Migen and LiteX

Wishbone Bus Basics

LiteX provides us with a Wishbone abstraction layer. There really is no reason we need to include a CPU with our design, but we can still reuse the USB Wishbone bridge in order to write HDL code.

We can use DummyUsb to respond to USB requests and bridge USB to Wishbone, and rely on LiteX to generate registers and wire them to hardware signals. We can still use wishbone-tool to read and write memory, and with a wishbone bridge we can actually have code running on our local system that can read and write memory on Fomu.

Go to the litex directory and build the design;

python3 workshop.py --board $FOMU_REV

Load it onto Fomu:

dfu-util -D build/gateware/top.dfu

If you get an error message about missing modules, check you have all submodules cloned and setup with;

git submodule update --recursive --init

Take a look at build/csr.csv. This describes the various regions present in our design. You can see memory_region,sram,0x10000000,131072, which indicates the RAM is 128 kilobytes long and is located at 0x10000000, just as when we had a CPU. You can also see the timer, which is a feature that comes as part of LiteX. Let’s try reading and writing RAM:

wishbone-tool 0x10000000
wishbone-tool 0x10000000 0x98765432
wishbone-tool 0x10000000

Wishbone Bus Extension

Aside from that, there’s not much we can do with this design. But there’s a lot of infrastructure there. So let’s add something we can see (workshop_rgb.py contains the completed example).

As explained in Hardware Description Languages the RGB LED driver is defined in Verilog/VHDL (as SB_RGBA_DRV), but we can import it as a Module into Migen:

class FomuRGB(Module, AutoCSR):
    def __init__(self, pads):
        self.output = CSRStorage(3)
        self.specials += Instance("SB_RGBA_DRV",
            i_CURREN = 0b1,
            i_RGBLEDEN = 0b1,
            i_RGB0PWM = self.output.storage[0],
            i_RGB1PWM = self.output.storage[1],
            i_RGB2PWM = self.output.storage[2],
            o_RGB0 = pads.r,
            o_RGB1 = pads.g,
            o_RGB2 = pads.b,
            p_CURRENT_MODE = "0b1",
            p_RGB0_CURRENT = "0b000011",
            p_RGB1_CURRENT = "0b000011",
            p_RGB2_CURRENT = "0b000011",

This will instantiate this Verilog block and connect it up. It also creates a CSRStorage object that is three bits wide, and assigns it to output. By having this derive from AutoCSR, the CSRStorage will have CSR bus accessor methods added to it automatically. Finally, it wires the pads up to the outputs of the block.

We can instantiate this block by simply creating a new object and adding it to self.specials in our design:

    # Add the LED driver block
    led_pads = soc.platform.request("rgb_led")
    soc.submodules.fomu_rgb = FomuRGB(led_pads)

Finally, we need to add it to the csr_map:


Now, when we rebuild this design and check build/csr.csv we can see our new register:


We can use wishbone-tool to write values to 0x60003000 (or whatever your build/csr.csv says) and see them take effect immediately.

wishbone-tool 0x60003000 0x1 # make LED green
wishbone-tool 0x60003000 0x2 # make LED red
wishbone-tool 0x60003000 0x3 # make LED yellow
wishbone-tool 0x60003000 0x4 # make LED blue
wishbone-tool 0x60003000 0x5 # make LED teal
wishbone-tool 0x60003000 0x6 # make LED pink
wishbone-tool 0x60003000 0x7 # make LED white

You can see that it takes very little code to take a Signal from HDL and expose it on the Wishbone bus.