Does anyone know how the baud rate error is calculated?

QWE4562009

Does anyone know how the baud rate error is calculated? [Copy link]

 

Does anyone know how the baud rate error is calculated?

maychang

You need to first understand what SMOD=0 and SMOD=1 mean, then understand how asynchronous serial communication is performed, and finally understand how the 51 microcontroller generates the baud rate for asynchronous serial communication.

maychang

Note that the two numbers connected by the red line are the same, one is when SMOD=0, the other is when SMOD=1, the baud rates of the two are exactly twice.

maychang

To calculate the baud rate error, you first need to know how to calculate the baud rate, right?

w494143467

You should first learn the formula for calculating the baud rate, and then substitute the corresponding parameters into it. The difference between the obtained baud rate and the desired baud rate can be obtained, and then the error can be obtained. And these have been sorted out by others. If you don't care about these, you can use them directly.

QWE4562009

maychang posted on 2020-10-12 19:32 If you want to calculate the baud rate error, you must first know how to calculate the baud rate, right?

I looked it up but I really don't understand. Some people say to use clock frequency/division number/timer initial value directly. For example, if the baud rate is 115200 and the clock frequency is 24M, if it is divided by 4, it is {[24000000/4]/[24000000/4/115200]-115200}/115200 *100%, right? ?

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn’t understand it. Some people said that they directly use the clock frequency/division number/timer initial value. For example, if the baud rate is 115200...

Don’t worry about Timer 2 for now. The 51 microcontroller only has Timer 0 and Timer 1, but no Timer 2. The 52 microcontroller has Timer 2.

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn't understand it. Some people say that you can use the clock frequency/division number/timer initial value directly. For example, if you need a baud rate of 115200...

The 51 single-chip microcomputer must be connected to a quartz crystal (piezoelectric ceramics can also be used) as a clock generator.

For 51 single-chip microcomputer, 12 quartz crystal oscillation cycles are one clock cycle. Don't ask why, the hardware is designed this way. When the quartz crystal frequency is 12MHz, one clock cycle is 1us. When the quartz crystal frequency is 6MHz, one clock cycle is 2us. When the quartz crystal frequency is 4MHz, one clock cycle is 3us. When the quartz crystal frequency is 3MHz, one clock cycle is 4us. And so on.

Timer 1 has several working modes. Used as a baud rate generator, Timer 1 usually works in 8-bit auto-reload mode (mode 2). When Timer 1 works in 8-bit reload mode, the upper 8 bits are equivalent to an 8-bit register, which can be written with data in advance, and the lower 8 bits are counters that count clock cycles. When the upper 8 bits overflow, the data stored in the lower 8 bits are automatically reloaded into the upper 8 bits, and the clock cycles are still counted.

So if the quartz crystal frequency is 12MHz, and the data stored in the upper 8 bits of Timer 1 is FFH (255), the lower 8 bits of Timer 1 will overflow once every clock cycle (1us) (only one number is counted from FFH to 00H, which is 256). When the data stored in the upper 8 bits of Timer 1 is FEH (254), the lower 8 bits of Timer 1 will overflow once every two clock cycles (2us).

If the quartz crystal frequency is 6MHz, the high 8 bits of timer 1 store data as FFH (255), and the low 8 bits of timer 1 overflow once every clock cycle (2us) (only one number is counted from FFH to 00H, which is 256). When the high 8 bits of timer 1 store data as FEH (254), the low 8 bits of timer 1 overflow once every two clock cycles (4us).

If the quartz crystal frequency is 4MHz, the high 8 bits of timer 1 store data as FFH (255), and the low 8 bits of timer 1 overflow once every clock cycle (3us) (only one number is counted from FFH to 00H, which is 256). When the high 8 bits of timer 1 store data as FEH (254), the low 8 bits of timer 1 overflow once every two clock cycles (6us).

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn't understand it. Some people say that you can use the clock frequency/division number/timer initial value directly. For example, if you need a baud rate of 115200...

