SAB80C517-M16 SIEMENS [Siemens Semiconductor Group], SAB80C517-M16 Datasheet - Page 104

• Manufacturer Part Number: SAB80C517-M16
• Description: 8-Bit CMOS Single-Chip Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

On-Chip Peripheral Components
Modulation Range of a PWM Signal and Differences between the Two Timer/Compare
Register Configurations in the CCU
There are two timer/compare register configurations in the CCU which can operate in compare
mode 0 (either timer 2 with a CCx (CRC and CC1 to CC4) register or the compare timer with a CMx
register). They basically operate in the same way, but show some differences concerning their
modulation range when used for PWM.
Generally it can be said that for every PWM generation with n-bit wide compare registers there are
n
2
different settings for the duty cycle. Starting with a constant low level (0% duty cycle) as the first
setting, the maximum possible duty cycle then would be
n
(1 – 1/2
) x 100 %
This means that a variation of the duty cycle from 0% to real 100% can never be reached if the
compare register and timer register have the same length. There is always a spike which is as long
as the timer clock period.
In the SAB 80C517 there are two different modulation ranges for the above mentioned two timer/
compare register combinations. The difference is the location of the above spike within the timer
period: at the end of a timer period or at the beginning plus the end of a timer period. Please refer
to the description of the relevant timer/register combination in section 7.5.5.1 or 7.5.5.2 for details.
Figure 7-39
Function of Compare Mode 0
Semiconductor Group
105