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8051 Microcontroller Timers: How to Generate Delay Example

In this tutorial, we will learn how to use timers on the 8051 microcontroller to generate a delay. The 8051 microcontroller has two 16-bit built-in timers. We can program each timer individually and configure them as either timers (to generate time delays) or counters (to count events occurring outside the microcontroller). The 8051 timer has three general functions, which are as follows:

1. Calculating the amount of time between events.
2. Counting events.
3. Generating the baud rate for the serial port.

Before learning about 8051 timer programming, you should know how to use KEIL for 8051 programming and how to use input and output ports of 8051 microcontroller

The 8051 microcontroller has two timers: Timer0 and Timer1. Both of these timers are 16-bit each, and we can also use them as counters. In the case of the 8051 microcontroller, its architecture is 8-bit, so the 16-bit timer is accessible as two separate 8-bit registers, low byte, and high byte.

It is a 16-bit register that is accessed as a low byte and a high byte. The low byte is referred to as TL0, and the high byte is referred to as TH0. These registers are accessible like any other register.

Timer1 is also a 16-bit register, which is split into two bytes: a low byte referred to as TL1 and a high byte referred to as TH1.

Note: Both the timers (Timer0 and Timer1) share two common SFRs (special function registers): TMOD and TCON. We will discuss these two SFRs below and see how they control Timer 0 and Timer 1.

This timer can set various timer modes for timer 0 and timer 1. In this 8-bit TMOD register, the 8051 microcontroller assigns the lower 4 bits to Timer1 and the upper 4 bits to Timer0. For each timer, the lower 2 bits set the timer mode, while the upper 2 bits specify the operation.

The last two bits for each timer, i.e., M1 and M0, select the timer mode. 8051 has four timer modes, which are as follows:

• This mode allows a 13-bit timer or counter.
• The Mode0 register can hold values up to 1FFFH in TH-TL.
• When the timer reaches its maximum of 1FFFH, it rolls over to 0000, and the TF (flag bit) is raised.
• Not used generally.
• Aside from these few differences, the rest is similar to Mode 1.

• This mode enables a 16-bit timer or counter.
• The Mode 1 registers (TL and TH) can store values from 0000 to FFFFH.
• The timer must be started after loading TH and TL with a 16-bit initial value.
• We can give initial values by using “SETB TR0” for timer0 and “SETB TR1” for timer1.
• When the timer starts, it keeps counting until it reaches its limit of FFFFH.
• Once the timer reaches its limit at 0000H, it sets a TF (timer flag) bit: TF0 for timer0 and TF1 for timer1.
• Now we can stop the timer using “CLR TR0” for timer0 and “CLR TR1” for timer1.
• To repeat the process, the original values must be reloaded in the TH and TL registers, and the timer flag must be reset to 0.

For time delay, the timer uses the clock source frequency of the crystal attached to 8051. The frequency of the timer will be 1/12th of the frequency of the external crystal oscillator.

• This mode enables an 8-bit timer or counter.
• This timer first loads the values up to FFH in the TH register.
• When TH is loaded, 8051 copies the same 8-bit values to TL.
• The timer starts by setting “SETB TR0” and “SETB TR1” instructions for timer0 and timer1, respectively.
• When the timer starts, it continues to increment the TL register until it reaches FFH; after that, it rolls over to 00 and raises the TF flag. Then the TL is automatically loaded with its original value, which is reserved by the register TH.
• The main feature of this mode is that it repeats the process. We just have to clear the TF register, and it will load the original value into the register. While in mode 1, the programmer reloads the values to TH and TL.

• This mode is also known as a split timer mode.
• It splits timer0 into two 8-bit timers.
• These 8-bit timers can count from 00H to FFH.
• This mode is used in applications where we require an additional 8-bit timer or counter.
• TL0 sets TF0, and TH0 sets TF1.
• We can program timer0 and timer1 independently in modes 0, 1, and 2. Each timer can have its own settings. But mode 3 can’t work independently.
• If we choose mode 3 for timer0 and place timer1 in mode 3, it will cause the timer to stop counting. Then timer 0 will use TR1 (timer1 register) and TF1 (timer1 flag), i.e., timer 0 has to depend on timer 1.

