T89C51RD2-SLRCM View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

T89C51RD2-SLRCM

0 to 40MHz Flash Programmable 8-bit Microcontroller

Atmel Corporation 

T89C51RD2

Figure 5. Auto-Reload Mode Up/Down Counter (DCEN = 1)

XTAL1

FXTAL

:12

FOSC

T2

0

1

C/T2

TR2

T2CONreg T2CONreg

(DOWN COUNTING RELOAD

FFh FFh

(8-bit) (8-bit)

TL2 TH2

(8-bit) (8-bit)

T2EX:

if DCEN=1, 1=UP

if DCEN=1, 0=DOWN

if DCEN = 0, up

TOGGL T2CONreg

EXF2

TF2

TIMER 2

INTERRUPT

T2CONreg

RCAP2L RCAP2H

(8-bit) (8-bit)

(UP COUNTING RELOAD VALUE)

Programmable Clock-Output

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

erator (See Figure 6) . 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 as a function of the

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

Clock – OutFrequency

=

----------------------------------------F----o---s----c----------------------------------------

4 × (65536 – RCAP2H ⁄ RCAP2L)

For a 16 MHz system clock, timer 2 has a programmable frequency range of 61 Hz

(FOSC/216) to 4 MHz (FOSC/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 a different one 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.

19

4243G–8051–05/03

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