1. Why 51 single-chip microcomputers like to use 11.0592MHZ crystal oscillator?

When designing the 51 series single-chip system, the 11.0592MHz crystal oscillator is generally used instead of the 12MHz crystal oscillator. Why? The 12M crystal oscillator has a significantly higher oscillation frequency than 11.0592M. It stands to reason that a 12MHz crystal oscillator can improve the performance of the microcontroller, so why don't we use a 12M one? This problem is related to the serial port baud rate of the microcontroller.

1. Because it can be accurately divided into clock frequency, it is related to the common baud rate of UART (Universal Asynchronous Receiver/Transmitter). Especially the higher baud rate (19600, 19200), no matter how weird the value, these crystal oscillators are accurate and often used.

2. The reason for using 11.0592 crystal oscillator is caused by the timer of 51 single-chip microcomputer. When using the 51 single-chip timer as the baud rate generator, if you use the 11.0592Mhz crystal oscillator, the value that needs to be set by the timer is an integer according to the formula; if you use the 12Mhz crystal oscillator, the baud rate will be deviated. For example, 9600, use the timer to take 0XFD, the actual baud rate is 10000, generally the baud rate deviation is about 4%, so you can also use STC90C516 crystal oscillator 12M baud rate 9600, the error rate is 6.99% when the multiple is not The error rate is 8.51% in multiples, and the data will definitely be wrong. This is the reason why everyone likes to use 11.0592MHz crystal oscillator for serial communication. When the baud rate is double speed, the highest can reach 57600, and the error rate is 0.00%. Using 12MHz, the highest is 4800, and there is an error rate of 0.16%, but it is within the allowable range, so there is not much effect.

2. Why is the crystal oscillator required to be next to the single-chip microcomputer when designing the 51 single-chip microcomputer system PCB?

The reason is as follows: The crystal oscillator generates mechanical vibration of a fixed frequency through electrical excitation, and the vibration will generate current feedback to the circuit. After the circuit receives the feedback, the signal is amplified, and the amplified electrical signal is used to excite the mechanical vibration of the crystal oscillator. The current generated by the vibration is fed back to the circuit, and so on. When the excitation electrical signal in the circuit is the same as the nominal frequency of the crystal oscillator, the circuit can output a sine wave with a strong signal and a stable frequency. The shaping circuit turns the sine wave into a square wave and sends it to the digital circuit for its use.

The problem is that the output capacity of the crystal oscillator is limited, it only outputs electrical energy in milliwatts. Inside the IC (Integrated Circuit), the signal can be used normally only if the signal is amplified hundreds of times or even thousands of times through an amplifier.

The crystal oscillator and the IC are generally connected by a copper wire. This wire can be regarded as a piece of wire or several pieces of wire. The wire will generate current when the magnetic line of force is cut. The longer the wire, the stronger the current generated. In reality, magnetic field lines are not common, but electromagnetic waves are everywhere, such as: radio broadcast transmission, TV tower transmission, mobile phone communication and so on. The connection between the crystal oscillator and the IC becomes the receiving antenna. The longer it is, the stronger the received signal and the stronger the electrical energy generated, until the received electrical signal strength exceeds or approaches the signal strength generated by the crystal oscillator. , The output of the amplifier circuit in the IC will no longer be a square wave with a fixed frequency, but a messy signal, causing the digital circuit to fail to work synchronously and make mistakes.