C509-L_97 SIEMENS [Siemens Semiconductor Group], C509-L_97 Datasheet - Page 147

• Manufacturer Part Number: C509-L_97
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

6.3.5
In compare mode 0, a 100% variation of the duty cycle of a PWM signal cannot be reached. A time
portion of 1/(2
compare register is set to the reload value (limiting the lower end of the modulation range) or it may
occur at the end of a timer period.
In a timer 2 / CCx register configuration in compare mode 0 this spike is divided into two halves:
one at the beginning when the contents of the compare register is equal to the reload value of the
timer; the other half when the compare register is equal to the maximum value of the timer register
(here: FFFF H ). Please refer to figure 6-35 where the maximum and minimum duty cycle of a
compare output signal is illustrated. Timer 2 is incremented with the processor cycle (
at 12-MHz operating frequency, these spikes are both approx. 250 ns long.
Figure 6-35
Modulation Range of a PMW Signal Generated with a Timer 2 / CCx Register Combination in
Compare Mode 0
The following example shows how to calculate the modulation range for a PWM signal. For the
calculation with reasonable numbers, a reduction of the resolution to 8-bit is used. Otherwise (for
the maximum resolution of 16-bit) the modulation range would be so severely limited that it would
be negligible.
Example:
Timer 2 in auto-reload mode; contents of reload register CRC = FF00 H
Restriction of modulation. range =
This leads to a variation of the duty cycle from 0.195% to 99.805% for a timer 2 / CCx register
configuration when 8 of 16 bits are used.
Semiconductor Group
Modulation Range in Compare Mode 0
n
) of an n-bit timer period is always left over. This "spike" may either appear when the
P1.x
P1.x
CCHx/CCLx = 0000 or = CRCH/CRCL (maximum duty cycle)
CCHx/CCLx = FFFF (minimum duty cycle)
256 x 2
H
H
1
6-69
Appr. 1/2 of a Machine Cycle
Appr. 1/2 of a Machine Cycle
x 100% = 0.195%
On-Chip Peripheral Components
MCT01851
H
L
H
L
f
1997-10-01
OSC
C509-L
/6), thus