AT89LP428 datasheet(60/149 Pages) ATMEL

60 / 149 page

3654A–MICRO–8/09

AT89LP428/828

Timer 2 as a baud rate generator is shown in Figure 12-7. This figure is valid only if RCLK or

TCLK = 1 in T2CON. Note that a rollover in TH2 does not set TF2 and will not generate an inter-

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

generator, T2EX can be used as an extra external interrupt. Also note that the Baud Rate and

Frequency Generator modes may be used simultaneously.

Figure 12-7. 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 12-8. 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 oscillator frequency and the reload value of Timer 2

capture registers (RCAP2H, RCAP2L), as shown in the following equations.

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.

÷TPS

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

T2CM = 00B

Clock Out Frequency

Oscillator Frequency

65536

RCAP2H,RCAP2L

()

–

[]

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

TPS

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

T2CM = 01B

Clock Out Frequency

Oscillator Frequency

RCAP2H,RCAP2L

() 1

[]

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

TPS

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

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