I try to construct the following:
CO
|
/carry\ ____
s2 ---(((---|I0 |------------ O
+------+((---|I1 |
| +-(+---|I2 |
| | +----|I3__|
| +-(-----------+
| | |
| /carry\ ____ |B ___ BQ
D -----+------(((---|I0 |-+-----| |-+
s0 --+((---|I1 | > | |
s1 ---(+---|I2 | s3 -|S | |
| +-|I3__| s4 -|CE_| |
| +--------------------+
|
/carry\
|||
I write in Verilog, and instantiates SB_LUT4, SB_CARRY, SB_DFFESS primitives. To try to get a LUT cascade, I edit a .pcf constraints file (set_cascading...). However, synthesis (Lattice IceCube 2017.01.27914) disregards the constraints: W2401: Ignoring cascade constraint for LUT instance 'filt.blk_0__a.cmbA.l.l', as it is packed with DFF/CARRY in a LogicCell
In the admirable Project IceStorm I can't see any reason why a combination of cascaded LUTs and the carry chain can't be used.
I am aware that a (slightly) newer IceCube2 is available. I know of the Yosys/arachne-pnr/icepack/iceprog toolchain. But before changing a toolchain, it seems prudent to ask if anyone solved this problem already, or if it is indeed not possible to combine the carry chain and LUT cascades?
Update - a quick install of Yosys/arachne-pnr/icetools synthesizes my design without warnings, but visualisation in ice40_viewer (and log output) indicates that the chained lut is not used.
