Specifications
63
3706C–MICRO–2/11
AT89LP3240/6440
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 Cou nter 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 a
n 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
RCAP2HRCAP2L
TL2 TH2
TF2
OSC
Table 12-5. Summary of Auto-Reload Modes
T2CM
1-0
DCEN T2EX Direction Behavior
00 0 X Up reload to BOTTOM
00 1 0 Down underflow to MAX
00 1 1 Up overflow to BOTTOM
01 0 X Up reload to MIN
01 1 0 Down underflow to TOP
01 1 1 Up overflow to MIN
10 X X Up-Down and repeat
11 X X 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
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TPS 1+()×=