AT89C55WD-33AU View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

AT89C55WD-33AU

8-bit Microcontroller with 20K Bytes Flash

Atmel Corporation 

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

AT89C55WD

TIMER 1 OVERFLOW

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

OSC

÷2

C/T2 = 0

T2 PIN

T2EX PIN

C/T2 = 1

TRANSITION

DETECTOR

TH2

TL2

CONTROL

TR2

RCAP2H RCAP2L

CONTROL

EXEN2

EXF2

÷2

"0" "1"

SMOD1

"1" "0"

"1" "0"

TIMER 2

INTERRUPT

RCLK

÷ 16

Rx

CLOCK

TCLK

÷ 16

Tx

CLOCK

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.

Clock-Out Frequency = 4-----x-----[--6---5O----5-s--3-c--6-i-l---l-a(--R-t--o--C--r--A-F---P-r--e-2--q--H--u--,-e-R--n--C-c---A-y---P----2----L---)--]

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.

15

1921D–MICRO–6/08

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