AT89C55WD-24AI datasheet(15/35 Pages) ATMEL

15 / 35 page

1921C–MICRO–3/05

AT89C55WD

Figure 13-1. Timer 2 in Baud Rate Generator Mode

14. Programmable Clock Out

A 50% duty cycle clock can be programmed to come out on P1.0, as shown in Figure 14-1. 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 for 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.

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.

OSC

SMOD1

RCLK

TCLK

CLOCK

T2EX PIN

T2 PIN

TR2

CONTROL

"1"

"0"

TIMER 1 OVERFLOW

NOTE: OSC. FREQ. IS DIVIDED BY 2, NOT 12

TIMER 2

INTERRUPT

RCAP2L

RCAP2H

TH2

TL2

C/T2 = 0

C/T2 = 1

EXF2

CONTROL

TRANSITION

DETECTOR

EXEN2

Clock-Out Frequency

Oscillator Frequency

4 x [65536-(RCAP2H,RCAP2L)]

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