
15
4431E–8051–04/06
AT/TS8xC54/8X2
Figure 8-1.
Auto-Reload Mode Up/Down Counter (DCEN = 1)
8.1.1
Programmable Clock-Output
In the clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock generator
(See
Figure 8-2) . 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 reg-
isters 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 :
For a 16 MHz system clock, 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.
(DOWN COUNTING RELOAD VALUE)
C/T2
TF2
TR2
T2
EXF2
TH2
(8-bit)
TL2
(8-bit)
RCAP2H
(8-bit)
RCAP2L
(8-bit)
FFh
(8-bit)
FFh
(8-bit)
T OG-
(UP COUNTING RELOAD VALUE)
TIMER 2
INTERRUPT
XTAL1
:12
F
OSC
FXTAL
0
1
T2CONreg
T2EX:
if DCEN=1, 1=UP
if DCEN=1, 0=DOWN
(:6 in X2 mode)
Clock OutFrequency
–
Fosc
4
65536
RCAP2H
–
RCAP2L
()
×
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