AT89C51ED2-SMRUM Datasheet, PDF (31/137 Pages) ATMEL Corporation – 8-bit Flash Microcontroller

AT89C51RD2/ED2
Figure 12-1. Auto-reload Mode Up/Down Counter (DCEN = 1)
F_{CLK PERIPH}
:6
0
1
T2
C/T2
T2CON
TR2
T2CON
(DOWN COUNTING RELOAD VALUE) T2EX:
FFh
(8-bit)
FFh
(8-bit)
If DCEN = 1, 1 = UP
If DCEN = 1, 0 = DOWN
If DCEN = 0, up counting
TOGGLE T2CON
EXF2
TL2
(8-bit)
TH2
(8-bit)
TF2
T2CON
TIMER 2
INTERRUPT
RCAP2L RCAP2H
(8-bit)
(8-bit)
(UP COUNTING RELOAD VALUE)
12.2
Programmable
Clock-output
In the clock-out mode, Timer 2 operates as a 50% duty-cycle, programmable clock generator
(See Figure 12-2). The input clock increments TL2 at frequency F_{CLK PERIPH}/2. The timer repeat-
edly 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 inter-
rupts. 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 = -₄----_×-----₍---₆---₅---₅----₃---₆----^F-–---^C-_R---^L--_C-^K----^P_A---^E-_P---^R-₂--^I--_H^P----^H--_⁄---_R----_C-----_A----_P-----₂---_L----₎-
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.
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