GPIO_IOCLR = 0xf << 4;                              // Put address on bus
  GPIO_IOSET = addr << 4;
  
  GPIO_IOCLR = 0xff << 16;                            // Write low order byte to data bus
  GPIO_IOSET = data << 16;

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOW;                                   // Toggle IOW-signal
  asm volatile ( "NOP" );
  GPIO_IOSET = IOW;
  asm volatile ( "NOP" );

  GPIO_IOCLR = 0xf << 4;
  GPIO_IOSET = ((addr | 1) << 4);                     // And put next address on bus

  GPIO_IOCLR = 0xff << 16;                            // Write high order byte to data bus
  GPIO_IOSET = data >> 8 << 16;

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOW;                                   // Toggle IOW-signal
  asm volatile ( "NOP" );
  GPIO_IOSET = IOW;
  asm volatile ( "NOP" );
}

// Reads a word in little-endian byte order from a specified port-address
unsigned
cs8900a_read(unsigned addr)
{
  unsigned int value;

  GPIO_IODIR &= ~(0xff << 16);                        // Data port to input

  GPIO_IOCLR = 0xf << 4;                              // Put address on bus
  GPIO_IOSET = addr << 4;

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOR;                                   // IOR-signal low
  asm volatile ( "NOP" );
  value = (GPIO_IOPIN >> 16) & 0xff;                  // get low order byte from data bus
  GPIO_IOSET = IOR;

  GPIO_IOSET = 1 << 4;                                // IOR high and put next address on bus

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOR;                                   // IOR-signal low
  asm volatile ( "NOP" );
  value |= ((GPIO_IOPIN >> 8) & 0xff00);              // get high order byte from data bus
  GPIO_IOSET = IOR;                                   // IOR-signal low
  
  return value;
}

// Reads a word in little-endian byte order from a specified port-address
unsigned
cs8900a_read_addr_high_first(unsigned addr)
{
  unsigned int value;

  GPIO_IODIR &= ~(0xff << 16);                        // Data port to input

  GPIO_IOCLR = 0xf << 4;                              // Put address on bus
  GPIO_IOSET = (addr+1) << 4;

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOR;                                   // IOR-signal low
  asm volatile ( "NOP" );
  value = ((GPIO_IOPIN >> 8) & 0xff00);               // get high order byte from data bus
  GPIO_IOSET = IOR;                                   // IOR-signal high

  GPIO_IOCLR = 1 << 4;                                // Put low address on bus

  asm volatile ( "NOP" );
  GPIO_IOCLR = IOR;                                   // IOR-signal low
  asm volatile ( "NOP" );
  value |= (GPIO_IOPIN >> 16) & 0xff;                 // get low order byte from data bus
  GPIO_IOSET = IOR;

  return value;
}

void
cs8900a_init(void)
{
  int i;

  // Reset outputs, control lines high
  GPIO_IOSET = IOR | IOW;

  // No LEDs on.
  GPIO_IOSET = LED_RED | LED_YELLOW | LED_GREEN;

  // Port 3 as output (all pins but RS232)
  GPIO_IODIR = ~0U; // everything to output.

  // Reset outputs
  GPIO_IOCLR = 0xff << 16;  // clear data outputs

  // Reset the CS8900A
  cs8900a_write(ADD_PORT, PP_SelfCTL);
  cs8900a_write(DATA_PORT, POWER_ON_RESET);

  // Wait until chip-reset is done
  cs8900a_write(ADD_PORT, PP_SelfST);
  while ((cs8900a_read(DATA_PORT) & INIT_DONE) == 0)
    ;

  // Configure the CS8900A
  for (i = 0; i < sizeof InitSeq / sizeof (TInitSeq); ++i)
    {
      cs8900a_write(ADD_PORT, InitSeq[i].Addr);
      cs8900a_write(DATA_PORT, InitSeq[i].Data);
    }
}

void
cs8900a_send(void)
{
  unsigned u;

  GPIO_IOCLR = LED_RED;  // Light RED LED when frame starting

  // Transmit command
  cs8900a_write(TX_CMD_PORT, TX_START_ALL_BYTES);
  cs8900a_write(TX_LEN_PORT, uip_len);

  // Maximum number of retries
  u = 8;
  for (;;)
    {
      // Check for avaliable buffer space
      cs8900a_write(ADD_PORT, PP_BusST);
      if (cs8900a_read(DATA_PORT) & READY_FOR_TX_NOW)
        break;
      if (u -- == 0)
        {
          GPIO_IOSET = LED_RED;  // Extinguish RED LED on end of frame
          return;
        }

      // No space avaliable, skip a received frame and try again
      skip_frame();
    }

