ECP5 Delay Line with ~50ps precision

delay_line.py

# This is an nmigen delay line for ECP5 FPGAs using the open source toolchain. It strings together a series of
# manually placed carry chains into a "thermometer." It returns a signal that's (length) long that represents
# the chain "snapshotted" at the primary clock domain (using the flip flops colocated in the slice).
#
# This can be used in a Time to Digital Converter (i.e. to measure the time between to events) or in
# an ADC by comparing (with LVDS) a signal to a reference signal.
#
# Note that the bit precision (read: delay per carry element) varies as a function of temperature. On 
# a LFE5U-25F-8MG285C, I've measure delay times of approximately 43ps on average. Due to assorted reasons,
# the delay time will vary between bits and due to variations in routing (even when manually places), you might 
# even end up with random bits you expect to be 1 actually be zero (and visa versa), so I recommend calibrating 
# your usage of this by first injecting a reference signal with predictable timing.
#
# For illustration purposes, let's say you inject a 100mhz signal and each carry element has a delay of 1ns
# (rather than 43ps). Then with a delay line of length 30 you'd expect something like this in the thermometer
#
# 111000000000011111111110000000

def create_delay_line(m, start: Signal, length):
    def get_slice():
        start_x = 10
        start_y = 11
        slice_i = 0
        while True:
            yield "X{}/Y{}/SLICE{}".format(start_x, start_y, ["A","B","C","D"][slice_i])
            if slice_i == 3:
                start_x += 1
            slice_i = (slice_i + 1) % 4
    gen_slice = get_slice()

    intoff = Signal(length + 1)
    thermometer = Signal(length)
    carry = Signal(length//2 + 1)
    m.submodules.delay_start = Instance("TRELLIS_SLICE",
        a_BEL=next(gen_slice),
        p_MODE="CCU2",
        p_CCU2_INJECT1_0="NO",
        p_CCU2_INJECT1_1="YES",
        p_LUT0_INITVAL=0x00FF,
        p_LUT1_INITVAL=0x00FF,
        i_D1=start,
        o_FCO=carry[0]
    )
    for i in range(length // 2):
        delay = Instance("TRELLIS_SLICE",
            a_BEL=next(gen_slice),

            p_MODE="CCU2",
            p_CCU2_INJECT1_0="NO",
            p_CCU2_INJECT1_1="NO",
            p_LUT0_INITVAL=0x3300,
            p_LUT1_INITVAL=0x3300,
            p_REG0_SD="1",
            p_REG1_SD="1",

            i_B0=0,
            i_B1=0,
            i_LSR=0,

            o_F0=intoff[2*i + 0], # Out of the LUT
            i_DI0=intoff[2*i + 0], # Into the FF
            o_Q0=thermometer[2*i + 0], # Out of the FF

            o_F1=intoff[2*i + 1],
            i_DI1=intoff[2*i + 1],
            o_Q1=thermometer[2*i + 1],

            i_FCI=carry[i],
            o_FCO=carry[i+1],

            i_CLK=ClockSignal()
        )
        setattr(m.submodules, "delay_" + str(i), delay)

    return thermometer