SM5953
8-Bit Micro-controller
15KB with ISP Flash
& 256B RAM embedded
7.2 Auto-reload (Up or Down Counter)
Timer 2 can be programmed to count up or down when configured in its 16-bit auto-reload mode. This feature is
invoked by a bit named DCEN (Down Counter Enable) located in the SFR T2MOD. Upon reset, the DCEN bit is set
to 0 so that Timer 2 will default to count up. When DCEN is set, Timer 2 can count up or down depending on the
value of the T2EX pin.
Fig. 7-2 shows Timer 2 automatically counting up when DCEN = 0. In this mode there are two options selected by
bit EXEN2 in T2CON. If EXEN2 = 0, Timer 2 counts up to FFFFh and then sets the TF2 bit upon overflow. The
overflow also causes the timer registers to be reloaded with the 16-bit value in RCAP2H and RCAP2L. The values
in RCAP2H and RCAP2L are preset by software. If EXEN2 = 1, a 16-bit reload can be triggered either by an
overflow or by a 1-to-0 transition at external input T2EX. This transition also sets the EXF2 bit. Both the TF2 and
EXF2 bits can generate an interrupt if enabled.
Setting the DCEN bit enables Timer 2 to count up or down as shown in Fig. 7-3. In this mode the T2EX pin controls
the direction of count. A logic 1 at T2EX makes Timer 2 count up. The timer will overflow at FFFFh and set the TF2
bit. This overflow also causes the 16-bit value in RCAP2H and RCAP2L to be reloaded into the timer registers, TH2
and TL2, respectively.
A logic 0 at T2EX makes Timer 2 count down. Now the timer underflows when TH2 and TL2 are equal to the values
stored in RCAP2H and RCAP2L. The underflow sets the TF2 bit and causes FFFFH to be reloaded into the timer
registers.
The EXF2 bit toggles whenever Timer 2 overflows or underflows. This bit can be used as a 17th bit of resolution if
desired. In this operating mode, EXF2 does not flag an interrupt.
Fosc/12
C/T2=0
TL2
TH2
TF2
(8 BITS) (8 BITS)
C/T2=1
T2 pin
TR2
RELOAD
Timer2 Interrupt
RCAP2L RCAP2H
Transition
Detector
T2 EX pin
EXF2
EXEN2
Timier2 in Auto Reload Mode
(DCEN=0)
Fig. 7-2：Timer 2 in auto reload mode (DCEN=0)
Specifications subject to change without notice contact your sales representatives for the most recent information.
ISSFD-M074 Ver B SM5953 12/27/2013
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SYNCMOS [ SYNCMOS TECHNOLOGIES,INC ]
