Altera RapidIO II MegaCore Function - Manual (Page 114)

4–72 Chapter 4: Functional Description
Transport Layer
RapidIO II MegaCore Function August 2014 Altera Corporation
User Guide
■ Routes packets with a tt value that does not match the RapidIO II IP core’s device
ID width support level according to the following rules:
■ If you turned on Enable 16-bit device ID width in the RapidIO II parameter
editor, routes packets with an 8-bit device ID to the Avalon-ST pass-through
interface, if the Avalon-ST pass-through interface is implemented in the IP core
variation. If this interface is not implemented in your variation, drops the
packet.
■ If you turned off Enable 16-bit device ID width in the RapidIO II parameter
editor, drops packets with a 16-bit device ID.
In any of the cases in which the packet is dropped, the Transport layer module
asserts the transport_rx_packet_dropped signal.
■ Request packets with a supported ftype and a tt value that matches the
RapidIO II IP core’s device ID width are routed to the Logical layer supporting the
ftype
. If the request packet has an unsupported ttype, the Logical layer module
then performs the following tasks:
■ Sends an ERROR response for request packets that require a response.
■ Records an unsupported_transaction error in the Error Management
extension registers.
■ Packets that would be routed to the Avalon-ST pass-through interface, in the case
that the RapidIO II IP core does not implement an Avalon-ST pass-through
interface, are dropped. In this case, the Transport layer module asserts the
transport_rx_packet_dropped
signal.
■ ftype =13 response packets are routed based on the value of their target
transaction ID (targetTID) field. Each Logical layer module is assigned a range of
transaction IDs (
Table 4–23 specifies these ranges). If the transaction ID of a
received response packet is not within one of the ranges assigned to one of the
enabled Logical layer modules, the packet is routed to the Avalon-ST pass-through
interface.
Packets marked as errored by the Physical layer (for example, packets with a CRC
error or packets that were stomped) are filtered out and dropped from the stream of
packets sent to the Logical layer modules or pass-through interface. In these cases, the
transport_rx_packet_dropped
output signal is not asserted.
Transmitter
On the transmit side, the Transport layer module uses a modified round-robin
scheduler to select the Logical layer module to transmit packets. The Physical layer
continuously sends Physical layer transmit buffer status information to the Transport
layer. Based on this information, the Transport layer either implements a standard
round-robin algorithm to select the Logical layer module from which to transmit the
next packet, or implements a modified algorithm in which the Transport layer only
considers packets whose priority field is set at or above a specified threshold. The
incoming status information from the Physical layer determines the current priority
threshold. The status information can also temporarily backpressure the Transport
layer, by indicating no packets of any priority level can currently be selected.