51 MCU Series Serial 5 - Timer

In the previous section, we mentioned that the most obvious application of interrupts is timers. Next, we will introduce timers. In fact, timers are a kind of counters. The essence of timers/counters is a plus-1 counter (16) bits, which consists of two registers, the high 8 bits and the low 8 bits, mainly the two registers TMOD and TCON.

 

TMOD is the working mode register of the timer/counter, which determines the working mode and function; TCON is the control register, which controls the start and stop of T0 and T1 and sets the overflow flag. 

 

         
    First, let's talk about the TMOD register. Its lower four bits are used for T0, and the upper four bits are used for T1. Its format is as follows:

 

 

Among them, GATE: gate bit. When GATE = 0, as long as TR0 or TR1 in TCON is set to 1 by software, the timer/counter can be started; when GATA = 1, TR0 or TR1 must be set to 1 by software, and the external interrupt pin or is also at a high level to start the timer/counter. That is, there is one more condition for starting the timer at this time. C/T: timing/counting mode selection bit. C/T = 0 is timing mode; C/T  = 1 is counting mode. M1M0: working mode setting bit. The timer/counter has four working modes, which are set by M1M0.

 

 

    The timer initialization procedure should complete the following tasks:

   (1) Assign values ​​to TMOD to determine the working mode of T0 and T1.

   (2) Calculate the initial value and write it into TH0, TL0 or TH1, TL1.

   (3) When in interrupt mode, assign a value to IE and enable interrupt.

   (4) Set TR0 or TR1 to start the timer/counter timing or counting.

 

    Next, let's focus on the calculation of the timer's initial value. Please remember the following formula:

                           t=T*(2L-C)

    Among them, t is the timing time, which is usually half of the machine cycle, T is the machine cycle, L is the length of the counter, and C is the counting constant, which is usually the initial value we need to find. Find an example to analyze and you will understand.

    Of course, this timer is used together with other contents, which will be introduced in more detail later, which is very interesting.