The AXI firewall in the NSU is similar to the one in the NMU. It protects the NoC network from failures of the AXI Master Interface (MI) connection, usually due to a misbehaving AXI slave endpoint. The firewall is triggered by timeouts on expected MI transactions and fatal AXI protocol violations observed on the MI. When a failure is detected, the firewall becomes blocked, preventing further downstream transfers from propagating. When a firewall becomes blocked, the NSU generates compliant NoC packet protocol requests for all outstanding transactions. Once blocked, the firewall continues blocking transactions until unblocked by way of a reset or NPI register unblock request.
The NSU firewall has a single block for both read and write transactions. The firewall adds no latency to the read or write paths. There is no firewall checking for AXI-Stream traffic.