8051 has two 16-bit programmable UP timers/counters. They can be configured to operate either as timers or as event counters. The names of the two counters are T0 and T1 respectively.  The timer content is available in four 8-bit special function registers, viz, TL0,TH0,TL1 and TH1 respectively.

In the "timer" function mode, the counter is incremented in every machine cycle. Thus, one can think of it as counting machine cycles. Hence the clock rate is 1/12 ^th of the oscillator frequency.

In the "counter" function mode, the register is incremented in response to a 1 to 0 transition at its corresponding external input pin (T0 or T1). It requires 2 machine cycles to detect a high to low transition. Hence maximum count rate is 1/24 ^th of oscillator frequency.

The operation of the timers/counters is controlled by two special function registers, TMOD and TCON respectively.

Timer Mode control (TMOD) Special Function Register:

TMOD
Address: 89 H

Various bits  of TMOD are described as follows -

Gate: This is an OR Gate enabled bit which controls the effect of on START/STOP of Timer. It is set to one ('1') by the program to enable the interrupt to start/stop the timer. If TR1/0 in TCON is set and signal on pin is high then the timer starts counting using either internal clock (timer mode) or external pulses (counter mode).

It is used for the selection of Counter/Timer mode.
Mode Select Bits:

M1 and M0 are mode select bits.

Timer/ Counter control logic:

Timer control (TCON) Special function register:

TF1 : Timer1 overflow flag. It is set when timer rolls from all 1s to 0s. It is cleared when processor vectors to execute ISR located at address 001BH.
TR1 : Timer1 run control bit. Set to 1 to start the timer / counter.
TF0 : Timer0 overflow flag. (Similar to TF1)
TR0 : Timer0 run control bit.
IE1 : Interrupt1 edge flag. Set by hardware when an external interrupt edge is detected. It is cleared when interrupt is processed.
IE0 : Interrupt0 edge flag. (Similar to IE1)
IT1 : Interrupt1 type control bit. Set/ cleared by software to specify falling edge / low level triggered external interrupt.
IT0 : Interrupt0 type control bit. (Similar to IT1)
As mentioned earlier, Timers can operate in four different modes. They are as follows

In this mode, the timer is used as a 13-bit UP counter as follows.

The lower 5 bits of TLX and 8 bits of THX are used for the 13 bit count.Upper 3 bits of TLX are ignored. When the counter rolls over from all 0's to all 1's, TFX flag is set and an interrupt is generated.

The input pulse is obtained from the previous stage. If TR1/0 bit is 1 and Gate bit is 0, the counter continues counting up. If TR1/0 bit is 1 and Gate bit is 1, then the operation of the counter is controlled by input. This mode is useful to measure the width of a given pulse fed to   input.

This mode is similar to mode-0 except for the fact that the Timer operates in 16-bit mode.

This is a 8 bit counter/timer operation. Counting is performed in TLX while THX stores a constant value. In this mode when the timer overflows i.e. TLX becomes FFH, it is fed with the value stored in THX. For example if we load THX with 50H then the timer in mode 2 will count from 50H to FFH. After that 50H is again reloaded. This mode is useful in applications like fixed time sampling.

Timer 1 in mode-3 simply holds its count. The effect is same as setting TR1=0. Timer0 in mode-3 establishes TL0 and TH0 as two separate counters.

Control bits TR1 and TF1 are used by Timer-0 (higher 8 bits) (TH0) in Mode-3 while TR0 and TF0 are available to Timer-0 lower 8 bits(TL0).