AT89C51RC-33AC ATMEL Corporation, AT89C51RC-33AC Datasheet - Page 12

• Manufacturer Part Number: AT89C51RC-33AC
• Description: 8-bit Microcontroller with 32K Bytes Flash
• Manufacturer: ATMEL Corporation

Timer 0 and 1
Timer 2
Capture Mode
Auto-Reload (Up or
Down Counter)
12
AT89C51RC
Timer 0 and Timer 1 in the AT89C51RC operate the same way as Timer 0 and Timer 1
in the AT89C51 and AT89C52.
Timer 2 is a 16-bit Timer/Counter that can operate as either a timer or an event counter.
The type of operation is selected by bit C/T2 in the SFR T2CON (shown in Table 2).
Timer 2 has three operating modes: capture, auto-reload (up or down counting), and
baud rate generator. The modes are selected by bits in T2CON, as shown in Table 3.
Timer 2 consists of two 8-bit registers, TH2 and TL2. In the Timer function, the TL2 reg-
ister is incremented every machine cycle. Since a machine cycle consists of 12
oscillator periods, the count rate is 1/12 of the oscillator frequency.
Table 3. Timer 2 Operating Modes
In the Counter function, the register is incremented in response to a 1-to-0 transition at
its corresponding external input pin, T2. In this function, the external input is sampled
during S5P2 of every machine cycle. When the samples show a high in one cycle and a
low in the next cycle, the count is incremented. The new count value appears in the reg-
ister during S3P1 of the cycle following the one in which the transition was detected.
Since two machine cycles (24 oscillator periods) are required to recognize a 1-to-0 tran-
sition, the maximum count rate is 1/24 of the oscillator frequency. To ensure that a given
level is sampled at least once before it changes, the level should be held for at least one
full machine cycle.
In the capture mode, two options are selected by bit EXEN2 in T2CON. If EXEN2 = 0,
Timer 2 is a 16-bit timer or counter which upon overflow sets bit TF2 in T2CON. This bit
can then be used to generate an interrupt. If EXEN2 = 1, Timer 2 performs the same
operation, but a 1-to-0 transition at external input T2EX also causes the current value in
TH2 and TL2 to be captured into RCAP2H and RCAP2L, respectively. In addition, the
transition at T2EX causes bit EXF2 in T2CON to be set. The EXF2 bit, like TF2, can
generate an interrupt. The capture mode is illustrated in Figure 2.
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 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.
RCLK +TCLK
X
0
0
1
CP/RL2
X
X
0
1
TR2
1
1
1
0
MODE
16-bit Auto-reload
16-bit Capture
Baud Rate Generator
(Off)
1920B–MICRO–11/02