Maxim Integrated DS33Z41 - Manual (Page 58)

DS33Z41 Quad IMUX Ethernet Mapper
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8.19 Committed Information Rate Controller
The DS33Z41 provides a CIR provisioning facility. The CIR can be used to restrict the transport of received MAC
data to the serial port at a programmable rate. This is shown in
Figure 8-16 . The CIR will restrict the data flow
from the Receive MAC to Transmit HDLC. This can be used for provisioning and billing functions towards the
WAN. The user must set the CIR register to control the amount of data throughput from the MAC to the
HDLC/X.86 transmitter. The CIR register is in granularity of 500kbps with a range of 0 to 52Mbps. The operation
of the CIR is as follows:
• The CIR block counts the credits that are accumulated at the end of every 125ms.
• If data is received and stored in the SDRAM to be sent to the Serial Interface, the interface will request
the data if there is a positive credit balance. If the credit balance is negative, transmit interface does not
request data.
• New credit balance is calculated: credit balance = old credit balance – frame size in bytes after the frame
is sent.
• The credit balance is incremented every 125ms by CIR/8.
• Credit balances not used in 250ms are reset to 0.
• The maximum value of CIR can not exceed the transmit line rate.
• If the data rate received from the Ethernet interface is higher than the CIR, the receive queue buffers will
fill and the high threshold water mark will invoke flow control to reduce the incoming traffic rate.
• CIR function is only available in data received at the Ethernet Interface to be sent to WAN. There is not
CIR functionality for data arriving from the WAN to be sent to the Ethernet Interface.
• Negative credits are not allowed, if there is not a credit balance, no frames are sent until there is a credit
balance again.