C51 MCU————Timer Counter

51 MCU timer/counter

Timing (timing control, measurement, delay, etc.) and counting (counting the number of external events).

Software Timing | Digital Circuit Timing | Programmable Timer/Counter

How are they achieved?

Uh! Guess

. . .

Software Timing

The machine executes a program that has no other use but to delay time! In order to achieve the purpose of timing.

Digital Circuit Hardware Timing

Devices such as 555 complete the timing

Programmable timer/counter

Hardware timing, but it is programmable and can be initialized by software to set the timing requirements.

1. Structure

The timer/counter is essentially an add-one counter that can work in two modes. In fact, both are counting pulses, but the source of the pulses is different!

1. Timing

  1. Add one to the count.

  2. The pulse comes from the oscillator's 12-divided pulse (Fosc/12), which simply means counting the machine cycles.

  3. The initial timing value (TH, TL) needs to be preset.

  4. The initial value setting needs to be a complement code. If the count is increased by one (the complement code is added, the original value is subtracted) and the overflow is added (the original value is reduced to 0), an overflow interrupt occurs.

2. Count

  1. Add one to the count.

  2. The initial counting value (TH, TL) needs to be preset. 

  2. The pulse comes from the T0 or T1 pin.

  2. When the control switch in the figure above is closed (control = 1), the counter starts counting the falling edge generated by the external pulse.

  3. When the counter is full, the initial value becomes 0 and an overflow interrupt is generated.

  4. The maximum frequency of the counting pulse cannot exceed Fosc/24

2. Timing counter related registers

We need to consider several aspects of interruption

Is interrupt turned on?

How to trigger an interrupt

Interrupted start and stop

How to respond after multiple interrupts are triggered

(1) TMOD timing counting mode register (address 89h)

GATE:

1. Gate signal, GATE=0, TRx=1 self-start mode

2. GATE=1, TRx=1, INTx=1 to start the timer/counter (a method that uses INTx as an external start condition)

C/-T:

Timing, counting selection—1 timing 0 counting

M1M0:

Four working modes of the timing counter

(2) TCON interrupt request and external interrupt selection (address 88h)

3. Timing counter working mode

Method 0:

When timer/counter T0 operates in mode 0, only 13 bits of the 16-bit counter are used, namely the upper 8 bits of TH0 and the lower 5 bits of TL0, forming a 13-bit timer/counter (maximum count value 8192).

Method 1:

Timer T0 working mode 1 is similar to working mode 0, the difference lies in the number of bits of the counter. Working mode 0 uses a 13-bit counter to participate in counting, while working mode 1 uses a 16-bit counter to participate in counting (maximum count value 65536).

Working mode 1 is a 16-bit counter. This is the only difference between working mode 1 and working mode 0 in the counting method.

Method 2:

The working mode of automatically reloading the preset number. At this time, M1M0=10.

Automatically reload THx to store the initial value (preset number), TLx is used for counting, and the value of THx is automatically assigned to TLx after counting is completed. (Except for reloading the initial value and the maximum count value, the rest is the same as mode 0)

Method 3: (only applicable to T0)

In this operation mode, timer/counter 0 is split into two independent timers/counters. Among them, TL0 can form an 8-bit timer or counter, while TH0 can only be used as a timer. We know that when used as a timer or counter, it needs control, and overflow flags are required when the count is full. T0 is divided into two, so two sets of control and overflow flags are needed. Where do they come from? TL0 still uses the original T0 flag, while TH0 borrows the T1 flag.

Normally, T0 is allowed to operate in mode 3 only when T1 is operating in mode 2 (when used as a baud rate generator).

3. Timing counter count initial value and initialization

Calculation of the initial value of the count:

Counting method: initial value C = modulus - X (number of pulses to be counted)

Timing method: initial value C = [t /MC] complement = modulus - [t /MC] (t timing time, MC machine cycle, MC = 12/fosc)

Count initial value example:

Mode 0: Initial value C = (64H) complement = 2000H - 64H = 1F9CH

Method 1: Initial value C = (64H) complement = 10000H - 64H = FF9CH

Method 2: Initial value C = (64H) complement = 100H - 64H = 9CH

initialization:

Calculate initial value

TMOD Settings

Write the initial count value THx, TLx

Start the timer, TRx = 1

EA = 1 (turn on the interrupt master switch)

ETx = 1 Enable interrupt

Writing an interrupt subroutine

#include

unsigned char i=0;

int0() interrupt 1 //Timer 0 interrupt function

{

  i++; //Calculate the number of interrupts

}

main()//Main function

{

        //Calculate initial value

    TMOD = 0x01; //TMOD setting

    TH0 = 0x3c;

    TL0 = 0xb0; //initial value

    TR0 = 1 //Start the timer, TRx = 1

    EA=1; //Interrupt main switch

    ET0=1//Timer 0 enables interrupt

}

ORG 0000H

AJMP MAIN

ORG 000bH ;Interrupt subroutine entry

AJMP COUNTING ;Interrupt function name

ORG 0030H

MAIN: 

    MOV TMOD,#01H ;TMOD settings

    MOV TH0 ,#3cH;

    TL0 = #0b0H; ; Initial value

    SETB TR0 ; Start timer, TRx = 1

    SETB EA ;Interrupt main switch

    SETB ET0 ;Timer 0 enables interrupt

HERE:AJMP HERE

COUNTING:

    INC A ;a++

    RETI

END