W78E365 Datasheet, PDF (14/40 Pages) Winbond – 8-BIT MICROCONTROLLER

W78E365
6.7 Pulse Width Modulated Outputs (PWM)
There are five 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 five 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 or PWM4 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
and PWM4. 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 and ENPWM4 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. The repetition frequency fpwm , at the PWM0/1/2/3/4
output is given by:
fpwm
=
2× (1+
fosc
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) 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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