PCI Express is a plug-and-play protocol meaning that at power up, the PCIe Host will enumerate the system. This process consists of the host reading the requested address size from each device and then assigning a base address to the device. As such, PCIe interfaces must be ready when the host queries them or they will not get assigned a base address. The PCI Express specification states that PERST# must deassert 100 ms after the power good of the systems has occurred, and a PCI Express port must be ready to link train no more than 20 ms after PERST# has deasserted. This is commonly referred to as the 100 ms boot time requirement.
Tandem Configuration utilizes a two-stage methodology that enables the IP to meet the configuration time requirements indicated in the PCI Express specification. Multiple use cases are supported with this technology:
• Tandem PROM : Load the single two-stage bitstream from the flash.
• Tandem PCIe : Load the first stage bitstream from flash, and deliver the second stage bitstream over the PCIe link to the MCAP.
• Tandem with Field Updates : After a Tandem PROM or Tandem PCIe initial configuration, update the entire user design while the PCIe link remains active. The update region (floorplan) and design structure are predefined, and Tcl scripts are provided.
• Tandem + Dynamic Function eXchange : This is a more general case of Tandem Configuration followed by Dynamic Function eXchange (DFX) of any size or number of dynamic regions.
• Dynamic Function eXchange over PCIe : This is a standard configuration followed by DFX, using the PCIe / MCAP as the delivery path of partial bitstreams.
To enable any of these capabilities, select the appropriate option when customizing the core. In the Basic tab:
1. Change the Mode to Advanced .
2. Change the Tandem Configuration or Dynamic Function eXchange option according to your particular case:
° Tandem for Tandem PROM, Tandem PCIe or Tandem + Dynamic Function eXchange use cases.
° Tandem with Field Updates ONLY for the predefined Field Updates use case.
° DFX over PCIe to enable the MCAP link for Dynamic Function eXchange, without enabling Tandem Configuration.
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X-Ref Target - Figure 3-4 
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The AXI DMA/Bridge Subsystem for PCI Express supports Tandem Configuration and Dynamic Function eXchange features for UltraScale devices, including Tandem with Field Updates. Device support details are documented in the DMA/Bridge Subsystem for PCI Express Product Guide (PG195) [Ref 5] , but the Tandem implementation details are presented thoroughly only here within this document.