AT89LP51RB2 datasheet(91/254 Pages) ATMEL

91 / 254 page

3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

14.3

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 14-4). 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 14-5.

14.3.1

Up Counter

Figure 14-2 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:

Note:

In Fast Mode, TPS applies only when the T2X2 bit in CKCON0 is set. TPS always

applies in Compatibility Mode, therefore setting T2X2 in Compatibility Mode will double

the timeout period.

Figure 14-2. Timer 2 Diagram: Auto-Reload Mode (DCEN = 0)

Table 14-5.

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

}

–

fSYS

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

TPS

()

CLKSYS

TL2

TH2

÷TPS

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