15

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

Rx

CLOCK

Tx

CLOCK

T2EX PIN

T2 PIN

TR2

CONTROL

"1"

"1"

"1"

"0"

"0"

"0"

TIMER 1 OVERFLOW

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

TIMER 2

INTERRUPT

2

2

16

16

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)]

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

=