The second special function register is the timer control register. It is an 8-bit register, and each bit has a special function. The bits, symbols, and functions of every bit of the TCON register are as follows:

The timer needs a clock source. If C/T = 0, the crystal frequency attached to the 8051 is the source of the clock for the timer. The value of the crystal frequency attached to the microcontroller determines the speed at which the timer ticks. Now suppose that the crystal frequency is 11.059 MHz.

The frequency for the timer is always 1/12th of the frequency of the crystal attached to the 8051.

• TF = 1/12 x 11.059 MHz = 921583 Hz.

The time delay of one machine cycle is given below. We use this to generate the delay.

• TP = 1/921583 = 1.085 µ sec.

Here we have explained in 3 steps how we can add a 10 ms Delay using timers:

1. Firstly, we divide the desired time delay value (10 ms) by the timer clock period.
   • N =  1/1.085us
   • N = 10ms/1.0859usec
   • N = 9216
2. Now we will subtract the value of N from the maximum number of counts possible for the 16-bit timer, i.e., 2¹⁶ = 65536.
   • M = 65536 – N
   • M = 65536 – 9216
   • M = 56320
3. Lastly, we convert this value to hexadecimal and write it in the TH and TL registers.
   • MH = DC00H
   • TH = D
   • TL = 00H

In this Proteus schematic, we have simulated this circuit by adding a delay of 10 ms between the on and off states of the LED. The code is provided in the section below.

We have set the crystal oscillator’s frequency at 11.059 MHz.

This sketch adds a delay between toggling LED states between on and off. This sketch initializes Port 2’s pin 0 as an LED pin. This sketch also uses mode 1 of timer 0 to add a 10 ms delay between the toggling of LED states.

#include<reg51.h>

sbit led = P2 ^ 0; // led at PORT 2 pin 0

void Delay(void); // Delay function declaration

void main() // main function
{
  led = 0; //output PORT

  while (1) // infinite loop
  {
    led = 1; // LED ON
    Delay();

    led = 0; // LED OFF
    Delay();
  }
}

void Delay()
{
  TMOD = 0x01; // Timer0 mode1
  TH0 = 0xDC; //initial value for 10ms
  TL0 = 0x00;
  TR0 = 1; // timer0 start

  while (TF0 == 0); // check overflow condition
  TR0 = 0; // Stop Timer
  TF0 = 0; // Clear flag
}

In this section, we will explain each part of the sketch provided above.

#include<reg51.h>
sbit led = P2 ^ 0; // led at PORT 2 pin 0
void Delay(void); // Delay function declaration

In the first part, we include the header file <reg51.h>. This will help us manage the registers of the 8051 microcontroller. In the next line, we have set the LED pin on PORT 2 of the 8051 microcontroller. On the last line, we have declared a function for the delay.

void main() // main function
{
  led = 0; //output PORT

  while (1) // infinite loop
  {
    led = 1; // LED ON
    Delay();

    led = 0; // LED OFF
    Delay();
  }
}

In the void main function, we have first set the LED state to off. Then, in the while loop, we will toggle between the on and off states of the LED. Between changing LED states, we are also calling the Delay function. Now let’s look at the Delay function.

void Delay()
{
  TMOD = 0x01; // Timer0 mode1
  TH0 = 0xDC; // High byte initial value for 10ms
  TL0 = 0x00; // Low byte initial value for 10ms
  TR0 = 1; // timer0 start

  while (TF0 == 0); // check overflow condition
  TR0 = 0; // Stop Timer
  TF0 = 0; // Clear flag
}

The void delay function is quite simple. First, we set the timer mode, which we have set to 1 for timer 0 in our case. Then we set the initial values for the low byte and the high byte to achieve a 10 ms delay. After this, we start this timer using TR0 = 1. Now in the while loop, we check for the overflow condition; once the timer reaches overflow, it goes back to the void main function.

In this tutorial, we have covered the following topics:

• 8051 microcontroller timers and their types.
• TMOD and TCON registers.
• Modes of timers and delay calculation.
• Proteus Simulation.
• Sketch and its explanation.

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This concludes our tutorial, if you face any issues while following this tutorial, let us know in the comment section below.

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