Embedded Lab

Using Timer 2 to Multiplex Seven Segment Displays

Rather than polling, a better way to use timers is to use their interrupts. Timer overflow interrupt is particularly useful for timing event accurately. Timer overflow interrupt will periodically interrupt main tasks and can perform certain things inside the interrupt and then get back to main tasks. In this way, several tasks will apparently seem to occur concurrently. Multiplexing seven segment displays is one such example. These displays need periodic scanning, multiplexing and updating. If these tasks are done swiftly and in short duration, human eyes can be tricked. If a CPU is occupied doing these tasks alone then it would not be able to do other tasks. If the tasks of seven segment displays are put inside a timer interrupt, the CPU can do other tasks in other interrupts and main code as seven segment displays need only periodic attention and not all code processing is spent on the displays alone.

Code

 #include "STC8xxx.h"
 #include "BSP.h"
  
 #define DAT_pin_HIGH                    P41_high
 #define DAT_pin_LOW                     P41_low
  
 #define CLK_pin_HIGH                    P42_high
 #define CLK_pin_LOW                     P42_low
  
 #define refresh_time_in_us              315
 #define max_tmr_cnt                     0xFFFF
 #define tmr_val                         (max_tmr_cnt - refresh_time_in_us)
 
 const unsigned char pos[0x08] =
 {
     0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80
 };
  
 const unsigned char num[0x0A] =
 {
     0x03, 0x9F, 0x25, 0x0D, 0x99, 0x49, 0x41, 0x1F, 0x01, 0x09
 };
  
 unsigned char i = 0x00;
  
 signed int value_1 = 0;
 signed int value_2 = 0;
  
 void setup(void);
 void Write_74HC164(unsigned char val, unsigned char seg);
  
 void ADC_ISR(void)          
 interrupt 5
 {
   value_1 = ((ADC_RES << 8) | ADC_RESL);
   value_2 = ((value_1 * 5000.0) / 4096.0);
   clear_ADC_flag;
 }
  
 void TMR_2_ISR(void)        
 interrupt 12
 {
     switch(i)
     {
         case 0:
         {
             Write_74HC164(num[(value_1 / 1000)], pos[3]);
             break;
         }
  
         case 1:
         {
             Write_74HC164((num[((value_1 % 1000) / 100)]), pos[2]);
             break;
         }
  
         case 2:
         {
             Write_74HC164(num[((value_1 % 100) / 10)], pos[1]);
             break;
         }
  
         case 3:
         {
             Write_74HC164(num[(value_1 % 10)], pos[0]);
             break;
         }
  
         case 4:
         {
             Write_74HC164(num[(value_2 / 1000)] & 0xFE, pos[7]);
             break;
         }
  
         case 5:
         {
             Write_74HC164((num[((value_2 % 1000) / 100)]), pos[6]);
             break;
         }
  
         case 6:
         {
             Write_74HC164(num[((value_2 % 100) / 10)], pos[5]);
             break;
         }
  
         case 7:
         {
             Write_74HC164(num[(value_2 % 10)], pos[4]);
             break;
         }
     }
  
     i++;
  
     if(i >= 8)
     {
         i = 0;
     }
     clear_TMR_2_overflow_flag;
 }  
  
 void main(void)
 {    
   setup();
     
     while(1)
     {
         ADC_set_channel(CH15);
         delay_ms(10);
         ADC_start_conversion;
         delay_ms(400);
     };
 }
  
 void setup(void)
 {
   CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
   
   P41_push_pull_mode;
   P42_push_pull_mode;
   
   ADC_enable;
   ADC_result_format_right_aligned;
   ADC_set_conversion_speed(ADC_conv_128_CLKs);
   _enable_ADC_interrupt;
   
   TMR2_setup(TMR2_sysclk, TMR2_clk_prescalar_12T);
   TMR2_load_counter_16(tmr_val);
   TMR2_start;
   _enable_TMR_2_interrupt;
   _enable_global_interrupt;
 }
 void Write_74HC164(unsigned char val, unsigned char seg)
 {
     unsigned char s = 0x10;
     unsigned int temp = 0x0000;
  
     temp = (unsigned int)seg;
     temp <<= 8;
     temp |= (unsigned int)val;
  
     while(s > 0)
     {
         if((temp & 0x0001) != 0x0000)
         {
             DAT_pin_HIGH;
         }
         else
         {
             DAT_pin_LOW;
         }
  
         CLK_pin_HIGH;
         temp >>= 1;
  
         s--;        
         CLK_pin_LOW;
     }
 } 

Schematic

Explanation

For demonstrating timer interrupt and seven segment displays, I used a simple dual 4-digit seven segment displays. It is made with a set of 74HC164 Serial-In-Parallel-Out (SPIO) shift register. In order to use it in real-time, its displays need to be scanned and updated at a fast rate without affecting other tasks.

