A partial BIT file can be downloaded to the FPGA in the same manner as a full BIT file. An external microprocessor determines which partial BIT file should be downloaded, where it exists in an external memory space, and directs the partial BIT file to a standard FPGA configuration port such as JTAG, Select MAP or serial interface. The FPGA processes the partial BIT file correctly without any special instruction that it is receiving a partial BIT file.
It is common to assert the INIT or PROG signals on the FPGA configuration interface before downloading a full BIT file. This must not be done before downloading a partial BIT file, as that would indicate the delivery of a full BIT file, not a partial one.
Any indication to the working design that a partial BIT file will be sent (such as holding enable signals and disabling clocks) must be done in the design—and not by means of dedicated FPGA configuration pins. System Design for Configuring an FPGA shows the process of configuring through a microprocessor.
In addition to the standard configuration interfaces, Dynamic Function eXchange supports configuration by means of the Internal Configuration Access Port (ICAP). The ICAP protocol is identical to SelectMAP and is described in the Configuration User Guide for the target device. The ICAP library primitive can be instantiated in the HDL description of the FPGA design, thus enabling analysis and control of the partial BIT file before it is sent to the configuration port. The partial BIT file can be downloaded to the FPGA through general purpose I/O or gigabit transceivers and then routed to the ICAP in the FPGA programmable logic.
Rules for DFX ports and formats:
- Encrypted partial bitstreams can be delivered to any port, but only when the initial configuration was also encrypted. The same key must be used for all bitstreams.
- If the initial configuration of a device is encrypted, unencrypted partial bitstreams can be used only if they are delivered to the ICAP, which is a trusted port.
- Bitstream authentication for partial bitstreams is not supported for Virtex or Kintex devices, only for Zynq devices. However, authentication can be used for the initial full-device programming.
- The ICAP must be used with an 8-bit bus only for Dynamic Function eXchange for encrypted 7 series BIT files.
- Reconfiguration through external configuration ports is permitted only when bitstream readback security is not set to Level 2.