Altera Cyclone V User Manual page 118

11–2
Each receiver also maintains a credit allocated counter which is initialized to the total
available space in the RX buffer (for the specific Flow Control class) and then
incremented as packets are pulled out of the RX buffer by the Application Layer. The
value of this register is sent as the FC Update DLLP value.
Figure 11–1. Flow Control Update Loop
Flow
Credit
Control
Limit
Gating
Logic
Credits
(Credit
Consumed
Check)
Counter
Allow
1 2
Incr
App Transaction
Layer Layer
Data Source
The following numbered steps describe each step in the Flow Control Update loop.
The corresponding numbers on
correspond.
1. When the Application Layer has a packet to transmit, the number of credits
required is calculated. If the current value of the credit limit minus credits
consumed is greater than or equal to the required credits, then the packet can be
transmitted immediately. However, if the credit limit minus credits consumed is
less than the required credits, then the packet must be held until the credit limit is
increased to a sufficient value by an FC Update DLLP. This check is performed
separately for the header and data credits; a single packet consumes only a single
header credit.
2. After the packet is selected for transmission the Credits Consumed Register is
incremented by the number of credits consumed by this packet. This increment
happens for both the header and data Credit Consumed Registers.
3. The packet is received at the other end of the link and placed in the RX buffer.
4. At some point the packet is read out of the RX buffer by the Application Layer.
After the entire packet is read out of the RX buffer, the Credit Allocated Register
can be incremented by the number of credits the packet has used. There are
separate Credit Allocated Registers for the header and data credits.
5. The value in the Credit Allocated Registers is used to create an FC Update
DLLP.
Cyclone V Hard IP for PCI Express
FC
Update
FC Update DLLP
DLLP
Decode
7
Data Packet
PCI
Data Link Physical
Express
Layer Layer
Link
Figure 11–1
FC
Update Credit
DLLP Allocated
6
Generate
5
3
Physical Data Link Transaction
Layer Layer
Data Sink
show the general area to which they
November 2011 Altera Corporation
Chapter 11: Flow Control
Throughput of Posted Writes
Incr
4
Rx
Data Packet
Buffer
App
Layer Layer