The number of overflows per second in the lower 8 bits of timer 1 is called the overflow rate of the lower 8 bits of timer 1. Therefore, when the quartz crystal frequency is 12MHz and the data stored in the upper 8 bits of timer 1 is 0xE6(230), the lower 8 bits of timer 1 overflow once every 26us, or 38461.54 times per second. When the quartz crystal frequency is 6MHz and the data stored in the upper 8 bits of timer 1 is 0xE6(230), the lower 8 bits of timer 1 overflow once every 52us, or 19230.77 times per second.

maychang

maychang published on 2020-10-13 15:24 Don’t worry about Timer 2 for now. The 51 microcontroller only has Timer 0 and Timer 1, but no Timer 2. Only the 52 microcontroller has Timer 2.

After the quartz crystal frequency is determined, to change the overflow rate of the lower 8 bits of timer 1, you can only change the preload value of the upper 8 bits of timer 1. The larger the preload value, the higher the overflow rate of the lower 8 bits of timer 1. The maximum value can only be FFH (255), at which time the overflow rate of the lower 8 bits of timer 1 is 1/12 of the quartz crystal frequency.

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn't understand it. Some people say that you can use the clock frequency/division number/timer initial value directly. For example, if you need a baud rate of 115200...

When SMOD=0, the overflow rate of the lower 8 bits of timer 1 will be divided by 32 (frequency division. Don't ask why, the hardware is designed this way), but when SMOD=1, the overflow rate of the lower 8 bits of timer 1 will be divided by 16, which is the actual baud rate you use.

For example, the quartz crystal frequency is 12MHz, the high 8 bits of timer 1 are preloaded with the value E6H, SMOD = 1, 38461.54/16 = 2403.85, which is the actual baud rate you use. In other words, the time taken by each bit of asynchronous serial communication is 1/2403.85 = 0.000416 seconds.

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn’t understand it. Some people said that they directly use the clock frequency/division number/timer initial value. For example, if the baud rate is 115200...

As for the baud rate error in this case (quartz crystal frequency 12MHz, timer 1 high 8 bits preloaded with value E6H, SMOD=1), you can calculate it yourself.

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn’t understand it. Some people said that they directly use the clock frequency/division number/timer initial value. For example, if the baud rate is 115200...

If the quartz crystal frequency is 6MHz, the high 8 bits of timer 1 are preloaded with the value E6H, and SMOD=1, then your actual baud rate is only 1201.92.

maychang

QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn’t understand it. Some people said that they directly use the clock frequency/division number/timer initial value. For example, if the baud rate is 115200...

"Some people say that we can use clock frequency/division number/timer initial value directly"

That's nonsense.

freebsder

What on earth does the OP do? Why do these things... So jumpy

QWE4562009

freebsder posted on 2020-10-13 21:00 What on earth does the OP do? And he does all this... So jumping

If you encounter it, ask about the hardware

QWE4562009

maychang posted on 2020-10-13 16:45 If the quartz crystal frequency is 6MHz, the high 8 bits of timer 1 are preloaded with the value E6H, and SMOD=1, then your actual baud rate is only 1201.92.

Where does this pre-loaded value come from?

QWE4562009

maychang posted on 2020-10-13 16:00 maychang posted on 2020-10-13 15:24 Don't worry about timer 2 for now. The 51 microcontroller only has timer 0 and timer 1, but no timer 2. The 52 microcontroller...

What is this overflow rate? 65535-initial value?

maychang

QWE4562009 posted on 2020-10-14 15:42 Where does this pre-installed value come from

“Where does this pre-loaded value come from?”

When the microcontroller is initialized, you write the high 8 bits of timer 1, that is, write TH1.

QWE4562009

maychang posted on 2020-10-13 15:59 QWE4562009 posted on 2020-10-13 15:02 I looked at it but I really didn’t understand it. Some people said that the clock frequency/division number/timing can be used directly...

What should I do if the overflow occurs? Overflow means full, right? Does it mean that 65535 is full?