51 Why is the crystal oscillator 11.0592

The commonly used baud rate is usually 1200, 2400, 4800, 9600 according to the specification. If a crystal oscillator is used at 12 MHz or 6 MHz, the calculated T1 timing initial value will not be an integer, which will cause accumulated errors during communication. In turn, a baud rate error is generated, which affects the synchronization performance of serial communication. The solution is to adjust the clock frequency of the microcontroller, usually 11.0592 MHz crystal oscillator. Because it can calculate the initial value of T1 timing very accurately, even for higher baud rates (19600, 19200), no matter how weird the value, as long as the standard communication rate, the use of 11.0592 MHz crystal oscillator can be very accurate The numerical value.

11.0592M is because it is not easy to achieve standard baud rates for serial communication at 12M frequency, such as 9600, 4800, and 11.0592M is just available when calculating. Therefore, it is generally selected for MCUs with communication interfaces. 11.0592 trillion.

The reason for using the 11.0592 crystal oscillator is caused by the timer of the 51 single-chip microcomputer. When using 51 single chip microcomputer timer as baud rate generator, if you use 11.0592 MHz crystal oscillator, according to the formula, the value that needs to be set by the timer is an integer; if you use 12 MHz crystal oscillator, the baud rate is all Deviation, such as 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 in multiples 6.99%, the error rate is 8.51% when it is not multiple, the data will definitely be wrong. This is the reason why everyone likes to use 11.0592 MHz crystal oscillator for serial communication. When the baud rate is doubled, the maximum can reach 57600, and the error rate is 0.00%. With 12 MHz, 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.

We assume that there is a number between 0-12_000_000 that satisfies the following conditions, this number is more suitable for the frequency of the crystal oscillator:

1. When the original value is 0-255, this number can divide more numbers (the more divisible numbers, the more types of baud rates can be obtained);

2. And this number should be larger, the faster the crystal oscillator frequency, the larger the baud rate, the faster the transmission speed;

3. In the case of doubling and not doubling SOMD, this number can divide more numbers.

Therefore, we have selected that in both cases of doubling and not doubling the SOMD, the higher frequency can be divided evenly (adding the two cases together).

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