components on the circuit board are also factors to consider. Because insulating layers are inserted into the metal wires during the manufacturing of metal film resistors to change the geometry of the metal wires to achieve an accurate resistance value, static electricity can easily penetrate from the insulating surface into the metal layer. Circuits constructed using this type of resistor have two consequences: first, the actual effective resistance value will be smaller than its nominal value when there is static voltage; second, it is easy to form ionization paths, changing the actual resistance value. Surface mount resistors have another problem: when there is electrostatic discharge (ESD), their solder joints with the metal layer can become hot spots due to uneven current density on the metal surface. This can cause the surface mount resistor to burn out due to ESD. When selecting resistors for a circuit, ordinary carbon film resistors are the best choice. Opto-isolators can also be used for ESD protection and interference prevention of digital input pins. They can isolate voltages of several kV, while input devices must provide an input current 1000 times greater than the current required by CMOS gate circuits to the opto-isolators. Opto-isolators have relatively slow switching speeds, and the design must also consider how to protect the LEDs within the opto-isolators from ESD damage. Appropriate selection must be made according to the design requirements. Conclusion Input interface design is a critical part of embedded controller systems because the reception of external data and feedback of external device status must be delivered to the processor through the interface. When designing input ports, two main practical considerations must be taken into account: first, load capacity, i.e., whether the input signal can be received by the controller; and second, ESD protection. Many processors now use CMOS packaging technology, which meets low power consumption requirements but also has higher ESD protection requirements.