AT89C51CC03_03 Datasheet, PDF (78/183 Pages) ATMEL Corporation – CAN Microcontrollers

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 F_OSC/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:
Figure 41. Clock-Out Mode
FT2
CLOCK
Clock – OutFrequency = ₄-----_×-----₍---₆---₅---₅----₃---₆-----–--^F--_R--^T---_C-²----_Ac---^l-_P-^o---₂-^c---^kH------_⁄--_R-----_C-----_A----_P----₂----_L----₎
For a 16 MHz system clock in x1 mode, timer 2 has a programmable frequency range of
61 Hz (F_OSC/2¹⁶⁾ 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.
0
1
CT/2
T2CON.1
TR2
T2CON.2
TL2
TH2
(8-bit) (8-bit)
OVERFLOW
T2
T2EX
1
0
C/T2
T2CON reg
:2
EXEN2
T2CON reg
RCAP2L RCAP2H
(8-bit) (8-bit)
T2OE
T2MOD reg
EXF2
T2CON reg
TIMER 2
INTERRUPT
78 AT89C51CC03
4182B–CAN–09/03