ADC to PWM functionality

Updating the PWM library

We need to add the a new prototype to the bbb_pwm.h and the body to the bbb_pwm.c files, that converts the ADC value to a the correct PWM duty cycle against the specified period.

1. Open the bbb_pwm.h file and modify to include the prototype below called map:

   Suppressed code here [8 lines]

   #ifndef BBB_PWM_H
   #define BBB_PWM_H
   
   ...
   
   unsigned int map(u_int16_t adc_value, unsigned int period_ns);
   
   #endif
   

2. To map we need to get the relationship between adc and the period to get the duty cycle

   \[ \text{duty_cycle_ns} = \frac{\text{adc_value} \cdot \text{period_ns}}{4095} \]

3. We now can implement above equation by modifying the bbb_pwm.c file to include the body of the previously defined prototype, place the function at the bottom of the file:

   Suppressed code here [15 lines]

   ... 
   
   // Map ADC value to PWM duty cycle with user-specified period
   unsigned int map(u_int16_t adc_value, unsigned int period_ns) {
   
       if (adc_value > 4095) {
           fprintf(stderr, "ADC value out of range\n");
           return 1;
       }
   
       // Map ADC value (0 to 4095) to PWM duty cycle (0 to period_ns)
       unsigned int duty_cycle_ns = (adc_value * period_ns) / 4095;
   
       return duty_cycle_ns;
   }
   

4. As the Makefile already exists and does not need updating, simply remake the library:

   Terminal

   make clean
   make uninstall
   make install
   

Implementing an example

We are going to create a script that utlises what we did for ~/examples_C/adc_read.c and examples_C/pwm_test.c, by utilising both libraries bbb_pwm.h and bbb_adc.h.

5. Create a new directory in the home directory and call it ~/examples_C/adc2pwm and inside a file called adc2pwm.c

   Terminal

   mkdir ~/examples_C/adc2pwm && cd ~/examples_C/adc2pwm && touch adc2pwm.c
   

6. Modify the contents of adc2pwm.c with the following code:

   Suppressed code here [61 lines]

   #include <bbb_pwm.h>
   #include <bbb_adc.h>
   #include <stdio.h>
   #include <unistd.h>
   #include <stdint.h>
   
   int main() {
       PWM pwm;
       unsigned int adc_reading = 0;
       unsigned int user_defined_period_ns = 1000000;
       unsigned int duty_cycle = 0;
   
       // Initialize PWM on a specified pin, for example "P9_14"
       pwm_init(&pwm, "P9_14");
   
       // Set initial PWM period
       pwm_set_period(&pwm, user_defined_period_ns);
   
       // Enable PWM output
       pwm_enable(&pwm);
   
       printf("Phy: %s\nChannel: %s\nChip: %s\nPeriod path: %s\nDuty Cycle path: %s\nEnable path: %s\n",
                                                                                   pwm.phy_pin,
                                                                                   pwm.channel,
                                                                                   pwm.chip,
                                                                                   pwm.period_path,
                                                                                   pwm.duty_cycle_path,
                                                                                   pwm.enable_path);
   
       // Loop to read ADC values and update PWM duty cycle
       for (int i = 0; i < 40; i++) {
           adc_get_value(0, &adc_reading); // Read ADC value from channel 0
           printf("ADC Reading: %u\n", adc_reading);
   
           // Map the ADC reading to the PWM duty cycle and update it
           duty_cycle = map(adc_reading, user_defined_period_ns);
   
           pwm_set_duty_cycle(&pwm, duty_cycle);
   
           sleep(1); // Update every 1 second
       }
   
       // Disable PWM and clean up
       pwm_disable(&pwm);
       pwm_cleanup(&pwm);
   
       return 0;
   }
   

7. Remember to create a Makefile file:

   Suppressed code here [25 lines]

   # Compiler and flags
   CC = gcc
   CFLAGS = -Wall -Werror
   
   # Target executable name
   TARGET = adc2pwm
   
   # Source files
   SRC = adc2pwm.c
   
   # Libraries to link against
   LIBS = -lbbb_pwm -lbbb_adc
   
   # Default target: build the executable
   all: $(TARGET)
   
   # Build the executable
   $(TARGET): $(SRC)
           $(CC) $(CFLAGS) $(SRC) $(LIBS) -o $(TARGET)
   
   # Clean up build artifacts
   clean:
           rm -f $(TARGET)
   
   # Phony targets to avoid conflicts with files of the same name
   .PHONY: all clean
   

8. Setup the following circuit, which is an amalgamation of ADC C Library and PWM C Library circuits

   
   

9. When running ./adc2pwm you may see this terminal output and you have connected up your circuity correctly, the LED should change it's brightness based on the resistance from the potentiometer:

   Output

   Phy: P9_14
   Channel: 0
   Chip: 0
   Period path: /sys/class/pwm/pwmchip0/pwm0/period
   Duty Cycle path: /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   Enable path: /sys/class/pwm/pwmchip0/pwm0/enable
   ADC Reading: 0
   ADC Reading: 0
   ADC Reading: 283
   ADC Reading: 697
   ADC Reading: 1313
   ADC Reading: 1674
   ADC Reading: 1907
   ADC Reading: 2454
   ADC Reading: 2968
   ADC Reading: 3367
   ADC Reading: 3687
   ADC Reading: 3989
   ADC Reading: 3920
   ADC Reading: 4001
   ADC Reading: 3932
   ADC Reading: 3985
   ADC Reading: 4002
   ADC Reading: 4002
   ADC Reading: 4001
   ...
   

10. Once you have done that, experiment with the code, can you get a faster response, or a longer run time of the program?