Minimal nMigen SERDES passthrough

serdesio.py

# Minimal nMigen code on a LFE5UM5G-25F-8BG381C (ECP5-5G Evaluation Kit) to get
# output on the TX of Channel 0 in Bank 0 straight from the FPGA fabric. The SERDES 
# DCUA instance has a bajillion parameters/inputs but these are the ones you actually 
# need for things to "work." Note that the output driver configuration is funky and 
# I see a way weaker output drive than I expect here.
#
# Original inspiration thanks to: https://lab.ktemkin.com/post/serdes-lfps/, but wanted
# to get things working absent a larger USB3/PCIe picture.

m.submodules.dcu0 = Instance("DCUA",
        a_LOC='DCU0',
        a_CHAN='CH0',
        a_BEL='X42/Y71/DCU',

        # DCU Power Management
        p_D_MACROPDB            = "0b1",
        p_D_IB_PWDNB            = "0b1",
        p_D_TXPLL_PWDNB         = "0b1",
        i_D_FFC_MACROPDB        = 1,

        # TX — power management
        p_CH0_TPWDNB            = "0b1",
        i_CH0_FFC_TXPWDNB       = 1,

        # Driver power
        p_CH0_TXAMPLITUDE="0d1000", # Apparently 1000mV, although I see 1/10th of that
        p_CH0_RTERM_TX="0d19", # 50 ohm termination

        p_CH0_TDRV_SLICE0_CUR   = "0b111",  # 800 uA
        p_CH0_TDRV_SLICE0_SEL   = "0b01",   # main data

        p_CH0_TDRV_SLICE1_CUR   = "0b111",  # 800 uA
        p_CH0_TDRV_SLICE1_SEL   = "0b01",   # main data

        p_CH0_TDRV_SLICE2_CUR   = "0b11",   # 3200 uA
        p_CH0_TDRV_SLICE2_SEL   = "0b01",   # main data

        p_CH0_TDRV_SLICE3_CUR   = "0b11",   # 3200 uA
        p_CH0_TDRV_SLICE3_SEL   = "0b01",   # main data

        p_CH0_TDRV_SLICE4_CUR   = "0b11",   # 3200 uA
        p_CH0_TDRV_SLICE4_SEL   = "0b01",   # main data

        p_CH0_TDRV_SLICE5_CUR   = "0b11",   # 800 uA
        p_CH0_TDRV_SLICE5_SEL   = "0b01",   # main data

        # TX Passthrough
        p_CH0_LDR_CORE2TX_SEL=1, # Either always enabled the passthrough
        #i_CH0_FFC_LDR_CORE2TX_EN=button,  # Or enable via a button
        i_CH0_LDR_CORE2TX=ClockSignal(), # The signal you actually want to send through

)