AT89S8252
Auto-reload (Up or Down 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
Counter)
the SFR T2MOD (see Table 9). 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.
Figure 2 shows Timer 2 automatically counting up when DCEN = 0. In this mode, two
options are selected by bit EXEN2 in T2CON. If EXEN2 = 0, Timer 2 counts up to
0FFFFH and then sets the TF2 bit upon overflow. The overflow also causes the timer
registers to be reloaded with the 16-bit value in RCAP2H and RCAP2L. The values in
RCAP2H and RCAP2L are preset by software. If EXEN2 = 1, a 16-bit reload can be trig-
gered 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 interrupt
if enabled.
Setting the DCEN bit enables Timer 2 to count up or down, as shown in Figure 3. In this
mode, the T2EX pin controls the direction of the count. A logic 1 at T2EX makes Timer 2
count up. The timer will overflow at 0FFFFH and set the TF2 bit. This overflow also
causes the 16-bit value in RCAP2H and RCAP2L to be reloaded into the timer registers,
TH2 and TL2, respectively.
A logic 0 at T2EX makes Timer 2 count down. The timer underflows when TH2 and TL2
equal the values stored in RCAP2H and RCAP2L. The underflow sets the TF2 bit and
causes 0FFFFH to be reloaded into the timer registers.
The EXF2 bit toggles whenever Timer 2 overflows or underflows and can be used as a
17th bit of resolution. In this operating mode, EXF2 does not flag an interrupt.
Figure 2. Timer 2 in Auto Reload Mode (DCEN = 0)
13
0401G–MICRO–3/06
ATMEL [ ATMEL ]
