USB 3.0 SKP order set sending criteria.

[mehul.gor]

Hi All,

As mention in USB 3.0 Specification section 10.7.10,

It is require to send SKIP order set when transmitted symbol count reaches nSkipSymbolLimit.
I am bit confused about sending of SKP oreder set. Have look at below example.

For Example.

Downstream Port is going to send Data Header Packet + Data Payload which is consist of 1056 symbols (including framing order set).

Initially TX State go to Tx header - At that time tx_symbol_count = 0,

Tx Header to TX Data - At that time tx_symbol_count = 20,

In TX Data,

After sending end framing set, tx_symbol_count = 1056. It is greater than nSkipSymbolLimit (354).

So i need to send SKP Symbol and need to reduce tx_symbol_count by nSkipSymbolLimit (354).
Now, tx_symbol_count = 702. Again send SKP order set and reduce tx_symbol_count by nSkipSymbolLimit (354).
Follow this step until tx_symbol_count become less than 354.

In this case it will become 348.

Am i right?

Suppose i am not reseting tx_symbol_count. Need to send another 20 bytes Header packet.

So it is require to go to TX Header state.

In that after sending whole header packet tx_symbol_count become 368 which is greater than 354 (nSkipSymbolLimit).

So i need to sent 1 SKP order set and reduce tx_symbol_count to 14.

Am i right?

after sending SKP Symbol need to go TX IDLE. in Tx IDLE whether i need to reset tx_symbol_count or not?

 

[prakashdrj]

The USB 3.0 standard provides that the transmitter send out an average of one SKP ordered set every 354 symbols. Since the once every 354 symbols is only an average, the transmitter has the flexibility to wait to insert a SKP ordered set until a packet (of varying length) has completed, so as not to insert the symbols in the middle of a packet. An eight symbol (four ordered set) buffer is kept on the transmission side to hold the SKP symbols until the current packet has completed.