AT89C5115
Programmable Clock-
Output
In clock-out mode, Timer 2 operates as a 50%-duty-cycle, programmable clock genera-
tor (Figure 30). 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 depending on the
system oscillator frequency and the value in the RCAP2H and RCAP2L registers:
2
4 × (65536
2
⁄
2 )
For a 16 MHz system clock in x1 mode, 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 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 30. Clock-Out Mode
TH2
(8-bit)
TL2
(8-bit)
FT2
CLOCK
OVERFLOW
TR2
T2CON.2
RCAP2H
(8-bit)
RCAP2L
(8-bit)
Toggle
T2
Q
Q
D
T2OE
T2MOD reg
TIMER 2
INTERRUPT
T2EX
EXF2
T2CON reg
EXEN2
T2CON reg
63
4128F–8051–05/06
ATMEL [ ATMEL ]
