Datasheet

67
A/T89C51CC01
4129N–CAN–03/08
Programmable Clock-
Output
In clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock genera-
tor (See Figure 37). 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:
For a 16 MHz system clock in x1 mode, timer 2 has a programmable frequency range of
61 Hz (F
OSC
/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.
• 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.
Figure 37. Clock-Out Mode
Clock OutFrequency–
FT2clock
4 65536 RCAP2H– RCAP2L⁄( )×
-----------------------------------------------------------------------------------------
=
EXEN2
EXF2
OVERFLOW
T2EX
TH2
(8-bit)
TL2
(8-bit)
TIMER 2
RCAP2H
(8-bit)
RCAP2L
(8-bit)
T2OE
T2CON reg
T2CON reg
T2MOD reg
INTERRUPT
TR2
T2CON.2
FT2
CLOCK
T2
Q D
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Q