code_8b10b.py

"""
IBM's 8b/10b Encoding

This scheme is used by a large number of protocols including Display Port, PCI
Express, Gigabit Ethernet, SATA and USB 3.

The encoding is built by combining an 5b/6b and 3b/4b encoding schemes and
guarantees both DC balance and enough bit transitions to recover the clock
signal.

Note: This encoding is *not* used by DVI/HDMI (that uses a *different* 8b/10b
scheme called TMDS).
"""

from migen import *


def disparity(word, nbits):
    n0 = 0
    n1 = 0
    for i in range(nbits):
        if word & (1 << i):
            n1 += 1
        else:
            n0 += 1
    return n1 - n0


def reverse_table_flip(inputs, flips, nbits):
    outputs = [None]*2**nbits

    for i, (word, flip) in enumerate(zip(inputs, flips)):
        if outputs[word] is not None:
            raise ValueError
        outputs[word] = i
        if flip:
            word_n = ~word & (2**nbits-1)
            if outputs[word_n] is not None:
                raise ValueError
            outputs[word_n] = i

    for i in range(len(outputs)):
        if outputs[i] is None:
            outputs[i] = 0

    return outputs


def reverse_table(inputs, nbits):
    outputs = [None]*2**nbits
    for i, word in enumerate(inputs):
        if outputs[word] is not None:
            raise ValueError
        outputs[word] = i
    for i in range(len(outputs)):
        if outputs[i] is None:
            outputs[i] = 0
    return outputs


# 5b6b

table_5b6b = [
    0b011000,
    0b100010,
    0b010010,
    0b110001,
    0b001010,
    0b101001,
    0b011001,
    0b000111,
    0b000110,
    0b100101,
    0b010101,
    0b110100,
    0b001101,
    0b101100,
    0b011100,
    0b101000,
    0b100100,
    0b100011,
    0b010011,
    0b110010,
    0b001011,
    0b101010,
    0b011010,
    0b000101,
    0b001100,
    0b100110,
    0b010110,
    0b001001,
    0b001110,
    0b010001,
    0b100001,
    0b010100,
]
table_5b6b_unbalanced = [bool(disparity(c, 6)) for c in table_5b6b]
table_5b6b_flip = list(table_5b6b_unbalanced)
table_5b6b_flip[7] = True

table_6b5b = reverse_table_flip(table_5b6b, table_5b6b_flip, 6)
# K.28
table_6b5b[0b001111] = 0b11100
table_6b5b[0b110000] = 0b11100

# 3b4b

table_3b4b = [
    0b0100,
    0b1001,
    0b0101,
    0b0011,
    0b0010,
    0b1010,
    0b0110,
    0b0001,  # primary D.x.7
]
table_3b4b_unbalanced = [bool(disparity(c, 4)) for c in table_3b4b]
table_3b4b_flip = list(table_3b4b_unbalanced)
table_3b4b_flip[3] = True

table_4b3b = reverse_table_flip(table_3b4b, table_3b4b_flip, 4)
# alternative D.x.7
table_4b3b[0b0111] = 0b0111
table_4b3b[0b1000] = 0b0111

table_4b3b_kn = reverse_table(table_3b4b, 4)
table_4b3b_kp = reverse_table([~x & 0b1111 for x in table_3b4b], 4)
# primary D.x.7 is not used
table_4b3b_kn[0b0001] = 0b000
table_4b3b_kn[0b1000] = 0b111
table_4b3b_kp[0b1110] = 0b000
table_4b3b_kp[0b0111] = 0b111


class SingleEncoder(Module):
    def __init__(self, lsb_first=False):
        self.d = Signal(8)
        self.k = Signal()
        self.disp_in = Signal()

        self.output = Signal(10)
        self.disp_out = Signal()

        # # #

        # stage 1: 5b/6b and 3b/4b encoding
        code5b = self.d[:5]
        code6b = Signal(6)
        code6b_unbalanced = Signal()
        code6b_flip = Signal()
        self.sync += [
            If(self.k & (code5b == 28),
                code6b.eq(0b110000),
                code6b_unbalanced.eq(1),
                code6b_flip.eq(1)
            ).Else(
                code6b.eq(Array(table_5b6b)[code5b]),
                code6b_unbalanced.eq(Array(table_5b6b_unbalanced)[code5b]),
                code6b_flip.eq(Array(table_5b6b_flip)[code5b])
            )
        ]

