C515 Datasheet, PDF (27/55 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C515
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 1/12 or 1/24 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. lf 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 (24 oscillator periods) to recognize
a 1-to-0 transition, the maximum count rate is 1/24 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 correspon-
ding input pin P1.5/T2EX. This transition will also set flag EXF2 if bit EXEN2 in SFR IEN1 has been
set.
Semiconductor Group
27
1997-08-01