Partial bitstreams are delivered during normal device operation to replace functionality in a pre-defined device region. These bitstreams have the same structure as full bitstreams but are limited to specific address sets to program a specific portion of the device. Dedicated DFX features such as per-frame CRC checks (to ensure bitstream integrity) and automatic initialization (so the region starts in a known state) are available, as well as full bitstream features such as encryption and compression.
The size of a partial bitstream is directly proportional to the size of the region it is reconfiguring. For example, if the RP is composed of 20% of the device resources, the partial bitstream is roughly 20% the size of the full design bitstream.
Partial bitstreams are fully self-contained, so they are delivered to an appropriate configuration port. All addressing, header, and footer details are contained within these bitstreams, just as they would be for full configuration bitstreams. You deliver partial bitstreams to the FPGA through any external non-master configuration mode, such as JTAG, Slave Serial, or Slave SelectMap. Internal configuration access includes the ICAP (all devices), PCAP (Zynq-7000 SoC devices), and MCAP (UltraScale and UltraScale+ devices through PCIe).
Partial bitstreams are automatically created when
write_bitstream is run on a DFX configuration. Each partial bitstream file
name references your top-level design name, plus the Pblock name for the RP, plus
_partial. For example, for a full design bit file
top_first.bit, a partial bit file could be
The Pblock instance is always the same, regardless of the RM contained within, so it is recommended that you use a descriptive base configuration name or rename the partial bit files to clarify which module it represents.
Downloading a Partial BIT File
A partially reconfigured FPGA is in user mode while the partial BIT file is loaded. This allows the portion of the FPGA logic not being reconfigured to continue functioning while the RP is modified. Partial Bitstreams illustrates this process.
The partial BIT file has a simplified header, and there is no startup sequence that brings the FPGA into user mode. The BIT file contains (essentially, and with default settings) only frame address and configuration data, plus a final checksum value. Additional CRC checks can be inserted, if desired, to perform bitstream integrity checking.
If Reset After Reconfiguration is used, the DONE pin pulls LOW when reconfiguration begins and pulls HIGH again when partial reconfiguration successfully completes, although the partial bitstream can still be monitored internally as well.
If Reset After Reconfiguration is not selected, you must monitor the data being sent to know when configuration has completed. The end of a partial BIT file has a DESYNCH word (0000000D) that informs the configuration engine that the BIT file has been completely delivered. This word is given after a series of padding NO OP commands, ensuring that once the DESYNCH has been reached, all the configuration data has already been sent to the target frames throughout the device. As soon as the complete partial BIT file has been sent to the configuration port, it is safe to release the reconfiguration region for active use.