Difficult points to note in MCU circuit design

A microcontroller, also known as a monolithic microcontroller, is not a chip that completes a certain logical function but a computer system integrated into a single chip. It is equivalent to a miniature computer. Compared with a computer, an MCU only lacks I/O devices. In a nutshell: a chip becomes a computer. It is small, light, inexpensive, and provides convenient learning, application, and development conditions. At the same time, learning to use MCU is the best choice to understand the principle and structure of the computer.

MCU has been used in many fields, such as smart meters, real-time industrial control, communication devices, navigation systems, home appliances, etc.

MCU is the core component of an embedded system. The circuit using MCU is much more complex. Still, when changing and adding new functions, the circuit with MCU is easier to implement, which is exactly why electrical devices use MCU. So what are the difficult points to note in the design of a microcontroller circuit?

1, The choice of microcontroller pull-up resistors

Reset circuit resistance R1 = 10k RST is high, and when R1 = 50 RST is low, it is clear that R1 = 10k is wrong. The microcontroller has been in the reset state and can not work. The reason for this is that the RST pin contains a triode. Even in the cutoff state, there will be a small amount of cutoff current when R is taken very large, the weak cutoff current through the high level.

2, The second is the LED series resistance calculation problem.

Usually, red SMD LED: voltage 1.6V-2.4V, current 2-20mA, in 2-5mA brightness has changed, 5mA above the brightness no change.

3, the port appears to be insufficient.

This can be achieved with the help of expansion chips, such as three eight decoder 74HC138 to expand.

4, the filter capacitor.

Filter capacitors are divided into high-frequency filter capacitors and low-frequency filter capacitors.

1, a high-frequency filter capacitor with 104 capacitance (0.1uF) is generally used. The purpose is to short-circuit the high-frequency component to protect the device from high-frequency interference. Ordinary IC (integrated) devices between the power supply and ground are added to remove high-frequency interference (air static).

2, low-frequency filter capacitors are generally used as electrolytic capacitors (100uF). The purpose is to remove low-frequency ripple, store part of the energy, and stabilize the power supply. Mostly connected to the power supply interface, next to high-power components, such as USB excuse, stepper motor, and 1602 backlight display. Withstand voltage value is at least 2 times higher than the maximum voltage of the system.

5, the role of transistors

1, the role of the switch.

LEds6 for the high level when the cutoff, for the low level when the on.

Calculation of current-limiting resistance: collector current for I, then the base current is I/100 (here involved in the role of amplification, the collector current is 100 times the base), PN junction voltage 0.7V, R = (5-0.7)/(I/100)

2, amplification: the collector current is 100 times the base current

3, level conversion: when the base is high, the transistor conducts, and the right side of the wire is grounded for low. When the base is low, the transistor is cut off, and the output high.

6, the current-voltage drive problem

Due to the limited output of the microcontroller, when the load takes a lot of time, we need to add another driver chip, such as 74HC245.

7, pull-up resistors

Pull-up resistor selection principles

1, from saving power consumption and the chip's ability to fill the current should be large enough to consider; large resistance, small current.

2, ensuring sufficient drive current should be small enough to consider; small resistance, large current.

3, too large a pull-up resistor for high-speed circuits may lead to edge flattening.

Comprehensive consideration: pull-up resistor commonly used value between 1K to 10K selected, pull-down the same.

Pull-up and pull-down resistors the pull-up is the uncertainty of the signal through a resistor embedded in the high level. Pull-down is the same.

1, level conversion, increase the output level parameter value.

2, OC gate must be added to the pull resistor to use.

3, increase the ordinary IO pin drive capability.

4、. Hanging pins up and down to pull anti-interference.

8, crystal oscillator and reset circuit.

Crystal circuit

1, crystal oscillator selection.

According to the current system requirements, 6M, 12M, 11.0592M, 20M, etc.

2, load capacitance.

Ground to connect two 10 to 30pF capacitors can be commonly used 20pF.

3, multimeter measurement crystal.

Directly with the red pen to the crystal pins, and the black pen to GND, measure the voltage can be. Reset circuit

The internal circuitry of the microcontroller is set to a definite state. All registers are initialized.

The reset time of 51 microcontrollers is about 2 mechanical cycles, which need to see in the chip datasheet.

Generally, through the reset chip or reset circuit, the specific resistance and capacitance parameters of the calculation can be found through google.