AT89LP51 datasheet(55/113 Pages) ATMEL

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3709B–MICRO–12/10

AT89LP51/52 - Preliminary

Figure 12-6. Timer 2 in Baud Rate Generator Mode

12.5

Frequency Generator (Programmable Clock Out)

Timer 2 can generate a 50% duty cycle clock on T2 (P1.0), as shown in Figure 13.. 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 toggle its output at every timer overflow. 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 system frequency and the reload value of Timer 2 capture registers (RCAP2H,

RCAP2L), as shown in the following equation.

In the frequency generator 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 generator 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 12-7. Timer 2 in Clock-out Mode

÷CDV

SMOD1

RCLK

TCLK

CLOCK

T2EX PIN

T2 PIN

TR2

"1"

"0"

TIMER 1 OVERFLOW

TIMER 2

INTERRUPT

RCAP2H

RCAP2L

TL2

TH2

C/T2 = 0

C/T2 = 1

EXF2

TRANSITION

DETECTOR

EXEN2

OSC

Clock Out Frequency

System Frequency

65536

RCAP2H,RCAP2L

()

–

[]

--------------------------------------------------------------------------------------------

OSC

T2EX PIN

T2 PIN

TR2

TIMER 2

INTERRUPT

RCAP2H

RCAP2L

TL2

TH2

C/T2

EXF2

TRANSITION

DETECTOR

EXEN2

÷CDV

÷2

T2OE

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