AT89C51AC3-RDTUM datasheet(75/143 Pages) ATMEL

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AT89C51AC3

4383B–8051–01/05

Programmable Clock-

Output

In clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock genera-

tor (See Figure 41). The input clock increments TL2 at frequency FOSC/2. The timer

repeatedly counts to overflow from a loaded value. At overflow, the contents of RCAP2H

and RCAP2L registers are loaded into TH2 and TL2. In this mode, timer 2 overflows do

not generate interrupts. The formula gives the clock-out frequency depending on the

system oscillator frequency and the value in the RCAP2H and RCAP2L registers:

For a 16 MHz system clock in x1 mode, timer 2 has a programmable frequency range of

61 Hz (FOSC/2

16) to 4 MHz (F

OSC/4). The generated clock signal is brought out to T2 pin

(P1.0).

Timer 2 is programmed for the clock-out mode as follows:

•

Set T2OE bit in T2MOD register.

•

Clear C/T2 bit in T2CON register.

•

Determine the 16-bit reload value from the formula and enter it in RCAP2H/RCAP2L

registers.

•

Enter a 16-bit initial value in timer registers TH2/TL2. It can be the same as the

reload value or different depending on the application.

•

To start the timer, set TR2 run control bit in T2CON register.

It is possible to use timer 2 as a baud rate generator and a clock generator simulta-

neously. For this configuration, the baud rates and clock frequencies are not

independent since both functions use the values in the RCAP2H and RCAP2L registers.

Figure 41. Clock-Out Mode

Clock OutFrequency

–

FT2clock

65536 RCAP2H

–

RCAP2L

⁄

()

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

EXEN2

EXF2

OVERFLOW

T2EX

TH2

(8-bit)

TL2

(8-bit)

TIMER 2

RCAP2H

(8-bit)

RCAP2L

(8-bit)

T2OE

T2CON reg

T2MOD reg

INTERRUPT

TR2

T2CON.2

FT2

CLOCK

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