//WRITE TO MII REGISTERS
void write_mii(unsigned char phyad,unsigned char regad,unsigned int mii_data)
{
 unsigned char mask8;
 unsigned int  i,mask16;
 mii_write;			//Macro that sends ST and OP for write operation		
 mask8 = 0x10;
 for(i=0;i<5;++i)		//Send 5 bits of PHY address
 {
  switch ((mask8 & phyad))
  {
   case 0:
    clr_mdo;		//Macro clears MDO bit
    break;
   default:
    set_mdo;		//Macro sets MDO bit
  }
    mii_clk;		//Macro clocks bits out of MDC by setting and clearing MDC bit
    mask8 >>= 1;	 
 }   
 mask8 = 0x10;
 for(i=0;i<5;++i)		//Send 5 bits of PHY register address
 {
  switch ((mask8 & regad))
  {
   case 0:
    clr_mdo;
    break;
   default:
    set_mdo;
  }
   mii_clk;
   mask8 >>= 1;	 
 }    					
 mii_w_ta;			//Macro that drives two turnaround bits 10
 mask16 = 0x8000;
 for(i=0;i<16;++i)		//Send 16 bits of data to PHY
 {
  switch (mask16 & mii_data)
  {
   case 0:
    clr_mdo;
    break;
   default:
    set_mdo;
  }
 mii_clk;	 
 mask16 >>= 1;	 
 }   			
}

//READ MII REGISTERS
unsigned int read_mii(unsigned char phyad,unsigned char regad)
{
 unsigned char mask8,i;
 unsigned int  mask16,result16;
 mii_read;			//Macro that sends ST and read OP bits
 mask8 = 0x10;
 for(i=0;i<5;++i)		//Send 5 bits of PHY address
 {
  switch (mask8 & phyad)
  {
   case 0:
    clr_mdo;
    break;
   default:
    set_mdo;
  }
 mii_clk;	 
 mask8 >>= 1;	 
 }  
 mask8 = 0x10;
 for(i=0;i<5;++i)		//Send 5 bits PHY register address
 {
  switch (mask8 & regad)
  {
   case 0:
    clr_mdo;
    break;
   default:
    set_mdo;
  }
 mii_clk;			//Sets then clears MDC bit-providing clock
 mask8 >>= 1;	 
 }  
 mii_r_ta;			//PHY output buffer turnaround time
 mask16 = 0x8000;
 result16 = 0x0000;
 for(i=0;i<16;++i)		//Read 16 bits of data from PHY
 {
  mii_clk;
  read_rtl(MEMR);
  switch (byte_read &= 0x04)
  {
   case 0:
    nop;
   break;
  default:
   result16 |= mask16;	
  }
 mask16 >>= 1;	 
 }  
 return result16;
}					

Listing 1—The ability to read and write the MII interface is a plus because you have access to information about the auto-negotiation process and what the PHY is really doing.