Exploring STC 8051 Microcontrollers – Coding

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Using PCA to Extend External Interrupt Capability

An alternative way of using the hardware PCA module is to use it to extend external interrupt capability of our STC micro. 

Code

 #include "STC8xxx.h"
 #include "BSP.h"
  
 void setup(void);
  
 void PCA_ISR(void)          
 interrupt 7
 {
   if(check_PCA_0_flag)
   {
     clear_PCA_0_flag;
     P54_low;
   }
   if(check_PCA_1_flag)
   {
     clear_PCA_1_flag;
     P54_high;
   }  
 }
  
 void main(void)
 {
  
   setup();
   
   while(1)
   {
     P55_toggle;
     delay_ms(400);
   };
 }
  
 void setup(void)
 {
   CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
  
   P54_push_pull_mode;
   P55_open_drain_mode;
   
   PCA_pin_option(0x00);
   
   PCA_setup(PCA_continue_counting_in_idle_mode, PCA_clk_sys_clk_div_1);
   
   PCA_0_mode(PCA_16_bit_falling_edge_capture);
   PCA_1_mode(PCA_16_bit_falling_edge_capture);
     
   PCA_load_counter(0);
   
   _enable_PCA_0_interrupt;
   _enable_PCA_1_interrupt;
   _enable_global_interrupt;
   
   PCA_start_counter;
 }

Schematic

Explanation

Perhaps this is the one and only PCA module example that doesn’t require precise selection of timing parameters and so we will directly go to PCA settings. In this example, a LED connected to pin P5.4 is either turned on or off using two buttons. These buttons are attached to default PCA channel 0 and 1 pins.

 PCA_pin_option(0x00);
   
 PCA_setup(PCA_continue_counting_in_idle_mode, PCA_clk_sys_clk_div_1);
   
 PCA_0_mode(PCA_16_bit_falling_edge_capture);
 PCA_1_mode(PCA_16_bit_falling_edge_capture);
     
 PCA_load_counter(0);
   
 _enable_PCA_0_interrupt;
 _enable_PCA_1_interrupt;
 _enable_global_interrupt;
   
 PCA_start_counter;

Both PCA channel input pins are set to capture falling edges. It is not mandatory to use falling edge capture. In this case however, I used so because the buttons have external pullup resistors and so pressing them would lead to a high-to-low pin transition or falling edge.

The whole PCA setup seem to look like PCA capture mode setup but the exception is the fact that we won’t be needing the capture counts this time.

The state of the P5.4 LED is altered inside PCA interrupt subroutine and everything will appear as if the things are being done with external interrupts. Note that though this external interrupt technique works just like ordinary external interrupts, there is no way of setting interrupt priorities amongst PCA channels themselves.

 void PCA_ISR(void)          
 interrupt 7
 {
   if(check_PCA_0_flag)
   {
     clear_PCA_0_flag;
     P54_low;
   }
   if(check_PCA_1_flag)
   {
     clear_PCA_1_flag;
     P54_high;
   }  
 }

The only task that is done in the main loop is a simple onboard LED blinking. This is not a must but this is only needed to show that PCA operations are done without blocking anything of the main loop.

 P55_toggle;
 delay_ms(400);

Demo

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