Question

I am learning SMSC smc91cx driver code, and I learned how to program test code for smc91c111 nic by the instructions of Application Note 9-6. I cannot understand the following instructions under “Transmitting A Packet”:

Write the destination address (three successive writes: bytes 10, bytes 32, bytes 54)  
Write 0xFFFF, 0xFFFF, 0xFFFF  
Write the source address (three successive writes: bytes 10, bytes32, bytes 54)  
Write 0x0000, 0x0000, 0x0000 

I cannot make sense of these instructions. Should I write 10 bytes size of 0xFF plus 32 bytes size plus 54 bytes size to the buffer, or just write 0xFF in 10th byte postion, 32th, 54th byte postion?
But if so, why would you write 0x0000 to the same position?

Answer 1

Rather than allocating several different registers to write to, that chip has you write to the same DATA register serially until you set all the info. The DATA register is 2 bytes wide, but a MAC address is 6 bytes, numbered 0-5. So you have to write it 2 bytes at a time: bytes number 1 and 0 first, followed by bytes number 3 and 2, then bytes number 5 and 4. Then write 0xFFFF 3 times to the DATA register, then repeat for the source address and the 0x0000s.