Common Features Description
2-87 Atmel 8051 Microcontrollers Hardware Manual
4316B–8051–02/04
but with the added feature that a 1-to-0 transition at external input T2EX will also trigger
the 16-bit reload and set EXF2.
2.12.1 Auto-reload Mode The auto-reload mode configures timer 2 as a 16-bit timer or event counter with auto-
matic reload. This feature is controlled by the DCEN bit in the T2MOD register (See
Table 2-9). Setting the DCEN bit enables timer 2 to count up or down as shown in Fig-
ure 2-17. In this mode the T2EX pin controls the counting direction.
When T2EX is high, timer 2 up-counts. Timer overflow occurs at FFFFh which sets the
TF2 flag and generates an interrupt request. The overflow also causes the 16-bit value
in the RCAP2H and RCAP2L registers to be loaded into the timer registers TH2 and
TL2.
When T2EX is low, timer 2 down-counts. Timer underflow occurs when the count in the
timer registers, TH2 and TL2, equals the value stored in the RCAP2H and RCAP2L reg-
isters. The underflow sets TF2 flag and reloads FFFFh into the timer registers.
The EXF2 bit toggles when timer 2 overflow or underflow occurs, depending on the
direction of the count. EXF2 does not generate an interrupt. This bit can be used to pro-
vide 17-bit resolution.
Figure 2-17. Auto-reload Mode Up/Down Counter
2.12.2 Programmable
Clock-output
In clock-out mode, timer 2 operates as a 50% duty-cycle, programmable clock generator
(See Figure 2-18). 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 the
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,
(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
Periph
CLOCK
To Next Page
Loading...
Need help?
General
