Tim output comparison - PWM

Catalog

Preface

One 、 What is output comparison

Two 、 What is? PWM

1. PWM Related parameters of ：

2.PWM Application scenarios of ：

3. Duty cycle

3、 ... and . Several modes of output comparison ：

Four , Polarity of output comparison

5、 ... and , In the output comparison ,TIM Common functions

6、 ... and , Different channel outputs of the same timer PWM Characteristics ：

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Preface

Briefly introduce the output comparison , This part is mainly the code part , The next one is passed PWM Breathing lights and PWM Control DC motor to introduce in detail TIM The output comparison function of

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One 、 What is output comparison

Output comparison （Output Compare）, By comparison CNT And CCR Relationship between register values , To set the output level 1, Set up 0 Or flip operation , Used to output a certain frequency and duty cycle PWM wave form , Every advanced timer and general timer have 4 Two output comparison channels

Be careful ： The first three channels of the advanced timer have additional functions of dead band generation and complementary output

Two 、 What is? PWM

The expression of output comparison lies in PWM, and PWM If you've learned 51 I probably know ,PWM It's a pulse width modulation , It is embodied in the system with inertia , The required analog parameters can be obtained equivalently by modulating the width of some columns of pulses , It can be used to adjust the speed of the motor

1. PWM Related parameters of ：

frequency ：1/T;

Duty cycle ：Ton/Ts

The resolution of the ： Duty cycle change step

2.PWM Application scenarios of ：

PWM The application scenario of must be an inertial system , With LED For example ,LED When it goes out , Due to the persistence of human vision ,LED It won't go out immediately , It has a certain inertia , It will take a short time to go out , For motors , Motor power failure , The rotation of the motor will not stop immediately , It has a certain inertia , It will stop after a while

According to the curve above ,PWM In fact, it can be regarded as the analog quantity represented by the dotted line in the middle

3. Duty cycle

The duty cycle determines PWM The equivalent analog voltage , The smaller the duty cycle , The equivalent analog voltage is closer to the low level , The larger the duty cycle , The equivalent analog voltage is closer to the high level

3、 ... and . Several modes of output comparison ：

Timing Freeze mode

Active Set the effective level when equal

Inacktive Set invalid level when equal

Toggle When equal, the level turns

Four , Polarity of output comparison

TIM_OCPolarity_High   ： High polarity , That is, polarity does not flip ,REF Waveform output directly , Or effective level high level ,REF Is valid , Output high level

TIM_OCPolarity_Low： Low polarity , Namely REF The level is reversed , Or the effective level is low ,

5、 ... and , In the output comparison ,TIM Common functions

four OCinit function , It initializes the output comparison unit with a structure

TIM_CtrlPWMOutputs function , Only use advanced timer , When using advanced timer output PWM You need to call this function , Enable main output , otherwise PWM Will not output normally

middle 1088 To 1100 Some small functions of timer , Below are some functions that change parameters at runtime

among , four TIM_SetCompare Functions are important

 TIM_OCStructInit function , Assign initial value to structure

TIM_SetCompare1 This function is used to change the channel separately 1 Of CCR It's worth it

6、 ... and , Different channel outputs of the same timer PWM Characteristics ：

Because different channels share a counter , So their frequencies must be equal ;

Their duty cycle , By their own CCR decision , So the duty cycle can be set separately ;

Their aspect , Due to counter update , all PWM Simultaneous jump , So their phases are synchronized ;

for example , If you drive multiple steering gears or DC motors , Use a timer for different channels PWM, That's all right.