ST & T UPSD3212C - Manual (Page 67)

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uPSD3212A, uPSD3212C, uPSD3212CV
PULSE WIDTH MODULATION (PWM)
The PWM block has the following features:
■ Four-channel, 8-bit PWM unit with 16-bit
prescaler
■ One-channel, 8-bit unit with programmable
frequency and pulse width
■ PWM Output with programmable polarity
4-channel PWM Unit (PWM 0-3)
The 8-bit counter of a PWM counts module 256
(i.e., from 0 to 255, inclusive). The value held in
the 8-bit counter is compared to the contents of the
Special Function Register (PWM 0-3) of the corre-
sponding PWM. The polarity of the PWM outputs
is programmable and selected by the PWML Bit in
PWMCON register. Provided the contents of a
PWM 0-3 register is greater than the counter val-
ue, the corresponding PWM output is set HIGH
(with PWML = 0). When the contents of this regis-
ter is less than or equal to the counter value, the
corresponding PWM output is set LOW (with
PWML = 0). The pulse-width-ratio is therefore de-
fined by the contents of the corresponding Special
Function Register (PWM 0-3) of a PWM. By load-
ing the corresponding Special Function Register
(PWM 0-3) with either 00H or FFH, the PWM out-
put can be retained at a constant HIGH or LOW
level respectively (with PWML = 0).
For each PWM unit, there is a 16-bit Prescaler that
are used to divide the main system clock to form
the input clock for the corresponding PWM unit.
This prescaler is used to define the desired repeti-
tion rate for the PWM unit. SFR registers B1h -
B2h are used to hold the 16-bit divisor values.
The repetition frequency of the PWM output is giv-
en by:
fPWM
8
= (f
OSC
/ prescaler0) / (2 x 256)
And the input clock frequency to the PWM
counters is = f
OSC
/ 2 / (prescaler data value + 1)
See I/O PORTS (MCU Module), page 40 for more
information on how to configure the Port 4 pin as
PWM output.