Area Repro

RISCBoy is significantly larger (in LUT4s) on ECP5 than on iCE40. This seems particularly true for the processor. To reproduce:

git clone --branch area_difference_repro --depth 1 https://github.com/Wren6991/RISCBoy.git riscboy
cd riscboy
. sourceme
cd synth
make -f HX8k-EVN.mk bit
make -f ECP5-EVN.mk bit

The two make command lines push identical RTL through build scripts for Lattice HX8k evaluation board and Lattice ECP5 evaluation board (not Versa, but the other EVN board) respectively. The RTL contains a tiny SoC with processor, 8 kiB of preloaded memory and some GPIO registers.

To confirm this works, you can replace the make bit target with make prog, and you should get a blinking LED. (Unfortunately the LEDs are inverted on the HX8k board.)

A prebuilt hex file has been included for blinky firmware, so no need for RISC-V toolchain. You will need:

• Yosys
• nextpnr-ice40
• nextpnr-ecp5

From Yosys I get these cell counts for iCE40:

=== riscboy_fpga ===

   Number of wires:               2947
   Number of wire bits:           8561
   Number of public wires:        2947
   Number of public wire bits:    8561
   Number of memories:               0
   Number of memory bits:            0
   Number of processes:              0
   Number of cells:               3091
     SB_CARRY                      172
     SB_DFF                        162
     SB_DFFE                       269
     SB_DFFER                      196
     SB_DFFES                        4
     SB_DFFR                       101
     SB_DFFS                         9
     SB_LUT4                      2158
     SB_RAM40_4K                    20

And these for ECP5:

=== riscboy_fpga ===

   Number of wires:               5573
   Number of wire bits:           9076
   Number of public wires:        5573
   Number of public wire bits:    9076
   Number of memories:               0
   Number of memory bits:            0
   Number of processes:              0
   Number of cells:               5517
     CCU2C                          92
     DP16KD                          4
     L6MUX21                       338
     LUT4                         3399
     PFUMX                         863
     TRELLIS_DPR16X4                32
     TRELLIS_FF                    789