Altera SerialLite II Protocol - Manual (Page 61)

Altera Corporation 61
SerialLite II Protocol Reference Manual
SerialLite II Specification
The 32 bits of the CRC value are placed in the CRC field so that X31 is the
left most bit of the first byte, and X0 is the right most bit of the fourth byte,
as shown in
Figure 2–40 .
Figure 2–40. Bit positioning for CRC-32
In the absence of bit errors, if the packet payload and CRC are sent to the
CRC-32 calculator, the remainder is
32'b11000111_00000100_11011101_01111011
(0xC704DD7B).
Flow Control (Optional)
Implementation of the flow control mechanism is optional.
For the purposes of this specification, the link is assumed to consist of a
near transmitter sending data to a far receiver. Flow control allows the far
receiver to exert backpressure on the data it is receiving if it is receiving
data faster than it can process it. This backpressure gives the receiver a
chance to catch up before the incoming data overflows the receiver buffer.
SerialLite II uses a ‘pause’ flow control scheme. When the far receiver is
not able to receive more data, the far transmitter sends a flow control
packet (pause packet) to the near receiver to instruct the near transmitter
to stop transmitting data for a user-defined period of time.
Table 2–20
specifies the flow control packet format, and Table 2–21 specifies the
pause packet parameter values.
Table 2–20. Flow Control Packet Format
Bit[23:20] Bit[19:16] Bit[15:12] Bit[11:8]
Link packet type Link packet parameter
0x1 CLASS FC_TIME
Table 2–21. Pause Packet Parameter Values
Parameter Bit Width Valid Settings
CLASS 4 0x0 = Priority
0x1 = Data
All other values are reserved for future use.
FC_TIME 8 Value from 0x00 to 0xFF
X
31
… …X
24
X
23
… …X
16
X
7
… …X
0
X
15
… …X
8