W79E548 Datasheet, PDF (47/79 Pages) Winbond – 8-BIT MICROCONTROLLER

W79E548/W79L548
11.3 Pulse Width Modulated Outputs (PWM)
There are six pulse width modulated output channels to generate pulses of programmable length and
interval. The repetition frequency is defined by an 8-bit prescaler PWMP, which supplies the clock for
the counter. The prescaler and counter are common to both PWM channels. The 8-bit counter counts
modular 255 (0 ~ 254). The value of the 8-bit counter compared to the contents of six registers:
PWM0, PWM1, PWM2, PWM3 and PWM4. Provided the contents of either these registers is greater
than the counter value, the corresponding PWM0, PWM1, PWM2, PWM3, PWM4 or PWM5 output is
set HIGH. If the contents of these registers are equal to, or less than the counter value, the output will
be LOW. The pulse-width-ratio is defined by the contents of the registers PWM0, PWM1, PWM2,
PWM3, PWM4 and PWM5. The pulse-width-ratio is in the range of 0 to 1 and may be programmed in
increments of 1/255. ENPWM0, ENPWM1, ENPWM2, ENPWM3, ENPWM4 and ENPWM5 bit will
enable or disable PWM output.
Buffered PWM outputs may be used to drive DC motors. The rotation speed of the motor would be
proportional to the contents of PWM0/1/2/3/4/5. The repetition frequency fpwm , at the
PWM0/1/2/3/4/5 output is given by:
fpwm =
fosc
2× (1+ PWMP) × 255
Prescaler division factor = PWM + 1
PWMn high/low ratio of PWMn = ^(PWMn)
255 - (PWMn)
This gives a repetition frequency range of 123 Hz to 31.4K Hz ( fosc = 16M Hz). By loading the PWM
registers with either 00H or FFH, the PWM channels will output a constant HIGH or LOW level,
respectively. Since the 8-bit counter counts modulo 255, it can never actually reach the value of the
PWM registers when they are loaded with FFH.
When a compare register (PWM0, PWM1, PWM2, PWM3, PWM4, PWM5) is loaded with a new value,
the associated output updated immediately. It does not have to wait until the end of the current
counter period. There is weakly pulled high on PWM output.
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Publication Release Date: December 12, 2005
Revision A1