        code3b = self.d[5:]
        code4b = Signal(4)
        code4b_unbalanced = Signal()
        code4b_flip = Signal()
        self.sync += [
            code4b.eq(Array(table_3b4b)[code3b]),
            code4b_unbalanced.eq(Array(table_3b4b_unbalanced)[code3b]),
            If(self.k,
                code4b_flip.eq(1)
            ).Else(
                code4b_flip.eq(Array(table_3b4b_flip)[code3b])
            )
        ]

        alt7_rd0 = Signal()  # if disparity is -1, use alternative D.x.7
        alt7_rd1 = Signal()  # if disparity is +1, use alternative D.x.7
        self.sync += [
            alt7_rd0.eq(0),
            alt7_rd1.eq(0),
            If(code3b == 7,
                If((code5b == 17) | (code5b == 18) | (code5b == 20),
                    alt7_rd0.eq(1)),
                If((code5b == 11) | (code5b == 13) | (code5b == 14),
                    alt7_rd1.eq(1)),
                If(self.k,
                    alt7_rd0.eq(1),
                    alt7_rd1.eq(1)
                )
            )
        ]

        # stage 2 (combinatorial): disparity control
        output_6b = Signal(6)
        disp_inter = Signal()
        self.comb += [
            disp_inter.eq(self.disp_in ^ code6b_unbalanced),
            If(~self.disp_in & code6b_flip,
                output_6b.eq(~code6b)
            ).Else(
                output_6b.eq(code6b)
            )
        ]

        output_4b = Signal(4)
        self.comb += [
            If(~disp_inter & alt7_rd0,
                self.disp_out.eq(~disp_inter),
                output_4b.eq(0b0111)
            ).Elif(disp_inter & alt7_rd1,
                self.disp_out.eq(~disp_inter),
                output_4b.eq(0b1000)
            ).Else(
                self.disp_out.eq(disp_inter ^ code4b_unbalanced),
                If(~disp_inter & code4b_flip,
                    output_4b.eq(~code4b)
                ).Else(
                    output_4b.eq(code4b)
                )
            )
        ]

        output_msb_first = Signal(10)
        self.comb += output_msb_first.eq(Cat(output_4b, output_6b))
        if lsb_first:
            for i in range(10):
                self.comb += self.output[i].eq(output_msb_first[9-i])
        else:
            self.comb += self.output.eq(output_msb_first)


class Encoder(Module):
    def __init__(self, nwords=1, lsb_first=False):
        self.d = [Signal(8) for _ in range(nwords)]
        self.k = [Signal() for _ in range(nwords)]
        self.output = [Signal(10) for _ in range(nwords)]
        self.disparity = [Signal() for _ in range(nwords)]

        # # #

        encoders = [SingleEncoder(lsb_first) for _ in range(nwords)]
        self.submodules += encoders

        self.sync += encoders[0].disp_in.eq(encoders[-1].disp_out)
        for e1, e2 in zip(encoders, encoders[1:]):
            self.comb += e2.disp_in.eq(e1.disp_out)

        for d, k, output, disparity, encoder in \
                zip(self.d, self.k, self.output, self.disparity, encoders):
            self.comb += [
                encoder.d.eq(d),
                encoder.k.eq(k)
            ]
            self.sync += [
                output.eq(encoder.output),
                disparity.eq(encoder.disp_out)
            ]


class Decoder(Module):
    def __init__(self, lsb_first=False):
        self.input = Signal(10)
        self.d = Signal(8)
        self.k = Signal()

        # # #

        input_msb_first = Signal(10)
        if lsb_first:
            for i in range(10):
                self.comb += input_msb_first[i].eq(self.input[9-i])
        else:
            self.comb += input_msb_first.eq(self.input)

        code6b = input_msb_first[4:]
        code5b = Signal(5)
        code4b = input_msb_first[:4]
        code3b = Signal(3)
        self.sync += [
            self.k.eq(0),
            If(code6b == 0b001111,
                self.k.eq(1),
                code3b.eq(Array(table_4b3b_kn)[code4b])
            ).Elif(code6b == 0b110000,
                self.k.eq(1),
                code3b.eq(Array(table_4b3b_kp)[code4b])
            ).Else(
                If((code4b == 0b0111) | (code4b == 0b1000),  # D.x.A7/K.x.7
                    If((code6b != 0b100011) &
                       (code6b != 0b010011) &
                       (code6b != 0b001011) &
                       (code6b != 0b110100) &
                       (code6b != 0b101100) &
                       (code6b != 0b011100), self.k.eq(1))
                ),
                code3b.eq(Array(table_4b3b)[code4b])
            ),
            code5b.eq(Array(table_6b5b)[code6b])
        ]

        self.comb += self.d.eq(Cat(code5b, code3b))