mc68hc05bs8 Freescale Semiconductor, Inc, mc68hc05bs8 Datasheet - Page 57

• Manufacturer Part Number: mc68hc05bs8
• Description: Mc68hc05 Family Of Low-cost Single-chip Microcontrollers.
• Manufacturer: Freescale Semiconductor, Inc

7
PULSE WIDTH MODULATION
The MC68HC05BS8 has two independent PWMs; one general purpose (GPWM) and one raster
positioning (RSPWM). They are controlled by the PWM registers located at $0010 and $0011.
7.1
General Purpose Pulse Width Modulator
The GPWM consists of a comparator and a 6-bit free-running counter driven by the internal bus
7
clock. The counter runs from $00 to $3E and rolls over back to $00. Whenever the GPWM value
in the GPWM register is greater than the running counter, the output of the GPWM pin will be
“high” (See Figure 7-1).
GPWM data = 16 = 01000
8 s
Internal clock = 2MHz
GPWM
23.5 s
Frame rate=32KHz
31.5 s
Figure 7-1 GPWM Timing Example
To reduce the occurrence of fast logic switching edges on the PCB, the GPWM output is
connected to the output pin via an on chip resistor of nominal value of 5K . In this way, for use
with DC controls the RC filter capacitor is connected externally directly to the output. A further
advantage is that the PWM logic signal has only to overcome modest on-chip capacitances and
thus supply current spikes are significantly reduced.
The GPWM output can be configured as an open drain output by setting ODE bit in the GPWM
register. It should be noted that the diode associated with the P-channel device is still connected
to the output pin and therefore the normal voltage limitations apply, i.e. up to V
. Figure 7-2
DD
shows a schematic of the GPWM output.
TPG
MC68HC05BS8
PULSE WIDTH MODULATION