Lattice Radiant: Why is the bitstream file size fixed (ex. Always 1.8Mb) even in small designs for Certus-Pro NX?
Certus Pro-NX, the BIT file size on simplest design is 1.8MB, while for Crosslink-NX it generates 790KB. Whenever design becomes more condensed (especially more usage of EBRs and LRAMs) bitstream size will increase from that 1.8MB.
The bitstream size primarily depends upon the CRAM (configuration RAM) array size of the device, which is related to the number of LUTs/logic cells in the device.
So the difference in bitstream size that is seen is expected. Note that regardless of the user’s design size we have to program the entire CRAM array in the bitstream, so the size of the design won’t have a big impact on the bitstream size, unless you turn on bitstream compression in which case a design with low utilization will be able to be compressed more effectively than a design with high utilization.