8051 Timers

8051 Timers

AU : May-08, Dec.-09, 11, 13

8051 has two timers, timer 0 and timer 1. Basically both, timer 0 and timer 1 are 16-bit registers. Since 8051 is an 8-bit microcontroller, each 16-bit register can be accessed as low-byte register(TL) and high-byte register(TH). Fig. 16.3.1 shows the timer 0 and timer 1 registers. These registers can be accessed like other registers (A, B, R0, R1 etc.) in 8051.

1. Structure of TMOD Register

Timer/counter mode control (TMOD) is the special function register in 8051 having format as shown in Fig. 16.3.2.

The TMOD register is responsible for configuring the timers for the following operations :

■ Select Timer 0 to operate as a counter or timer

■ Select Timer 1 to operate as a counter or timer

■ Select the mode in which timer should operate.

M1, M0 : These bits select the timer mode. There are four different modes of timer, mode 0, mode 1, mode 2 and mode 3. All these modes are discussed in the further section.

 This bit is cleared (C/= 0) for selecting 'timer' operation and is set (C/ = 1) for selecting 'counter' operation.

GATE : Gating control when set. Timer/Counter "x" is enabled only while "INTx" pin is high and "TRx" control bit is set. When cleared Timer "x" is enabled whenever "TRx" control bit is set.

2. Structure of TCON Register

The Fig. 16.3.3 shows the format for the TCON register of 8051.

The TCON register controls the following timer operations :

• Start and stop timer 0 and timer 1

• Provides status of timer/counter overflows

• Provides status of external interrupts

• Configures external interrupts as either low level triggered or falling edge triggered.

Example 16.3.1 Indicate the effect of following 8051 instructions.

a. MOV TMOD, #00010000B b. MOV TMOD, #00000001B c. MOV TMOD, #04

Solution :

a. MOV TMOD, #00010000B : The effect of this instruction is to set Timer 1 in mode 1 and Gate = 0 for internal clocking.

b. MOV TMOD, #00000001B : The effect of this instruction is to set Timer 0 in mode

1 and Gate = 0 for internal clocking.

c. MOV TMOD, #04 : The effect of this instruction is to select timer 0 to run in the counter mode.

Example 16.3.2 Perform the following operations using bit addressable instructions

Solution : a) SETB TRI ; starts timer 1 by setting TCON.6 = 1

b) CLR TRO ; stops timer 0 by clearing TCON.4 = 0

Review Questions

1. Explain the timer/counter functional unit of microcontroller 8051 with relevant diagrams.  AU : May-08, Marks 16        

2. Discuss in detail the on chip timers supported by 8051, bringing out the various modes of operation of these timers. AU : Dec.-09, Marks 16        

3. Draw the TMOD register format and explain.   AU : Dec.-11, Marks 4

4. Discuss about the timers in 8051 with suitable examples.     AU : Dec.-13, Marks 16