Altera SerialLite II IP Core - Manual (Page 38)

Chapter 3: Parameter Settings 3–15
Link Layer Configuration
January 2014 Altera Corporation SerialLite II MegaCore Function
User Guide
Retry-on-error Operation Example
Figure 3–10 shows an example of the retry-on-error operation.
Flow Control
The SerialLite II MegaCore function provides the Enable flow control parameter as
an optional means of exerting backpressure on a data source when data consumption
is too slow. Use this parameter to ensure that the receive FIFO buffers do not overflow.
1 Flow control is only needed when the system logic on the receiving end of the link is
reading the data slower than the system logic on the transmitting end of the link is
sending data.
The flow control feature in the SerialLite II Megacore function operates by having the
receiving end of the link issue a PAUSE instruction to the transmitting end of the link
when threshold of the receiver’s FIFO buffer is breached. The instruction causes the
transmitter to cease transmission for specified pause duration. Once the pause
duration has expired, the transmission resumes.
Flow Control Operation
When flow control is used, the FIFO buffer is structured as two sections, threshold
and headroom.
Figure 3–10. Retry-On-Error Example
Notes to Figure 3–10 :
(1) Device A transmits Seg_A, Seg_B, and Seg_C to Device B.
(2) At the same time, Device B transmits Seg_S, Seg_T, and Seg_U to Device A.
(3) Device B properly receives Seg_A, but detects an error with Seg_B.
(4) Device B returns positive acknowledge for Seg_A, but requests retransmission of Seg_B. Device B discards all
subsequently received segments until Seg_B is received again.
(5) Device A acknowledges the proper reception of Seg_S; Seg_T; and Seg_U.
(6) Device A resends all segments starting from Seg_B.
(7) Finally, Device B acknowledges the proper reception of Seg_B and Seg_C.
ACK_S; AC
K_T; ACK_U
Seg
_S; Se
g_T
; Se
g_U
ACK_A; N
ACK_B
ACK_B;
ACK_C
Resend S
eg_B; Seg_C
1
5
6
7
3
4
2
Device A Device B
Seg_A; Seg_B; Seg_C