We are switching from STM32 to the Russian K1986BE92QI microcontroller. Practical application: control LED brightness (PWM)

Introduction

In the two previous articles, we generated a clock signal of the frequency we need with the help of PWM, receiving equal intervals of luminescence and its absence on the LED. This task takes place in practice (in one of the subsequent articles we will definitely encounter it). But most often PWM is used for another purpose. One of the most common - control the brightness of the LEDs or the speed of rotation of the motors. Also with the help of PWM you can generate sound (what the next article will be about). And in this article I would like to tell you how you can control the brightness of the LED on our controller.

Let's start: changing the settings of the main timer

We will take the project from this article as a basis. Based on the initTimerPWMled function, we will create the initTimerPWMconstPeriod function. The parameter of the function will not be PWM_speed (PWM frequency), but timeEnabledState will be the duration of the pulse. The theory of voltage generation at the output by the PWM method is perfectly described by this short article. To begin with, we will define the parameters:

• Let the duration of the whole period of the function be 0xFFFF ticks of the timer (you can choose any value, I chose this value for convenience and not to change the code for the next article on generating PWM sound)
• Then the parameter of the timeEnabledState function will show how many clock cycles from 0xFFFF on the output is "1". All the rest of the time on channel "0". For example, if timeEnabledState = 0x4000, the signal will look like this.
  
  
  ')
  At 0x8000 will be almost equal.
  
  
  
  Well, at 0xF000, the signal "1" will almost always be.
  
  

The shorter the period, the better. We have chosen a period of 0xFFFF cycles. To achieve good results, we need to maximize the frequency of these periods. To do this, turn off the clock divider in the initTimerPWMconstPeriod function.

MDR_TIMER1->PSG = 0; //    . 

Next you need to change the way the signal is generated. As we remember from the previous article, the OCCM bit group of the CH1_CNTRL register is responsible for this. Then we selected the invert mode of the REF signal with CNT == CCR1. Since CCR was = 0 by default, by adjusting the ARR of the main counter, we got the same effect. Now we are recording the number of ticks of the whole period in ARR (0xFFFF), and in CRR we will write the number of unit ticks at the output (pulse duration). And the mode will be 6 (0b110: 1, if DIR = 0 (up), CNT <CCR, otherwise 0;).

 MDR_TIMER1->ARR = 0xFFFF; //  .        0  0xFFFF. MDR_TIMER1->CCR1 = timeEnabledState; //    1     0 - . MDR_TIMER1->CH1_CNTRL = 6<<TIMER_CH_CNTRL_OCCM_Pos; // REF = 1,  CNT < CCR1, 0 - CNT >= CCR1; 

In fact, at this stage we have already obtained the function that sets the PWM signal's duty cycle. But you need to take care that when changing the value of CCR1 the counter does not go beyond the limits. To do this, set the CRRRLD bit in the CH1_CNTRL2 register. It allows changing the value of CCR1 and CCR2 only with CNT == 0.

 MDR_TIMER1->CH1_CNTRL2 = TIMER_CH_CNTRL2_CCRRLD; // CCR1    CNT = 0.    . 

Finish: change interrupt settings

With the PWM setting, we figured it out. It remains only to slightly change the interrupt by polling keys. Let us set a new variable characterizing the pulse duration in the period. Let's call it PWM_time . By default, let it be 0xFFFF (PWM is not at this value, the LED is on). And then we'll see at what interval to take a step when pressing the key. To do this, we decompose the number of cycles of the period into prime factors.

 // 65535 = 3·5·17·257. 257        0 -    257 -   . =>  3*5*17 = 255. 

From the calculation we see that walking with an interval of 255, we can go through the whole scale from 0 to 257. So we proceed. We get the following code.

 // 65535 = 3·5·17·257. 257        0 -    257 -   . =>  3*5*17 = 255. if (UP_FLAG == 0) PWM_time+=255; // ,   - .   - -   . else if (DOWN_FLAG == 0) PWM_time-=255; else if (LEFT_FLAG == 0) PWM_time-=255; else if (RIGHT_FLAG == 0) PWM_time+=255; if (PWM_time < 0) PWM_time = 0; // ,   ""        . else if (PWM_time > 0xFFFF) PWM_time = 0xFFFF; 

It remains only to assign a new value.

 MDR_TIMER1->CCR1 = PWM_time; //  . 

I will note once again that we don’t touch the period. He is permanent. We change only the pulse duration. The whole interruption will be as follows.

 int PWM_time = 0xFFFF; //    . void Timer2_IRQHandler (void) { MDR_TIMER2->STATUS = 0; //  .   . //LED1_FLAG = !LED1_FLAG; // ,    . // 65535 = 3·5·17·257. 257        0 -    257 -   . =>  3*5*17 = 255. if (UP_FLAG == 0) PWM_time+=255; // ,   - .   - -   . else if (DOWN_FLAG == 0) PWM_time-=255; else if (LEFT_FLAG == 0) PWM_time-=255; else if (RIGHT_FLAG == 0) PWM_time+=255; if (PWM_time < 0) PWM_time = 0; // ,   ""        . else if (PWM_time > 0xFFFF) PWM_time = 0xFFFF; MDR_TIMER1->CCR1 = PWM_time; //  . } 

I deliberately hid an LED inversion string indicating that an interrupt is occurring. Because there is not enough current for 2 LEDs and unwanted ripples appear.

Conclusion

In this article, we looked at how you can configure and control PWM. In the next article, we will try to generate a sound using PWM.

Project files to the article.