Programmable clock output – Rainbow Electronics AT89C5131 User Manual
Page 65
65
AT89C5131
4136A–USB–03/03
Figure 26. Auto-reload Mode Up/Down Counter (DCEN = 1)
Programmable Clock
Output
In the Clock-out mode, Timer 2 operates as a 50%-duty-cycle, programmable clock gen-
erator (See Figure 27). The input clock increments TL2 at frequency F
CLK PERIPH
/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 following formula gives the Clock-out fre-
quency 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
CLK PERIPH
/2
16)
to 4 MHz (F
CLK PERIPH
/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 a different one depending on the application.
•
To start the timer, set TR2 run control bit in T2CON register.
(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)
TOGGLE
(UP COUNTING RELOAD VALUE)
Timer 2
INTERRUPT
F
CLK PERIPH
0
1
T2CON
T2CON
T2CON
T2CON
T2EX:
if DCEN = 1, 1 = UP
if DCEN = 1, 0 = DOWN
if DCEN = 0, up counting
: 6
Clock
OutFrequency
–
F
CLKPERIPH
4
65536
RCAP2H
–
RCAP2L
⁄
(
)
Ч
--------------------------------------------------------------------------------------------
=