ym3014b_read.c

// read a sample sent to the YM3014B                                                                                                                                                             
// it's a 13.3 floating point format, 3 MSBs are exponent,                                                                                                                                       
// 13 LSBs are mantissa, except only 10 of the mantissa bits are used                                                                                                                            
uint16_t ym3014b_read16(void) {                                                                                                                                                                  
  uint16_t data = 0;                                                                                                                                                                             
  uint8_t last_port_state = GPIOC_PDIR;                                                                                                                                                          
                                                                                                                                                                                                 
  // wait for rising clock edge                                                                                                                                                                  
  // if we see a falling sync edge, return data                                                                                                                                                  
                                                                                                                                                                                                 
  while (true) {                                                                                                                                                                                 
    uint8_t port_state = GPIOC_PDIR;                                                                                                                                                             
    uint8_t change = port_state ^ last_port_state;                                                                                                                                               
    // rising clock edge, shift in a data bit                                                                                                                                                    
    if ((change & mask_clk) && (port_state & mask_clk)) {                                                                                                                                        
      data = (data >> 1) | ((port_state & mask_data) << shift_data);                                                                                                                             
    }                                                                                                                                                                                            
    // falling sync edge; we're done                                                                                                                                                             
    if ((change & mask_sync) && (last_port_state & mask_sync)) {                                                                                                                                 
      return data;                                                                                                                                                                               
    }                                                                                                                                                                                            
    last_port_state = port_state;                                                                                                                                                                
  }                                                                                                                                                                                              
}