Transaction Model of the SLD Infrastructure
The SLD infrastructure mimics the IR/DR paradigm defined by the JTAG protocol. Each application implements an Instruction Register, and a set of Data Registers that operate similarly to the Instruction Register and Data Registers in the JTAG standard. Note that the Instruction Register and the Data Register banks implemented by each application are a subset of the USER1 and USER0 Data Register chains. The SLD infrastructure consists of three subsystems: the JTAG TAP controller, the SLD hub, and the SLD nodes.
The SLD hub acts as the arbiter that routes the TDI pin connection between each SLD node, and is a state machine that mirrors the JTAG TAP controller state machine.
The SLD nodes represent the communication channels for the end applications. Each instance of IP requiring a JTAG communication resource, such as the Signal Tap logic analyzer, would have its own communication channel in the form of a SLD node interface to the SLD hub. Each SLD node instance has its own Instruction Register and bank of DR chains. Up to 255 SLD nodes can be instantiated, depending on resources available in your device.
Together, the sld_hub and the SLD nodes form a virtual JTAG scan chain within the JTAG protocol. It is virtual in the sense that both the Instruction Register and DR transactions for each SLD node instance are encapsulated within a standard DR scan shift of the JTAG protocol.
The Instruction Register and Data Registers for the SLD nodes are a subset of the USER1 and USER0 Data Registers. Because the SLD Node IR/DR register set is not directly part of the IR/DR register set of the JTAG protocol, the SLD node Instruction Register and Data Register chains are known as Virtual IR (VIR) and Virtual DR (VDR) chains. The figure below shows the transaction model of the SLD infrastructure.
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