The microcontroller is the core component of embedded systems, and circuits using microcontrollers are much more complex. However, when modifying and adding new features, circuits with microcontrollers are easier to implement, which is why electronic devices use microcontrollers. So, what are the challenges to pay attention to in the design of microcontroller circuits?
1. Selection of Pull-Up Resistors for Microcontrollers
As we can see, when the resistor R1=10k, the RST pin is high, while when R1=50, the RST pin is low. Clearly, R1=10k is incorrect, as the microcontroller remains in a reset state and cannot function at all. This occurs because the RST pin contains a transistor, which allows a small cutoff current even when in cutoff state. When R is very large, the weak cutoff current can create a high level.
2. Calculation of Series Resistors for LEDs
Typically, for red surface mount LEDs: the voltage is 1.6V-2.4V, and the current is 2-20mA. Brightness varies between 2-5mA, and remains relatively constant above 5mA.
3. Insufficient Ports
In this case, expansion chips can be used, such as the 74HC138 decoder for expansion.
4. Filter Capacitors
Filter capacitors are divided into high-frequency and low-frequency filter capacitors.
1. High-frequency filter capacitors are generally 104 (0.1uF), aimed at short-circuiting high-frequency components to protect devices from high-frequency interference. Ordinary ICs should have these capacitors between power and ground to eliminate high-frequency interference (such as electrostatic discharge).
2. Low-frequency filter capacitors are typically electrolytic capacitors (100uF), aimed at removing low-frequency ripple and storing some energy to stabilize the power supply. These are mostly connected at the power interface, near high-power components, such as: USB ports, stepper motors, and 1602 backlight displays. The voltage rating should be at least twice the maximum system voltage.
5. The Role of Transistors
1. Switching Function:
When LEDS6 is high, it is off; when low, it is on.
Calculating the current-limiting resistor: If the collector current is I, then the base current is I/100 (this involves amplification, where the collector current is 100 times the base current), with a PN junction voltage of 0.7V, R=(5-0.7)/(I/100)
2. Amplification Function: The collector current is 100 times the base current.
3. Level Conversion:
When the base is high, the transistor conducts, grounding the right-side wire to low; when the base is low, the transistor is off, outputting high.
6. Issues Related to Seven-Segment Displays
The digits formed by lighting the seven-segment display consist of a, b, c, d, e, f, g, and dp (decimal point), as shown in the truth table above.
7. Current and Voltage Drive Issues
Due to the limited output of microcontrollers, when there are many loads, additional driver chips are needed, such as the 74HC245.
8. Pull-Up Resistors
Principles for selecting pull-up resistors:
1. To save power and consider the chip’s sinking current capability, the resistor should be sufficiently large; a larger resistor means lower current.2. To ensure sufficient driving current, the resistor should be sufficiently small; a smaller resistor means higher current.3. For high-speed circuits, excessively large pull-up resistors may cause the edges to become sluggish.In summary: Pull-up resistors are commonly selected between 1K and 10K, and the same applies to pull-down resistors.
Pull-up and pull-down resistors are used to set uncertain signals to high or low levels through a resistor.
1. Level conversion, improving output level parameters.2. OC gates must have pull-up resistors to function.3. Increase the driving capability of ordinary IO pins.4. Pull-up and pull-down on floating pins to resist interference.
9. Crystal Oscillator and Reset Circuit
Crystal Oscillator Circuit
1. Crystal selection: Choose based on actual system requirements, such as 6M, 12M, 11.0592M, 20M, etc.
2. Load capacitance: Connect two 10 to 30pF capacitors to ground, commonly 20pF.
3. Measuring the crystal with a multimeter: Directly connect the red probe to the crystal pin and the black probe to GND to measure the voltage.
Reset Circuit
Sets the internal circuit of the microcontroller to a defined state, initializing all registers.
The reset time for the 51 microcontroller is approximately 2 machine cycles, but this depends on the chip’s datasheet.
Typically, this is done through a reset chip or reset circuit, with specific resistor-capacitor parameters calculated via Google search.
10. Key Bounce and Elimination
Keys are also mechanical devices, and when pressed or released, they can produce bounce, as shown in the figure below:
There are two methods to eliminate this: software debouncing and hardware debouncing, where hardware debouncing uses capacitors to short high-frequency signals.
Software debouncing detects the key closure and executes a delay program, creating a 5ms to 10ms delay to allow the front edge bounce to disappear before checking the key state again. If it remains closed, it is confirmed that the key is genuinely pressed.
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