81 lines
2.5 KiB
Python
81 lines
2.5 KiB
Python
from __future__ import annotations
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from dataclasses import dataclass
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from typing import Iterable
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P9_ACK_BIT = 0x80
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P9_STROBE_BIT = 0x02
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@dataclass(frozen=True)
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class P9StrobeEvent:
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edge: str
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ddr: int
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dr: int
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data_bit: int
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bit7_output: bool
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class P9Bus:
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"""Small model for the ROM's P9 bit-banged serial handshake."""
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def __init__(self, ddr: int = 0x00, dr: int = 0x00, input_bits: Iterable[int] = ()) -> None:
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self.ddr = ddr & 0xFF
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self.dr_latch = dr & 0xFF
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self.input_bits: list[int] = [1 if bit else 0 for bit in input_bits]
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self.default_input_bit = 0
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self.strobe_edges: list[P9StrobeEvent] = []
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self.transmitted_bits: list[int] = []
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self.byte_candidates: list[int] = []
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def write_ddr(self, value: int) -> int:
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self.ddr = value & 0xFF
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return self.ddr
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def write_dr(self, value: int) -> int:
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previous = self.dr_latch
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self.dr_latch = value & 0xFF
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previous_strobe = bool(previous & P9_STROBE_BIT)
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current_strobe = bool(self.dr_latch & P9_STROBE_BIT)
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if previous_strobe != current_strobe:
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edge = "rising" if current_strobe else "falling"
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data_bit = 1 if self.dr_latch & P9_ACK_BIT else 0
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bit7_output = bool(self.ddr & P9_ACK_BIT)
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self.strobe_edges.append(P9StrobeEvent(edge, self.ddr, self.dr_latch, data_bit, bit7_output))
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if edge == "rising" and bit7_output:
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self._record_transmitted_bit(data_bit)
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return self.dr_latch
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def read_ddr(self) -> int:
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return self.ddr
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def read_dr(self) -> int:
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value = self.dr_latch
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if not (self.ddr & P9_ACK_BIT):
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if self.input_bits:
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input_bit = self.input_bits.pop(0)
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else:
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input_bit = self.default_input_bit
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if input_bit:
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value |= P9_ACK_BIT
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else:
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value &= ~P9_ACK_BIT
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return value & 0xFF
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def queue_input_bits(self, bits: Iterable[int]) -> None:
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self.input_bits.extend(1 if bit else 0 for bit in bits)
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def set_default_input_bit(self, bit: int) -> None:
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self.default_input_bit = 1 if bit else 0
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def _record_transmitted_bit(self, bit: int) -> None:
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self.transmitted_bits.append(bit)
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if len(self.transmitted_bits) % 8 == 0:
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byte = 0
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for data_bit in self.transmitted_bits[-8:]:
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byte = (byte << 1) | data_bit
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self.byte_candidates.append(byte)
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