text: //CC-by-www.Electrosmash.com open-source project
  
  #include <bcm2835.h>
  #include <stdio.h>
  
  //Define input pins
  #define PUSH1          RPI_GPIO_P1_08     //GPIO14
  #define PUSH2          RPI_V2_GPIO_P1_38  //GPIO20
  #define TOGGLE_SWITCH  RPI_V2_GPIO_P1_32  //GPIO12
  #define FOOT_SWITCH    RPI_GPIO_P1_10     //GPIO15
  #define LED            RPI_V2_GPIO_P1_36  //GPIO16
  
  uint32_t read_timer = 0;
  uint32_t input_signal = 0;
  uint32_t output_signal = 0;
  uint32_t booster_value = 2047;
  
  uint8_t FOOT_SWITCH_val;
  uint8_t TOGGLE_SWITCH_val;
  uint8_t PUSH1_val;
  uint8_t PUSH2_val;
  
  int main(int argc, char **argv) {
      //Start the BCM2835 library to access GPIO
      if (!bcm2835_init()) {
          printf("bcm2835_init failed. Are you running as root ?\n");
          return 1;
      }
      //Start the SPI BUS
      if (!bcm2835_spi_begin()) {
          printf("bcm2835_spi_begin failed. Are you running as root ?\n");
          return 1;
      }
  
      //Define PWM
      bcm2835_gpio_fsel(18, BCM2835_GPIO_FSEL_ALT5 );  //PWM0 signal on GPIO18
      bcm2835_gpio_fsel(13, BCM2835_GPIO_FSEL_ALT0 );  //PWM1 signal on GPIO13
      bcm2835_pwm_set_clock(2);       //Max clk frequency (19.2MHz/2 = 9.6MHz)
      bcm2835_pwm_set_mode(0, 1, 1);  //Channel 0, markspace mode, PWM enabled
      bcm2835_pwm_set_range(0, 64);   //Channel 0, 64 is max range (6bits): 9.6MHz/64 = 150KHz switching PWM freq
      bcm2835_pwm_set_mode(1, 1, 1);  //Channel 1, markspace mode, PWM enabled
      bcm2835_pwm_set_range(1, 64);   //Channel 0, 64 is max range (6bits): 9.6MHz/64 = 150KHz switching PWM freq
  
      //Define SPI bus configuration
      bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST);    //The default
      bcm2835_spi_setDataMode(BCM2835_SPI_MODE0);                 //The default
      bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_64);  //4MHz clock with _64
      bcm2835_spi_chipSelect(BCM2835_SPI_CS0);                    //The default
      bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, LOW);    //The default
  
      uint8_t mosi[10] = {0x01, 0x00, 0x00};  //12 bit ADC read 0x08 ch0,-0c for ch1
      uint8_t miso[10] = {0};
  
      //Define GPIO pins configuration
      bcm2835_gpio_fsel(PUSH1, BCM2835_GPIO_FSEL_INPT);          //PUSH1 button as input
      bcm2835_gpio_fsel(PUSH2, BCM2835_GPIO_FSEL_INPT);          //PUSH2 button as input
      bcm2835_gpio_fsel(TOGGLE_SWITCH, BCM2835_GPIO_FSEL_INPT);  //TOGGLE_SWITCH as input
      bcm2835_gpio_fsel(FOOT_SWITCH, BCM2835_GPIO_FSEL_INPT);    //FOOT_SWITCH as input
      bcm2835_gpio_fsel(LED, BCM2835_GPIO_FSEL_OUTP);            //LED as output
  
      bcm2835_gpio_set_pud(PUSH1, BCM2835_GPIO_PUD_UP);          //PUSH1 pull-up enabled
      bcm2835_gpio_set_pud(PUSH2, BCM2835_GPIO_PUD_UP);          //PUSH2 pull-up enabled
      bcm2835_gpio_set_pud(TOGGLE_SWITCH, BCM2835_GPIO_PUD_UP);  //TOGGLE_SWITCH pull-up enabled
      bcm2835_gpio_set_pud(FOOT_SWITCH, BCM2835_GPIO_PUD_UP);    //FOOT_SWITCH pull-up enabled
  
      while(1) {  //Main Loop
          //Read 12 bits ADC
          bcm2835_spi_transfernb(mosi, miso, 3);
          input_signal = miso[2] + ((miso[1] & 0x0F) << 8);
  
          //Read the PUSH buttons every 50000 times (0.25s) to save resources
          read_timer++;
          if (read_timer == 5000) {
              read_timer = 0;
              PUSH1_val = bcm2835_gpio_lev(PUSH1);
              PUSH2_val = bcm2835_gpio_lev(PUSH2);
              TOGGLE_SWITCH_val = bcm2835_gpio_lev(TOGGLE_SWITCH);
              FOOT_SWITCH_val = bcm2835_gpio_lev(FOOT_SWITCH);
              //Light the effect when the footswitch is activated
              bcm2835_gpio_write(LED, !FOOT_SWITCH_val);
  
              //Update booster_value when the PUSH1 or 2 buttons are pushed
              if (PUSH2_val == 0) {
                  bcm2835_delay(100);  //100ms delay for buttons debouncing
                  if (booster_value < 4095) booster_value = booster_value + 500;
              }
              else if (PUSH1_val == 0) {
                  bcm2835_delay(100);  //100ms delay for buttons debouncing
                  if (booster_value > 500) booster_value = booster_value - 500;
              }
          }
  
          //*** BOOSTER EFFECT ***//
          //The input_signal is attenuated depending on the value of booster_value
          //I am using a simplified version of the Arduino "map" function, more info in: https://www.arduino.cc/en/reference/map
          output_signal = (int)((float)(input_signal) * (float)((float) booster_value / (float) 4095.0));
  
          //Generate output PWM signal 6 bits
          bcm2835_pwm_set_data(1, output_signal & 0x3F);
          bcm2835_pwm_set_data(0, output_signal >> 6);
      }
  
      //Close all and exit
      bcm2835_spi_end();
      bcm2835_close();
      return 0;
  }
  
  
  