Rainbow Electronics T89C51AC2 - Manual (Page 60)

60
T89C51AC2
Rev. B – 19-Dec-01
Figure 27. Auto-Reload Mode Up/Down Counter
13.2 Programmable
Clock-Output In clock-out mode, timer 2 operates as a 50%-duty-cycle, programmable clock genera-
tor (See Figure 28). 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.
(DOWN COUNTING RELOAD VALUE)
TF2
T2
EXF2
TH2
(8-bit)
TL2
( 8-bit)
RCAP2H
(8-bit)
RCAP2L
(8-bit)
FFh
(8-bit)
FFh
( 8-bit)
TOGGLE
(UP COUNTING RELOAD VALUE)
TIMER 2
INTERRUPT
:6
T2CONreg
T2CONreg
T2EX:
1=UP
2=DOWN
0
1
CT/2
T2CON.1
TR2
T2CON.2
FT2
CLOCK
see section “Clock”
Clock OutFrequency –
FT2clock
4 65536 RCAP2H – RCAP2L ⁄ ( ) Ч
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