Freescale Semiconductor, Inc.
MC68HC05BD7 Rev. 2.0
GENERAL RELEASE SPECIFICATION
SECTION 8
PULSE WIDTH MODULATION
There are 16 PWM channels. Channel 0 to channel 7 are dedicated PWM channels with
5V open-drain option. Channel 8 to channel 15 are shared with ports C under the control
of the corresponding configuration register. The channel 8 and channel 9 are 12V open-
drain outputs.
8.1
Operation of 8-Bit PWM
Each 8-Bit PWM channel is composed of an 8-bit register which contains a 5-bit PWM in
MSB portion and a 3-bit binary rate multiplier (BRM) in LSB portion. There are 16 data
registers as shown in Figure 8-1 located from $20 to $2F. The value programmed in the 5-
bit PWM portion will determine the pulse length of the output. The clock to the 5-bit PWM
portion is the MCU clock and the repetition rate of the output is hence 62.5 KHz at 2 MHz
MCU clock.
The 3-bit BRM will generate a number of narrow pulses which are equally distributed
among an 8-PWM-cycle frame. The number of pulses generated is equal to the number
programmed in the 3-bit BRM portion. An example of the waveform is shown in Figure 8-2.
Combining the 5-bit PWM together with the 3-bit BRM, the average duty cycle at the output
will be (M+N/8)/32, where M is the content of the 5-bit PWM portion, and N is the content
of the 3-bit BRM portion. Using this mechanism, a true 8-bit resolution PWM type DAC with
reasonably high repetition rate can be obtained.
7
0PWM4
0
6
0PWM3
0
5
0PWM2
0
4
0PWM1
0
3
0PWM0
0
2
0BRM2
0
1
0BRM1
0
0
0BRM0
0
R
PWMR
$20-$2F
W
reset
Figure 8-1: PWM Data Register
The value of each PWM Data Register is continuously compared with the content of an
internal counter to determine the state of each PWM channel output pin. Double buffering
is not used in this PWM design.
SECTION 8: PULSE WIDTH MODULATION
Page 37
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