AT89C51RC_05 Datasheet, PDF (17/36 Pages) ATMEL Corporation – 8-bit Microcontroller with 32K Bytes Flash

AT89C51RC
rupt. Note too, that if EXEN2 is set, a 1-to-0 transition in T2EX will set EXF2 but will not cause a
reload from (RCAP2H, RCAP2L) to (TH2, TL2). Thus when Timer 2 is in use as a baud rate gen-
erator, T2EX can be used as an extra external interrupt.
Note that when Timer 2 is running (TR2 = 1) as a timer in the baud rate generator mode, TH2 or
TL2 should not be read from or written to. Under these conditions, the Timer is incremented
every state time, and the results of a read or write may not be accurate. The RCAP2 registers
may be read but should not be written to, because a write might overlap a reload and cause
write and/or reload errors. The timer should be turned off (clear TR2) before accessing the Timer
2 or RCAP2 registers.
15. Programmable Clock Out
A 50% duty cycle clock can be programmed to come out on P1.0, as shown in Figure 15-1. This
pin, besides being a regular I/O pin, has two alternate functions. It can be programmed to input
the external clock for Timer/Counter 2 or to output a 50% duty cycle clock ranging from 61 Hz to
4 MHz at a 16 MHz operating frequency.
To configure the Timer/Counter 2 as a clock generator, bit C/T2 (T2CON.1) must be cleared and
bit T2OE (T2MOD.1) must be set. Bit TR2 (T2CON.2) starts and stops the timer.
The clock-out frequency depends on the oscillator frequency and the reload value of Timer 2
capture registers (RCAP2H, RCAP2L), as shown in the following equation.
Clock-Out Frequency = ---------------^O-----^s---^c---ⁱ-^l--^l-^a---^t--^o---^r----^F----^r--^e---^q---^u----^e---ⁿ---^c----^y----------------
4 x [65536-(RCAP2H,RCAP2L)]
In the clock-out mode, Timer 2 roll-overs will not generate an interrupt. This behavior is similar to
when Timer 2 is used as a baud-rate generator. It is possible to use Timer 2 as a baud-rate gen-
erator and a clock generator simultaneously. Note, however, that the baud-rate and clock-out
frequencies cannot be determined independently from one another since they both use
RCAP2H and RCAP2L.
Figure 15-1. Timer 2 in Clock-Out Mode
OSC
÷2
TL2
TH2
(8-BITS) (8-BITS)
TR2
P1.0
(T2)
P1.1
(T2EX)
TRANSITION
DETECTOR
EXEN2
C/T2 BIT
RCAP2L RCAP2H
÷2
T2OE (T2MOD.1)
EXF2
TIMER 2
INTERRUPT
17
1920C–MICRO–03/05