AT89C51AC3-RDTIM datasheet(66/143 Pages) ATMEL

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AT89C51AC3

4383B–8051–01/05

Timers/Counters

The AT89C51AC3 implements two general-purpose, 16-bit Timers/Counters. Such are

identified as Timer 0 and Timer 1, and can be independently configured to operate in a

variety of modes as a Timer or an event Counter. When operating as a Timer, the

Timer/Counter runs for a programmed length of time, then issues an interrupt request.

When operating as a Counter, the Timer/Counter counts negative transitions on an

external pin. After a preset number of counts, the Counter issues an interrupt request.

The various operating modes of each Timer/Counter are described in the following

sections.

Timer/Counter

Operations

A basic operation is Timer registers THx and TLx (x = 0, 1) connected in cascade to

form a 16-bit Timer. Setting the run control bit (TRx) in TCON register (see Figure 30)

turns the Timer on by allowing the selected input to increment TLx. When TLx overflows

it increments THx; when THx overflows it sets the Timer overflow flag (TFx) in TCON

ters can be accessed to obtain the current count or to enter preset values. They can be

read at any time but TRx bit must be cleared to preset their values, otherwise the behav-

ior of the Timer/Counter is unpredictable.

The C/Tx# control bit selects Timer operation or Counter operation by selecting the

divided-down peripheral clock or external pin Tx as the source for the counted signal.

TRx bit must be cleared when changing the mode of operation, otherwise the behavior

of the Timer/Counter is unpredictable.

For Timer operation (C/Tx# = 0), the Timer register counts the divided-down peripheral

clock. The Timer register is incremented once every peripheral cycle (6 peripheral clock

periods). The Timer clock rate is FPER/6, i.e. FOSC/12 in standard mode or FOSC/6 in X2

mode.

For Counter operation (C/Tx# = 1), the Timer register counts the negative transitions on

the Tx external input pin. The external input is sampled every peripheral cycles. When

the sample is high in one cycle and low in the next one, the Counter is incremented.

Since it takes 2 cycles (12 peripheral clock periods) to recognize a negative transition,

the maximum count rate is FPER/12, i.e. FOSC/24 in standard mode or FOSC/12 in X2

mode. There are no restrictions on the duty cycle of the external input signal, but to

ensure that a given level is sampled at least once before it changes, it should be held for

at least one full peripheral cycle.

Timer 0

Timer 0 functions as either a Timer or event Counter in four modes of operation.

Figure 35 to Figure 38 show the logical configuration of each mode.

Timer 0 is controlled by the four lower bits of TMOD register (see Figure 31) and bits 0,

1, 4 and 5 of TCON register (see Figure 30). TMOD register selects the method of Timer

gating (GATE0), Timer or Counter operation (T/C0#) and mode of operation (M10 and

M00). TCON register provides Timer 0 control functions: overflow flag (TF0), run control

bit (TR0), interrupt flag (IE0) and interrupt type control bit (IT0).

For normal Timer operation (GATE0 = 0), setting TR0 allows TL0 to be incremented by

the selected input. Setting GATE0 and TR0 allows external pin INT0# to control Timer

operation.

Timer 0 overflow (count rolls over from all 1s to all 0s) sets TF0 flag generating an inter-

rupt request.

It is important to stop Timer/Counter before changing mode.

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