T89C51AC2-RLSI-M datasheet(65/117 Pages) ATMEL

65 / 117 page

Draft.A - March 30, 2001

Preview - Confidential

T89C51AC2

Figure 53. Auto-Reload Mode Up/Down Counter

13.3. Programmable Clock-Output

In clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock generator (See Figure 54). 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:

NOTE: X2 bit is located in CKCON register.

In X2 mode, FOSC=FXTAL. In standard mode, FOSC=FXTAL/2.

For a 16 MHz system clock, 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.

(DOWN COUNTING RELOAD VALUE)

TF2

EXF2

TH2

(8-bit)

TL2

(8-bit)

RCAP2H

(8-bit)

RCAP2L

(8-bit)

FFh

(8-bit)

FFh

(8-bit)

TOGGLE

(UP COUNTING RELOAD VALUE)

TIMER 2

INTERRUPT

T2CONreg

T2EX:

1=UP

2=DOWN

CT/2

T2CON.1

TR2

T2CON.2

FT2

CLOCK

Clock

OutFrequency

–

osc

2x2

65536

RCAP2H

–

RCAP2L

⁄

()

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