Embedded Lab

Using PCA to Measure Frequency

Frequency measurement is often a challenging task as it requires good understanding of timers and input capture modules. Frequency measurement becomes very important while designing power electronics devices like generator controllers, inverters, power line measurement meters, etc. In STC8A8K64S4A12, there is no dedicated input capture hardware module but there is a PCA module that can be used to replenish this absence of a dedicated input capture peripheral.

Code

 #include "STC8xxx.h"
 #include "BSP.h"
 #include "LCD.c"
 #include "lcd_print.c"
  
 unsigned int first_edge = 0x0000;
 unsigned int second_edge = 0x0000;
  
 unsigned long clks = 0x00000000;
 unsigned long ov_cnt = 0x00000000;
  
 void setup(void);
  
 void PCA_ISR(void)          
 interrupt 7
 {
     if(check_PCA_Counter_overflow_flag)
     {
       ov_cnt++;;
       clear_PCA_Counter_overflow_flag;
     }
     
     if(check_PCA_0_flag)
     {
       second_edge = PCA_get_CCAP0();
       clks = ((65536 * ov_cnt) + second_edge - first_edge);
       ov_cnt = 0;
       first_edge = second_edge;
       second_edge = 0;
       clear_PCA_0_flag;
     }
 }
  
 void main(void)
 {
   float f = 0.0;
   
   unsigned char s = 0;
   
   setup();
  
   LCD_goto(0, 0);
   LCD_putstr("Period/ms:");
   
   LCD_goto(0, 1);
   LCD_putstr("Freq./kHz:");
  
   while(1)
   {
     PWM_stop_counter;
  
     switch(s)
     {
       case 1:
       {
         PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_1);  
         break;
       }
       
       case 2:
       {
         PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_4);      
         break;
       }
       
       case 3:
       {
         PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_3);  
         break;
       }      
       
       default:
       {
         PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_2);  
         break;
       }
 }
  
     PWM_start_counter;
     delay_ms(1000);
     
     f = (12000.0 / ((float)clks));
     print_I(11, 0, clks);  
     print_F(11, 1, f, 1);
     
     delay_ms(1000);
     
     s++;
     if(s > 3)
     {
       s = 0;
     }
   };
 }
  
 void setup(void)
 {
   CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
   
   PWM0_setup(PWM_pin_is_PWM_output, \
                    PWM_init_lvl_low, \
                    PWM_0_pin_P10, \
                    PWM_level_normal);
   
   PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_2);  
   PWM_set_counter(500);  
         
   PWM_set_PWM0_T1(0);
   PWM_set_PWM0_T2(200); 
   
   PWM_start_counter;
  
   PCA_pin_option(0x00);
   
   PCA_setup(PCA_continue_counting_in_idle_mode, PCA_clk_sys_clk_div_1);
   PCA_load_counter(0x0000);
   
   PCA_0_mode(PCA_16_bit_falling_edge_capture);
   
   _enable_PCA_0_interrupt;
   _enable_PCA_counter_interrupt;
   _enable_global_interrupt;  
   
   PCA_start_counter;
   
   LCD_init();
   LCD_clear_home();
 } 

Schematic

Explanation

Measurement of frequency can easily be done by timing the time difference between two successive rising or falling edge captures. Input capture hardware along with a timer would scan and sample incoming waveform. Thus, input capture hardware must have at least twice the sampling frequency than the frequency of the waveform coming to the capture input. This is simply the Nyquist criterion. 

To demonstrate frequency measurement with PCA, I have used two hardware. Firstly, I used a PWM generator to generate waveform of different frequencies and secondly, a PCA module to capture the PWM waveform.

To begin with, note that the micro is running at 12MHz.

 CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);   

The PWM peripheral is setup with P1.0 pin as PWM channel 0. The initial frequency of the PWM is 12kHz and the duty cycle is about 60%.

 PWM0_setup(PWM_pin_is_PWM_output, \
            PWM_init_lvl_low, \
            PWM_0_pin_P10, \
            PWM_level_normal);
   
 PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_2);  
 PWM_set_counter(500);  
         
 PWM_set_PWM0_T1(0);
 PWM_set_PWM0_T2(300); 
   
 PWM_start_counter; 

The following equations show us how we fabricated this frequency and duty cycle:

In the main, the PWM frequency if changed four times and these frequencies are 24kHz, 12kHz, 8kHz and 6kHz. The change is done about every two seconds.

Now let us see how we have to setup the PCA’s capture part. P1.7 pin (PCA0) is setup as the capture pin that would look for falling edges. PCA hardware clock is running at full system clock speed of 12MHz. The Nyquist criterion is fulfilled because PWM out frequency is not more than 24kHz.

 PCA_pin_option(0x00);
  
 PCA_setup(PCA_continue_counting_in_idle_mode, PCA_clk_sys_clk_div_1);
 PCA_load_counter(0x0000);
   
 PCA_0_mode(PCA_16_bit_falling_edge_capture);
   
 _enable_PCA_0_interrupt;
 _enable_PCA_counter_interrupt;
 _enable_global_interrupt;  
  
 PCA_start_counter; 

PCA interrupt will be triggered when a falling edge is detected or when there is PCA counter overflows. Both of these events will be needed. Though under a same interrupt subroutine, these events are differentiated by two separate flags. The first will be needed to make sure that we took account of any PCA overflow in between two successive captures and the second will be needed to snap PCA counter count at the moment of falling-edge detection.

 void PCA_ISR(void)          
 interrupt 7
 {
     if(check_PCA_Counter_overflow_flag)
     {
       ov_cnt++;;
       clear_PCA_Counter_overflow_flag;
     }
     
     if(check_PCA_0_flag)
     {
       second_edge = PCA_get_CCAP0();
       clks = ((65536 * ov_cnt) + second_edge - first_edge);
       ov_cnt = 0;
       first_edge = second_edge;
       second_edge = 0;
       clear_PCA_0_flag;
     }
 } 

When PCA interrupt occurs, two flags are checked. Firstly, the PCA counter overflow is checked and secondly, PCA0 interrupt flag is checked. If PCA counter overflow occurs then it is saved in a variable. This will mark that an overflow event occurred during a snap.

Since frequency measurement is done by measuring the PCA counter’s count difference between two consecutive falling edges, we need to assume that the first capture is basically the second capture instead of the first falling edge capture. When the second capture is recorded, it is measured against the first one. In this measurement, we also have to take note if any PCA overflow occurred during the captures. Note that no timer has been used so far and that is because the PCA module is doubling as a timer here.

In the main, the PWM frequencies are changed periodically and the PWM frequencies are measured. The measured frequency is displayed on an LCD.

 PWM_stop_counter;
  
 switch(s)
 {
   case 1:
   {
     PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_1);  
     break;
   }
       
   case 2:
   {
     PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_4);      
     break;
   }
       
   case 3:
   {
     PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_3);  
     break;
   }      
       
 default:
   {
     PWM_clk_set(PWM_clk_sys_PS, PWM_clk_ps_sys_clk_div_2);  
     break;
   }
 }
  
 PWM_start_counter;
 delay_ms(1000);
     
 f = (12000.0 / ((float)clks));
 print_I(11, 0, clks);  
 print_F(11, 1, f, 1);
     
 delay_ms(1000);
     
 s++;
 if(s > 3)
 {
   s = 0;
 } 

Variable clks represents the count difference between capture snaps and so dividing PCA clock frequency with this count gives capture waveform frequency in hertz. I have coded 12000 instead of 12000000 as clock frequency because I want measurements to be in kHz instead of Hz.  

Demo

Pages: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35