AT89C55WD-33AC ATMEL Corporation, AT89C55WD-33AC Datasheet - Page 12

• Manufacturer Part Number: AT89C55WD-33AC
• Description: 8-bit Microcontroller with 20K Bytes Flash
• Manufacturer: ATMEL Corporation

Figure 5. Timer in Capture Mode
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OSC
C/T2 = 0
TH2
TL2
TF2
OVERFLOW
CONTROL
TR2
C/T2 = 1
CAPTURE
T2 PIN
RCAP2H
RCAP2L
TRANSITION
TIMER 2
DETECTOR
INTERRUPT
T2EX PIN
EXF2
CONTROL
EXEN2
Figure 6 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 Timer in Capture ModeRCAP2H
and RCAP2L are preset by software. 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 interrupt if enabled.
Setting the DCEN bit enables Timer 2 to count up or down, as shown in Figure 6. 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.
AT89C55WD
12
1921B–MICRO–09/02