AT89C5131 Datasheet, PDF (50/176 Pages) ATMEL Corporation – 8-bit Flash Microcontroller with Full Speed USB Device

Figure 23. Auto-reload Mode Up/Down Counter (DCEN = 1)
F_{CLK PERIPH}
:6
T2
0
1
C/T2
T2CON
TR2
T2CON
(DOWN COUNTING RELOAD VALUE) T2EX:
FFh FFh
(8-bit) (8-bit)
if DCEN = 1, 1 = UP
if DCEN = 1, 0 = DOWN
if DCEN = 0, up counting
TOGGLE T2CON
EXF2
TL2 TH2
(8-bit) (8-bit)
TF2
T2CON
Timer 2
INTERRUPT
Programmable Clock
Output
RCAP2L RCAP2H
(8-bit)
(8-bit)
(UP COUNTING RELOAD VALUE)
In the Clock-out mode, Timer 2 operates as a 50%-duty-cycle, programmable clock gen-
erator (See Figure 24). 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
Clock – OutFrequency
=
-------------------------------^F---^C----^L--^K---^P---^E---^R---^I--^P---^H-------------------------------
4 × (65536 – RCAP2H ⁄ RCAP2L)
For a 16 MHz system clock, Timer 2 has a programmable frequency range of 61 Hz
(F_{CLK PERIPH}/2¹⁶⁾ 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.
50 AT89C5131
4136B–USB–09/03