C511_1 Datasheet, PDF (48/128 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Units
6.2 Timers/Counters
The SAB-C511/513 microcontrollers contains two (SAB-C511/C511A) or three (SAB-C513/C513A/
C513A-H) 16-bit timers/counters which are useful in many applications for timing and counting
functions.
In timer function, the register is incremented every machine cycle. Thus one can think of it as
counting machine cycles. Since a machine cycle consists of 12 oscillator periods, the counter rate
is 1/12 of the oscillator frequency.
In counter function, the register is incremented in response to a 1-to-0 transition (falling edge) at
its corresponding external input pin, T0 or T1 (alternate functions of P3.4 and P3.5, resp.). In this
function the external input is sampled during S5P2 of every machine cycle. When the samples show
a high in one cycle and a low in the next cycle, the counter is incremented. The new count value
appears in the register during S3P1 of the cycle following the one in which the transition was
detected. 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.
Semiconductor Group
6-15