FPGA Simulation

An FPGA simulation is provided using the Xilinx ISim simulator. However, due to the lack of behavioural models for all peripherals, this FPGA simulation covers only part of the whole SoC.

Normally FPGA simulation is used only for debugging the initialization of peripherals.

Run FPGA simulation

Use the hello world test as an example:

cd $TOP/fpga/board/$FPGA_BOARD
# compile and load the test
make hello
# run the simulation
make simulation

The result of simulation is recorded in $TOP/fpga/board/$FPGA_BOARD/simulate.log.

Also, the waveform of the simulation is automatically recorded to
$TOP/fpga/board/$FPGA_BOARD/lowrisc-chip-imp/lowrisc-chip-imp.sim/sim_1/behav/lowrisc-chip.vcd. If no VCD is needed, remove related lines in $TOP/fpga/board/$FPGA_BOARD/script/simulate.tcl. When VCD is enabled, be careful with disk space as the VCD file can grow very large.

Simulate other programs

The program to be executed in simulation is stored in the on-chip boot BRAM (src/boot.mem). To load simulation with an arbitrary program, it is needed to compile the program and replace the BRAM image. elf2hex should be used to translate an executable into a memory image. See [RTL simulation] for more information.

Enable the full DDR simulation model

It is possible to simulate the actual behaviour of the DDR interface but it is extremely slow.

To enable the full DDR simulation model, revise script/make_project.tcl, changing

#set_property verilog_define [list FPGA FPGA_FULL KC705] $obj
set_property verilog_define [list FPGA] $obj


set_property verilog_define [list FPGA FPGA_FULL KC705] $obj
#set_property verilog_define [list FPGA] $obj

which effectively defines the FPGA_FULL and KC705 macros for FPGA simulation. For NEXYS4-DDR, replace macro KC705 with NEXYS4.

Please rebuild the whole FPGA project to update the change:

make cleanall
make project

Unless the DDR interface is the debugging target, it is strongly suggested NOT to enable this option.