AT89LP51 datasheet(53/113 Pages) ATMEL

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3709C–MICRO–5/11

AT89LP51/52 - Preliminary

12.2

Capture Mode

In the Capture mode, Timer 2 is a fixed 16-bit timer or counter that counts up from MIN to MAX.

An overflow from MAX to MIN sets bit TF2 in T2CON. If EXEN2 = 1, a 1-to-0 transition at exter-

nal input T2EX also causes the current value in TH2 and TL2 to be captured into RCAP2H and

RCAP2L, respectively. In addition, the transition at T2EX causes bit EXF2 in T2CON to be set.

The EXF2 and TF2 bits can generate an interrupt. Capture mode is illustrated in Figure 12-1.

The Timer 2 overflow rate in Capture mode is given by the following equation:

Figure 12-1. Timer 2 Diagram: Capture Mode

12.3

Auto-Reload Mode

Timer 2 can be programmed to count up or down when configured in its 16-bit auto-reload

mode. This feature is invoked by the DCEN (Down Counter Enable) bit located in the SFR

T2MOD (see Table 12-4). Upon reset, the DCEN bit is set to 0 so that timer 2 will default to

count up. When DCEN is set, Timer 2 can count up or down, depending on the value of the

T2EX pin. A summary of the Auto-Reload behaviors is listed in Table 12-5.

12.3.1

Up Counter

Figure 12-2 shows Timer 2 automatically counting up when DCEN = 0. In this mode Timer 2

counts up to MAX and then sets the TF2 bit upon overflow. The overflow also causes the timer

registers to be reloaded with BOTTOM, the 16-bit value in RCAP2H and RCAP2L. If EXEN2 = 1,

a 16-bit reload can be triggered either by an overflow or by a 1-to-0 transition at external input

T2EX. This transition also sets the EXF2 bit. Both the TF2 and EXF2 bits can generate an inter-

rupt. The Timer 2 overflow rate for this mode is given in the following equation:

Capture Mode:

Time-out Period

65536

System Frequency

--------------------------------------------------

TPS

()

÷TPS

EXF2

T2EX PIN

T2 PIN

TR2

EXEN2

C/T2 = 0

C/T2 = 1

CAPTURE

OVERFLOW

TRANSITION

DETECTOR

TIMER 2

INTERRUPT

RCAP2H

RCAP2L

TL2

TH2

TF2

OSC

÷CDV

Table 12-5.

Summary of Auto-Reload Modes

DCEN

T2EX

Direction

Behavior

reload to BOTTOM

Down

underflow to MAX

overflow to BOTTOM

BOTTOM

MAX

→

MAX

BOTTOM

→

BOTTOM

MAX

→

Auto-Reload Mode:

DCEN = 0

Time-out Period

65536

RCAP2H RCAP2L

}

–

System Frequency

-------------------------------------------------------------------------------

TPS

()

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