Twenty-one questions about 51 single-chip microcomputer crystal oscillator

Of course it does have an impact. The faster the microcontroller works, the greater the power consumption and the more interference it will suffer. In short, the maximum speed that can run is 40M. There is no problem if it does not exceed 40M. It just requires much higher related technologies (such as the selection of PCB design components, etc.).

10. The reset circuit of 89c51 microcontroller often uses a 12MHZ crystal oscillator, but in fact, the crystal oscillator on the market is slightly smaller than 12MHZ. Why?

Answer: When serial communication is required, 11.0582MHZ is generally used so that the baud rate is easier to calculate.

It is easy to calculate using a 12MHZ duty cycle.

11. The microcontroller crystal oscillator does not oscillate when powered on, but it oscillates when touched by hand. Why? How to determine whether the microcontroller crystal oscillator is oscillating?

Check whether the capacitors for the crystal oscillator are soldered and whether the values ​​are correct.

The simplest way is to use an oscilloscope, and you can also check whether the power supply is normal.

12. How to judge whether the external crystal oscillator of the microcontroller is oscillating? My STC89C52 microcontroller was working fine at first, but then it stopped working. I changed the crystal oscillator and it worked. But after a few hours, it stopped working again. What happened? Also, how to judge whether the crystal oscillator is oscillating?

① Try to replace a single-chip microcomputer first, if the problem still exists, then exclude the single-chip microcomputer; ② It may be caused by poor soldering, so pay attention to this; ③ I have encountered similar problems with STC89C52, and it is OK after replacing a crystal oscillator. It seems that the oscillation of STC is not as smooth as that of AT89S52. In fact, for STC89C52, you can directly look at the 30th pin (ALE), connect a light, and you can see the oscillation immediately.

Thirteen, how to choose the capacitor size connected to the 51 MCU crystal? Is the larger the crystal, the larger the capacitance value? What is the commonly used value? Some people say that the commonly used value is from 15-33pf. How to choose the best effect? ​​For example, if I use a 6M and a 12M crystal respectively, what capacitor size is more suitable?

15-33pf is fine, 15P and 30P are commonly used. The size of the crystal oscillator has little effect. The commonly used 4M and 12M and 11.0592M and 20M 24M all use 30P. There is a corresponding shaping circuit inside the microcontroller, so don't worry.

14. Can an external crystal oscillator start a blank MCU without a program?

For microcontrollers without internal crystal oscillators, external crystal oscillators can be used to start oscillation. For example, for microcontrollers with internal crystal oscillators such as the traditional MS51 series, external crystal oscillators will not start oscillation and need to be configured before the external crystal oscillator can start oscillation. If the external crystal oscillator is not configured and the internal crystal oscillator is still used, such as the silicon lab series C8051F020 microcontroller

15. Why does at89c52 P1.0 output 2.5v voltage, the microcontroller seems not to work, and the crystal oscillator waveform is an irregular sine wave? Is it possible? The circuit board does not achieve the expected effect, the LED is always on, and it seems to be a problem with the microcontroller. P1.0 outputs 2.5v voltage, and the watchdog uses X5045. What's going on?

Remove the watchdog and temporarily make it a minimum system, which only has a power supply, 8952, a crystal oscillator and two 30P capacitors.

① Set the P1.0 port to 1 and test whether the voltage of the port is above 2.5V;

② Set P1.0 port to 0 and test whether the port voltage is approximately 0V.

If yes, it is OK, otherwise you need to check the power supply voltage, crystal oscillator, and 8952. The power supply voltage is 5+, -0.25V, and the ripple must be small

16. When making a max232 download microcontroller, the working voltage is normal. Do I need to add an external crystal oscillator?

Of course it needs to be added. If there is no external crystal oscillator, the clock circuit of the microcontroller will be gone, causing the microcontroller serial port to be unable to transmit data, and ultimately the download device will be unable to download programs.

17. If the 89c52 microcontroller uses an external crystal oscillator, how should it be set?

Connect a 20~30pf capacitor to each of the two pins of the crystal oscillator and then connect them to the XTAL1 and XTAL2 of the microcontroller respectively. Connect the other ends of the two capacitors in parallel and then ground them. No further settings are required.

18. What is the principle of crystal oscillator? How to generate sinusoidal signal? Can you explain it in detail and analyze it from the circuit aspect?

The crystal can be equivalent to an inductor, forming an oscillation loop with the capacitor inside. The energy slowly flows from the inductor to the capacitor, and then from the capacitor to the inductor, and the oscillation is formed over and over again. The positive half cycle is the charging and discharging process of the capacitor, and the negative half cycle is the charging and discharging process of the inductor.

19. Now we need to use a 52-bit single-chip microcomputer to make a traffic light circuit. The requirement is that the red light is on, the green light is on for 30 seconds, and the yellow light is on for 3 seconds. It changes in a cycle. So how to choose the external crystal oscillator? What is the appropriate single instruction cycle? What is the function of the two external capacitors in the figure? What is the appropriate size?

If you choose a crystal oscillator, you can choose two capacitor values: 30 plus or minus 10PF (frequency between 0 and 33MHZ);

If you choose a ceramic crystal oscillator, the capacitance value can be selected as: 40 plus or minus 10PF (frequency between 1.2 and 12 MHZ). The oscillator should be as close to the capacitor as possible. The instruction cycle can be calculated, and there is a formula for this!

20. The crystal frequency of 89c52 microcontroller is only 12MHz, which is too small. How can I increase the crystal frequency?

Connect an external 18.432 or 24MHz crystal oscillator. Or replace the 4T W77E58 microcontroller, which is equivalent to increasing the operating frequency by 3 times. Or replace the 1T DS89C4XX microcontroller, which is equivalent to increasing the operating frequency by 8 times! Using the 1T STC12C5A60S2 microcontroller also has this effect.

21. The microcontroller does not work properly. Is it a crystal oscillator problem? How do I check whether the crystal oscillator is normal or not? In addition, I saw that the crystal oscillator and the two small capacitors should be very close. I almost didn't cut the pins (just the length they were when I bought them) and just plugged them in. Does this have anything to do with it?

Use a multimeter to measure the two pins of the microcontroller connected to the crystal oscillator. Under normal oscillation, the voltage is slightly lower than 1/2 of the power supply voltage. If one or all of the pins are at the power supply voltage or zero, it means that there is no oscillation. Which pin is longer generally has no effect. In comparison, grounding is more critical. The grounding ends of the two resonant capacitors should be as close to the power supply ground of the microcontroller as possible.