In this tutorial we will see 8051 timers. We will use the 8051 timers to generate a precise delay of 1 sec.

8051 timers/counters

The 8051 has 2 timers/counters.

  • They can be used to generate precise timing, i.e., we can measure time between events. The unit is then called timer.
  • It can also be used to count external events, known as counter.
  • Timer 1 is also used for generating baud rate in serial communication, which we will discuss in the next tutorial

Timer Operation

The Timer 0 is a 16 bit registers as shown. This can be accessed as 2 eight bit registers TL0 and TL1. Same applies to Timer 1.
The 8051 timer and counter is the same unit, but in this tutorial we will discuss only the timer unit to simplify the discussion.

fig 1:Basic Timer

Timer Tick

Fig 1, shows the basic 8051 timer unit. The registers TCON and TMOD affect the timer operation. The clock frequency is divided by 12 and used by the timer unit. Thus if a 11.0592MHz external crystal is used, the timer uses a frequency of 921KHz. Thus timer increments every 1.085μ seconds.

  • The C/Ṫ = 0 bit of TMOD register selects operation of Timer/counter unit as timer.
  • The TR bit of TCON register is used to start the timer.

Timer Registers

Table. 1: Timer Register
T0
TH0 TL0
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0


The TMOD Register specifies the operational mode of the two timers. The higher nibble is used for Timer 1 and Lower for the timer 0 as shown below.

Table 3. Timer Modes
M1 M0 Operation
0 0 13 bit Timer
0 1 16 bit Timer
1 0 8 bit Auto Reload
1 1 Split Mode
Table 2. TMOD Register
TMOD
D7 D6 D5 D4 D3 D2 D1 D0
Gate C/T M1 M0 Gate C/T M1 M0
Timer1 Timer 0
fig 2:Mode 0
fig 3:Mode 1
fig 4:Mode 2