AT89C51ID2_0309 Datasheet, PDF (37/159 Pages) ATMEL Corporation – 8-bit Flash Microcontroller

AT89C51ID2
Figure 13. Auto-Reload Mode Up/Down Counter (DCEN = 1)
F_{CLK PERIPH}
:6
0
1
T2
C/T2
TR2
T2CON
T2CON
(DOWN COUNTING RELOAD VALUET) ^2EX:
FFh FFh
if DCEN=1, 1=UP
(8-bit) (8-bit)
if DCEN=1, 0=DOWN
if DCEN = 0, up counting
TOGGLE T2CON
EXF2
TL2
(8-bit)
TH2
(8-bit)
TF2
T2CON
TIMER 2
INTERRUPT
Programmable Clock-
Output
4289A–8051–09/03
RCAP2L RCAP2H
(8-bit)
(8-bit)
(UP COUNTING RELOAD VALUE)
In the clock-out mode, Timer 2 operates as a 50%-duty-cycle, programmable clock gen-
erator (See Figure 14). The input clock increments TL2 at frequency F_{CLK PERIPH}/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----^C----^L---^K---^P----^E----^R---^I--^P----^H-------------------------------
4 × (65536 – RCAP2H ⁄ RCAP2L )
For a 16 MHz system clock, Timer 2 has a programmable frequency range of 61 Hz
(F_{CLK PERIPH}/2¹⁶) to 4 MHz (F_{CLK PERIPH}/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.
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