Altera SerialLite II Protocol - Manual (Page 62)

62 Altera Corporation
SerialLite II Protocol Reference Manual
Data Link Layer Description
Flow control time
The FC_TIME parameter is defined in units of a burst of 8 columns. The
parameter value is valid from 0x00 to 0xFF. For example, if FC_TIME is
set to 0x10, the pause period is 128 columns. Reception by the near
receiver of a flow control packet causes the near transmitter to stop data
transmission for the specified period (IDLE control characters are
transmitted by the transmitter during the pause period). The duration of
this specified period is monitored with a pause timer. If an additional
flow control packet arrives before the current pause time has expired, its
parameter replaces the current pause time. To extend the flow control
period, the far receiver must periodically refresh its request by having the
far transmitter send another flow control packet before the pause timer
expires. The far receiver can request traffic to resume immediately by
sending, via the far transmitter, a pause packet with an FC_TIME of 0x00.
To avoid the flow control packet being corrupted and lost, the near
transmitter periodically refreshes the transmission of a request based on
a flow control resend timer. The near transmitter periodically sends the
refresh when flow control is required to reliably stop transmission of data
from the far end. As the FC_TIME expires, the injection of additional link
management packets to reliably resume data transmission is not required.
Flow control has no effect on management packets. Meaning that link
management packets can still be transmitted during the pause period. It
also means that there is no way to flow control link management packets.
If a flow control packet is received during the transmission of a link
management packet, the transmission of the link management packet
continues irrespective of the pause packet.
If a flow control packet is received during a user packet transfer, the near
transmitter stops sending data immediately after the current column of
the data packet is transmitted. If a priority packet is in transfer, flow
control begins immediately after the completion of the priority packet. If
the flow control type is both priority and data packets, the new
transmitter stops sending data immediately after the current column,
whether a priority or data packet transfer has begun or not. Once the
pause has completed, the packet is resumed where it was previously
stopped.
The transmission of data packets and priority packets use two
independent timers to flow control the streams separately. The timer
begins to decrement upon the last transfer of data for data transmission
and completion of priority packet for priority data transmission. The
timers are cycle-based and are not affected by transmission of link
management packets or other types of user packet.