Datasheet

C515C
Data Sheet 36 2003-02
Timer 2 Operating Modes
The timer 2, which is a 16-bit-wide register, can operate as timer, event counter, or gated
timer. A roll-over of the count value in TL2/TH2 from all 1’s to all 0’s sets the timer
overflow flag TF2 in SFR IRCON, which can generate an interrupt. The bits in register
T2CON are used to control the timer 2 operation.
Timer Mode: In timer function, the count rate is derived from the oscillator frequency. A
prescaler offers the possibility of selecting a count rate of 1/6 or 1/12 of the oscillator
frequency.
Gated Timer Mode: In gated timer function, the external input pin T2 (P1.7) functions as
a gate to the input of timer 2. If T2 is high, the internal clock input is gated to the timer.
T2 = 0 stops the counting procedure. This facilitates pulse width measurements. The
external gate signal is sampled once every machine cycle.
Event Counter Mode: In the event counter function. the timer 2 is incremented in
response to a 1-to-0 transition at its corresponding external input pin T2 (P1.7). In this
function, the external input is sampled every machine cycle. Since it takes two machine
cycles (12 oscillator periods) to recognize a 1-to-0 transition, the maximum count rate is
1/12 of the oscillator frequency. There are no restrictions on the duty cycle of the external
input signal, but to ensure that a given level is sampled at least once before it changes,
it must be held for at least one full machine cycle.
Reload of Timer 2: Two reload modes are selectable:
In mode 0, when timer 2 rolls over from all 1’s to all 0’s, it not only sets TF2 but also
causes the timer 2 registers to be loaded with the 16-bit value in the CRC register, which
is preset by software.
In mode 1, a 16-bit reload from the CRC register is caused by a negative transition at the
corresponding input pin P1.5/T2EX. This transition will also set flag EXF2 if bit EXEN2
in SFR IEN1 has been set.