AT89C51AC3-RDTUM datasheet(74/143 Pages) ATMEL

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AT89C51AC3

4383B–8051–01/05

Timer 2

The AT89C51AC3 timer 2 is compatible with timer 2 in the 80C52.

It is a 16-bit timer/counter: the count is maintained by two eight-bit timer registers, TH2

and TL2 that are cascade- connected. It is controlled by T2CON register (See Table )

and T2MOD register (See Table 38). Timer 2 operation is similar to Timer 0 and Timer

1. C/T2 selects FT2 clock/6 (timer operation) or external pin T2 (counter operation) as

timer clock. Setting TR2 allows TL2 to be incremented by the selected input.

Timer 2 includes the following enhancements:

•

Auto-reload mode (up or down counter)

•

Programmable clock-output

Auto-Reload Mode

The auto-reload mode configures timer 2 as a 16-bit timer or event counter with auto-

matic reload. This feature is controlled by the DCEN bit in T2MOD register (See

Table 38). Setting the DCEN bit enables timer 2 to count up or down as shown in

Figure 40. In this mode the T2EX pin controls the counting direction.

When T2EX is high, timer 2 counts up. Timer overflow occurs at FFFFh which sets the

TF2 flag and generates an interrupt request. The overflow also causes the 16-bit value

in RCAP2H and RCAP2L registers to be loaded into the timer registers TH2 and TL2.

When T2EX is low, timer 2 counts down. Timer underflow occurs when the count in the

timer registers TH2 and TL2 equals the value stored in RCAP2H and RCAP2L registers.

The underflow sets TF2 flag and reloads FFFFh into the timer registers.

The EXF2 bit toggles when timer 2 overflow or underflow, depending on the direction of

the count. EXF2 does not generate an interrupt. This bit can be used to provide 17-bit

resolution.

Figure 40. Auto-Reload Mode Up/Down Counter

(DOWN COUNTING RELOAD VALUE)

TF2

EXF2

TH2

(8-bit)

TL2

(8-bit)

RCAP2H

(8-bit)

RCAP2L

(8-bit)

FFh

(8-bit)

FFh

(8-bit)

TOGGLE

(UP COUNTING RELOAD VALUE)

TIMER 2

INTERRUPT

T2CONreg

T2EX:

1=UP

2=DOWN

CT/2

T2CON.1

TR2

T2CON.2

FT2

CLOCK

see section “Clock”

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