This demo is a very important one as here multiple interrupts are employed and literally there is nothing inside the main loop. Things are done in such a way as if everything is parallelly done and in real-time.   Firstly, the system clock and GPIOs are set. The system clock is set to 12 MHz. P4.1 and P4.2 are GPIOs that are used for driving the display. These drive data and clock lines of the display respectively. 

 CLK_set_sys_clk(IRC_24M, 2, MCLK_SYSCLK_no_output, MCLK_out_P54);
   
 P41_push_pull_mode;
 P42_push_pull_mode; 

ADC setup is as simple as the past ADC example. The conversion rate is set to about 3 kHz and the ADC interrupt is enabled. Note no external ADC pin is used as the goal here is to read the internal reference voltage source, i.e., ADC Channel 15.

 ADC_enable;
 ADC_result_format_right_aligned;
 ADC_set_conversion_speed(ADC_conv_128_CLKs);
 _enable_ADC_interrupt; 

ADC interrupt is triggered when a conversion is completed. Inside this interrupt all we have to do is to read the ADC and clear the AD conversion completion flag.

 void ADC_ISR(void)          
 interrupt 5
 {
   value_1 = ((ADC_RES << 8) | ADC_RESL);
   value_2 = ((value_1 * 5000.0) / 4096.0);
   clear_ADC_flag;
 } 

Timer 2 is used in this example and so let us now see how it is configured. There are eight seven segment displays and we would want them to be multiplexed quickly in order to make them appear to update simultaneously. We cannot afford to stay long for each display. Thus, this task should be completed within 2 – 3ms time. Due to this reason, the timer is set to interrupt every 315µs. The total time for all displays is therefore about 2.52ms (8 × 0.315ms).

 #define refresh_time_in_us              315
 #define max_tmr_cnt                     0xFFFF
 #define tmr_val                         (max_tmr_cnt - refresh_time_in_us)
  
 ....
  
 TMR2_setup(TMR2_sysclk, TMR2_clk_prescalar_12T);
 TMR2_load_counter_16(tmr_val);
 TMR2_start;
 _enable_TMR_2_interrupt; 

Timer 2 is a 16-bit timer with auto-reload feature. In this demo, it is clocked internally with the 12 MHz system clock. This clock is further scaled down by a factor of 12 and so the timer’s ticks have a frequency of 1 MHz, i.e., the timer ticks are 1µs apart.

Lastly, the timer is started with its interrupt enabled.

Inside the timer interrupt the displays are updated by writing to the 74HC164 chips and clearing the timer overflow interrupt flag.

 void TMR_2_ISR(void)        
 interrupt 12
 {
     switch(i)
     {
         case 0:
         {
             Write_74HC164(num[(value_1 / 1000)], pos[3]);
             break;
         }
  
         case 1:
         {
             Write_74HC164((num[((value_1 % 1000) / 100)]), pos[2]);
             break;
         }
  
         case 2:
         {
             Write_74HC164(num[((value_1 % 100) / 10)], pos[1]);
             break;
         }
  
         case 3:
         {
             Write_74HC164(num[(value_1 % 10)], pos[0]);
             break;
         }
  
         case 4:
         {
             Write_74HC164(num[(value_2 / 1000)] & 0xFE, pos[7]);
             break;
         }
  
         case 5:
         {
             Write_74HC164((num[((value_2 % 1000) / 100)]), pos[6]);
             break;
         }
  
         case 6:
         {
             Write_74HC164(num[((value_2 % 100) / 10)], pos[5]);
             break;
         }
  
         case 7:
         {
             Write_74HC164(num[(value_2 % 10)], pos[4]);
             break;
         }
     }
  
     i++;
  
     if(i >= 8)
     {
         i = 0;
     }
  
     clear_TMR_2_overflow_flag;
 }   

The function below is the function for writing the 74HC164 shift-register ICs. Since the shift-registers are cascaded and we need them to work simultaneously, it is better to bit-bang seven segment data as 16-bit chunks – each word consisting of segment position and segment value data.

 void Write_74HC164(unsigned char val, unsigned char seg)
 {
     unsigned char s = 0x10;
     unsigned int temp = 0x0000;
  
     temp = (unsigned int)seg;
     temp <<= 8;
     temp |= (unsigned int)val;
  
     while(s > 0)
     {
         if((temp & 0x0001) != 0x0000)
         {
             DAT_pin_HIGH;
         }
         else
         {
             DAT_pin_LOW;
         }
  
         CLK_pin_HIGH;
  
         temp >>= 1;
         s--;  
       
         CLK_pin_LOW;
     }
 } 

Note interrupt priorities are not changed and are left in their default conditions.

In the main, the channel to be read is selected and AD conversion is triggered. This operation is cycled every 400ms. Channel 15 is read here. Channel 15 is the internal reference voltage source. The internal reference voltage source has a typical value of 1.344V. The seven segment displays show ADC counts and voltage value.

 ADC_set_channel(CH15);
 delay_ms(10);
 ADC_start_conversion;
 delay_ms(400); 

Demo

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