  GPIO_IODIR |= 0xff << 16;                           // Data port to output

  // Send 40+14=54 bytes of header
  for (u = 0; u < 54; u += 2)
    {
      GPIO_IOCLR = 0xf << 4;                              // Put address on bus
      GPIO_IOSET = TX_FRAME_PORT << 4;

      GPIO_IOCLR = 0xff << 16;                            // Write low order byte to data bus
      GPIO_IOSET = uip_buf[u] << 16;                      // write low order byte to data bus

      asm volatile ( "NOP" );
      GPIO_IOCLR = IOW;                                   // Toggle IOW-signal
      asm volatile ( "NOP" );
      GPIO_IOSET = IOW;

      GPIO_IOCLR = 0xf << 4;                              // Put address on bus
      GPIO_IOSET = (TX_FRAME_PORT | 1) << 4;              // and put next address on bus

      GPIO_IOCLR = 0xff << 16;                            // Write low order byte to data bus
      GPIO_IOSET = uip_buf[u+1] << 16;                    // write low order byte to data bus

      asm volatile ( "NOP" );
	  GPIO_IOCLR = IOW;                                   // Toggle IOW-signal
      asm volatile ( "NOP" );
      GPIO_IOSET = IOW;
    }

  if (uip_len <= 54)
    {
      GPIO_IOSET = LED_RED;  // Extinguish RED LED on end of frame
      return;
    }

  // Send remainder of packet, the application data
  uip_len -= 54;
  for (u = 0; u < uip_len; u += 2)
    {

      GPIO_IOCLR = 0xf << 4;                          // Put address on bus
      GPIO_IOSET = TX_FRAME_PORT << 4;

      GPIO_IOCLR = 0xff << 16;                        // Write low order byte to data bus
      GPIO_IOSET = uip_appdata[u] << 16;              // write low order byte to data bus

      asm volatile ( "NOP" );
	  GPIO_IOCLR = IOW;                               // Toggle IOW-signal
      asm volatile ( "NOP" );
      GPIO_IOSET = IOW;

      GPIO_IOCLR = 0xf << 4;                          // Put address on bus
      GPIO_IOSET = (TX_FRAME_PORT | 1) << 4;          // and put next address on bus

      GPIO_IOCLR = 0xff << 16;                        // Write low order byte to data bus
      GPIO_IOSET = uip_appdata[u+1] << 16;            // write low order byte to data bus

      asm volatile ( "NOP" );
	  GPIO_IOCLR = IOW;                               // Toggle IOW-signal
      asm volatile ( "NOP" );
      GPIO_IOSET = IOW;
    }

  GPIO_IOSET = LED_RED;  // Extinguish RED LED on end of frame
}

static void
skip_frame(void)
{
  // No space avaliable, skip a received frame and try again
  cs8900a_write(ADD_PORT, PP_RxCFG);
  cs8900a_write(DATA_PORT, cs8900a_read(DATA_PORT) | SKIP_1);
}

u8_t
cs8900a_poll(void)
{
  u16_t len, u;

  // Check receiver event register to see if there are any valid frames avaliable
  cs8900a_write(ADD_PORT, PP_RxEvent);
  if ((cs8900a_read(DATA_PORT) & 0xd00) == 0)
    return 0;

  GPIO_IOCLR = LED_GREEN;  // Light GREED LED when frame coming in.

  // Read receiver status and discard it.
  cs8900a_read_addr_high_first(RX_FRAME_PORT);

  // Read frame length
  len = cs8900a_read_addr_high_first(RX_FRAME_PORT);

  // If the frame is too big to handle, throw it away
  if (len > UIP_BUFSIZE)
    {
      skip_frame();
      return 0;
    }

  // Data port to input
  GPIO_IODIR &= ~(0xff << 16);

  GPIO_IOCLR = 0xf << 4;                          // put address on bus
  GPIO_IOSET = RX_FRAME_PORT << 4; 

  // Read bytes into uip_buf
  u = 0;
  while (u < len)
    {
      GPIO_IOCLR = 1 << 4;                            // put address on bus

      GPIO_IOCLR = IOR;                               // IOR-signal low
      uip_buf[u] = GPIO_IOPIN >> 16;                // get high order byte from data bus
      asm volatile ( "NOP" );
      GPIO_IOSET = IOR;                               // IOR-signal high

      GPIO_IOSET = 1 << 4;                            // put address on bus

      GPIO_IOCLR = IOR;                               // IOR-signal low
      asm volatile ( "NOP" );
      uip_buf[u+1] = GPIO_IOPIN >> 16;                  // get high order byte from data bus
      GPIO_IOSET = IOR;                               // IOR-signal high
      u += 2;
    }

  GPIO_IOSET = LED_GREEN;  // Extinguish GREED LED when frame finished.
  return len;
}