UPSD323X datasheet(76/176 Pages) STMICROELECTRONICS

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µPSD323X

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PWM 4 Channel Operation

The 16-bit Prescaler1 divides the input clock

(fOSC/2) to the desired frequency, the resulting

clock runs the 8-bit Counter of the PWM 4 chan-

nel. The input clock frequency to the PWM 4

Counter is:

f PWM4 = (fOSC/2)/(Prescaler1 data value +1)

When the Prescaler1 Register (B4h, B3h) is set to

data value ’0,’ the maximum input clock frequency

to the PWM 4 Counter is fOSC/2 and can be as high

as 20MHz.

The PWM 4 Counter is a free-running, 8-bit

counter. The output of the counter is compared to

the Compare Registers, which are loaded with

data from the Pulse Width Register (PWM4W,

ABh) and the Period Register (PWM4P, AAh). The

Pulse Width Register defines the pulse duration or

the Pulse Width, while the Period Register defines

the period of the PWM. When the PWM 4 channel

is enabled, the register values are loaded into the

Comparator Registers and are compared to the

Counter output. When the content of the counter is

equal to or greater than the value in the Pulse

Width Register, it sets the PWM 4 output to low

(with PWMP Bit = 0). When the Period Register

equals to the PWM4 Counter, the Counter is

cleared, and the PWM 4 channel output is set to

logic ’high’ level (beginning of the next PWM

pulse).

The Period Register cannot have a value of “00”

and its content should always be greater than the

Pulse Width Register.

The Prescaler1 Register, Pulse Width Register,

and Period Register can be modified while the

PWM 4 channel is active. The values of these reg-

isters are automatically loaded into the Prescaler

Counter and Comparator Registers when the cur-

rent PWM 4 period ends.

The PWMCON Register (Bits 5 and 6) controls the

enable/disable and polarity of the PWM 4 channel.

Figure 38. PWM 4 With Programmable Pulse Width and Frequency

AI07090

PWM4

Defined by Pulse

Width Register

Switch Level

RESET

Counter

Defined by Period Register

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