ST72774 datasheet(116/144 Pages) STMICROELECTRONICS

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ST72774/ST727754/ST72734

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4.9 PWM/BRM GENERATOR (DAC)

4.9.1 Introduction

This PWM/BRM peripheral includes two types of

PWM/BRM outputs, with differing step resolutions

based on the Pulse Width Modulator (PWM) and

Binary Rate Multiplier (BRM) Generator technique

are available. It allows the digital to analog

conversion

(DAC)

when

used

with

external

filtering.

4.9.2 Main Features

Fixed frequency: fCPU/64

Resolution: T

CPU

10-Bit PWM/BRM generator with a step of

DD/2

10 (5mV if V

DD=5V)

4.9.3 Functional Description

4.9.3.1 PWM/BRM

The 10 bits of the 10-bit PWM/BRM are distributed

as 6 PWM bits and 4 BRM bits. The generator

consists of a 12-bit counter (common for all

channels), a comparator and the PWM/BRM

generation logic.

PWM Generation

The counter increments continuously, clocked at

internal

CPU

clock.

Whenever

the

least

significant bits of the counter (defined as the PWM

counter) overflow, the output level for all active

channels is set.

The state of the PWM counter is continuously

compared to the PWM binary weight for each

channel, as defined in the relevant PWM register,

and when a match occurs the output level for that

channel is reset.

This Pulse Width modulated signal must be

filtered, using an external RC network placed as

close as possible to the associated pin. This

provides an analog voltage proportional to the

average charge passed to the external capacitor.

Thus for a higher mark/space ratio (High time

much greater than Low time) the average output

voltage is higher. The external components of the

RC network should be selected for the filtering

level required for control of the system variable.

Each output may individually have its polarity

inverted by software, and can also be used as a

logical output.

Figure 69. PWM Generation

COUNTER

COMPARE

VALUE

OVERFLOW

000

PWM OUTPUT

CPU x 64

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