Altera Stratix GX Transceiver - Manual (Page 163)

Altera Corporation 6–9
January 2005 Stratix GX Transceiver User Guide
GigE Mode
Rate Matcher
The GigE mode operates in multi-crystal environments, which can
tolerate a frequency variation of
± 100 ppm between crystals. Stratix GX
devices have embedded circuitry to perform clock rate compensation by
inserting or removing the /I2/ code group from the interpacket gap
(IPG) or idle stream. This process is called “rate matching” or “clock rate
compensation.”
The IEEE 802.3 standard, clause 36, specifies two idle order sets (/I1/
and /I2/) for the transmitter. The /I1/ ordered set consists of a negative
disparity /K28.5/ (10’h283) followed by a /D5.6/ code group. (A
/D5.6/
has the same value, 10’h1A5, for the positive and negative
disparity versions and has a balanced 10-bit code.) The /I1/ ordered set
should be transmitted only once if the running disparity before the idle is
positive.
The /I2/ ordered set consists of a positive disparity /K28.5/
(10’h17C) followed by a negative disparity /D16.2/ (10’h289) code
group. The /I2/ ordered set can start the idle sequence if the disparity
before the idle sequence is negative. Otherwise, /I2/ follows an /I1/
ordered set and is continually transmitted, maintaining a negative
running disparity until the end of the IPG.
Figure 6–8 shows a case in which the idle stream starts with an /I1/
followed by /I2/ ordered sets. The running disparity before the idle state
is positive, as indicated by the positive disparity /D30.1/.
Figure 6–8. Idle Generation With /I1/ Ordered Set
Figure 6–9 shows cases in which only /I2/ ordered sets are generated.
The running disparity is negative before the start of the idle generation,
as indicated by the negative disparity /D30.1/. The /D30.1/ code
group in
Figure 6–8 and Figure 6–9 is intended only for illustrating
disparity and is not intended to signify an end of frame (EOF), nor is it
required prior to idle generation.
clock
tx_out 261h 283h 1A5h 17Ch 289h 17Ch 289h 17Ch
GMII Idle
Code Group D30.1+ K28.5- D5.6 K28.5+ D16.2- K28.5+ D16.2- K28.5+
/I2/ /I2/ /I2/ /I1/
289h
D16.2-