Learning notes: how to time 10ms for 51 single chip microcomputer (STC89C52)

1. How to time the timer

First of all, describe the timing principle of the timer , In fact, the essence is just one sentence ： Every machine cycle , Add... To the register 1.

Here we have to explain what a clock cycle is , What is a mechanical cycle . our 51 Whether it is the development board or the smallest system board , There will be an electronic component called “ Crystal oscillator ”, This crystal oscillator is a single-chip computer “ The heart ”. Usually in order to make the serial port baud rate stable to transmit data , Most of 51 The crystal oscillator frequency on the single chip microcomputer is 11.05922MHz. Clock cycle The calculation method is the reciprocal of the crystal vibration frequency , That is to say 11.0592 One second . The clock cycle is the most basic in a computer 、 The smallest unit of time . In a clock cycle ,CPU Just complete one basic action . A clock cycle is an amount of time . Smaller clock cycles mean higher operating frequencies . With the clock cycle, we can then calculate the mechanical cycle , One Mechanical cycle = 12 x Clock cycle =12 x (1/ clock frequency ) second = 12 / clock frequency second = 12 / 11059200 second = 12 000 000 / 11059200 Microsecond = 1.085 Microsecond . To sum up , The process of timer timing is equivalent to counting several mechanical cycles , After counting the mechanical cycle, the timer will overflow and end the timing .

2. How to schedule 10ms

  Use 51 SCM timing has to understand TCON register ,TR0： Control start timing ,TF0： Judge whether the timer overflows （ This position when overflowing 1）,TL0 The lower eight bits of the initial value of the timer ,TH0 The initial value of the timer is eight bits higher （ Do not configure the initial value. The default value is from 0 Start counting ）. That is to say, the timer mode is in 16 In case of automatic reloading of bit , from 0 Start counting , Count up 65536 A mechanical cycle, that is 71ms The timer will overflow . So if we want to schedule 10ms Just from 61ms Time to start counting , Count to 71ms The timer just overflows . At this time, you only need to configure the upper eight bits and the lower eight bits of the initial value of the timer .

The process of configuring the upper octet and the lower octet ：

The configuration process is not complicated , First determine 10ms Several mechanical cycles are involved ：

10ms1.085us=9216（ Pay attention to unit conversion here ）

If from 0 From the beginning of counting to overflow, the total number is 65536 A mechanical cycle , Well, if you go from 61ms Start timing is also from 65536-9216=56320 Start counting , The conversion 16 The binary system is shown in the following figure ：

  So the high order is DC, Low position is 0. Perfect scattered flowers ！！！

  Of course, now some software can automatically generate after inputting the timing time C Code , for example ISP. But I want to contact the early days of SCM “ shortcut ” Try not to walk , It is very helpful to learn more about the underlying things .