Media Interfaces

For every kind of network medium we need an dedicated media interface. For example there is an lvds interface (200MBit), an optical link interface for use with TLK2501 on the TRB and one for the internal serdeses of the lattice chips.

Functionality

Although the media interfaces reside on the lowest level of the network structure, they have to provide some additional functionality. The implementation of these mechanisms is up to the programmer of the media interfaces.

Synchronization to packet boundary

The interface must provide the correct packet number to the multiplexer. The network protocol relies on an accurate counting of the 16Bit packets to ensure that the packet type of each packet can be retrieved. Starting value of the counter is 4, counting for each packet 4-0-1-2-3.

The media interface has to provide a detection for the start of each packet to assure that the counter is aligned properly. This can be done in two ways: On LVDS interfaces with some free lines, one control line can be used to mark the first word of each packet.

For example with the optical link we don't have this opportunity with extra lines. To detect when a word of a packet has been lost, idles may only be sent between two packets, but not between the words of one packet.

If such a wrongly placed idle (or any other code violation, or a not-yet-defined komma character) is detected, the start-up sequence described below is started again, the bits in stat_op are set accordingly and the own transmitter has to send some kind of errors (misplaced idles, a (not yet defined) komma character...) as well.

FIFO sizes

The FiFos inside the media interfaces should be kept to an absolute minimum, especially on interfaces running slower than the internal logic. The reason is, that a big fifo that might run full prohibits us from effectively switching between channels. (A 16 word deep fifo on a POF-interface might introduce up to 1.2us additional latency!)

Encoding for secure transmission

The media interfaces also have to provide a mechanism to secure the network from bit errors. On the optical link, 8/10bit encoding is used, which is sufficient due the very low bit error rate. On electrical connections this might be too weak. In these cases for example a 8/12 bit encoding might be used to be able to correct single bit errors.

Serdes settings

Please check whether the Elastic Buffers of ECP2M can be kept switched off since they produce a massive amount of latency.

Interface to internal logic

The interface to the medium can not be standardized, but there is a fixed interface layout on the internal connections:

Name Width Description
RESET 1 common reset
CLK 1 internal clock
MED_DATAREADY_IN 1 data is offered by the internal side
MED_READ_OUT 1 media interface is reading offered data
MED_DATA_IN 16 data from internal side
MED_PACKET_NUM_IN 2 packet number from internal side
MED_DATAREADY_OUT 1 data is offered for the internal side
MED_READ_IN 1 internal side is reading offered data (should always be 1)
MED_DATA_OUT 16 data output
MED_PACKET_NUM_OUT 2 packet number output
STAT_OP 16 operational status of the interface (see below)
CTRL_OP 16 ctrl input for the media interface

MED_ERROR_OUT

Name Description
ERROR_OK Everything is fine
ERROR_RECOV There was a bit error, but it was recovered by internal encoding
ERROR_FATAL Some serious problem occured - restart is needed
ERROR_WAIT Link is starting up / waiting for connection / sending resync
ERROR_NC No cable is connected

STAT_OP

Bit Name Meaning
2..0 MED_ERROR_OUT Error status of the interface
3    
4 LSM[0] Link State Machine status bit 0
5 LSM[1] Link State Machine status bit 1
6 LSM[2] Link State Machine status bit 2
7 LSM[3] Link State Machine status bit 3
8    
9 status_led LED output showing link status: flashing during start-up, on when link is established
10 rx_led LED output showing RX status: pulsing high during receiving
11 tx_led LED output showing TX status: pulsing high during sending
12 link error strobe signal, when code violation or similar errors happen
13 reset_sequence_received reset trbnet logic
14 no_signal reset internal logic
15 send_reset_sequence send reset over network

CTRL_OP

Bit Meaning
14 reset media interface. switches off link state machine and transmitter
15 send reset over network

Startup of the optical links

Marek and I (Jan) agreed on the following mechanism to start up an optical link:
  • Start sending idle - words acording to data sheet of TLK2501 that is used on the trb
  • After receving idles for 650ms, 16bit word alignment is made
  • After 1300ms (27 bit counter overflow) receiving of data is enabled
  • After 2600ms sending data is enabled
This sequence gives enough time to properly connect a cable and allow the transmitters to synchronize on the probably shaky signal while connecting the cable.

-- JanMichel - 09 Mar 2009
Topic revision: r10 - 23 Jul 2010, JanMichel
 
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