AT89LP6440 datasheet(62/194 Pages) ATMEL

62 / 194 page

3706A–MICRO–9/09

AT89LP6440 - Preliminary

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. The overflow and reload values depend on the Timer 2 Count Mode bits, T2CM

1-0 in

T2MOD. 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 and T2CM

1-0 = 00B. 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 tran-

sition at external input T2EX. This transition also sets the EXF2 bit. Both the TF2 and EXF2 bits

can generate an interrupt. The Timer 2 overflow rate for this mode is given in the following

equation:

Timer 2 may also be configured to count from MIN to TOP instead of BOTTOM to MAX by set-

ting T2CM

1-0 = 01B. In this mode Timer 2 counts up to TOP, the 16-bit value in RCAP2H and

÷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

Table 12-5.

Summary of Auto-Reload Modes

T2CM1-0

DCEN

T2EX

Direction

Behavior

reload to BOTTOM

Down

underflow to MAX

overflow to BOTTOM

reload to MIN

Down

underflow to TOP

overflow to MIN

Up-Down

and repeat

Up-Down

and repeat

BOTTOM

MAX

→

MAX

BOTTOM

→

BOTTOM

MAX

→

MIN

TOP

→

TOP

MIN

→

MIN

TOP

→

MIN

TOP

MIN

→→

MIN

TOP

MIN

→→

Auto-Reload Mode:

DCEN = 0, T2CM = 00B

Time-out Period

65536

RCAP2H RCAP2L

}

–

Oscillator Frequency

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

TPS

()

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