// ============================================================================
// f245.v — 54F/74F245 Octal Bidirectional Transceiver (3-State Inputs/Outputs)
//
// Fairchild FAST (Advanced Schottky TTL)
// Source: docs/devices/54F74F245.txt (1985 Fairchild FAST Data Book,
//         pages 4-158 ... 4-159; the 1980 data book carries the 'F245 in its
//         Section 3 selection guide only. Released data sheet.)
//
// Eight channels, each a back-to-back pair of non-inverting 3-state buffers
// between the bidirectional bus pins A0..A7 and B0..B7. A single T//R input
// selects which buffer of the pair is active (HIGH = Transmit = A -> B,
// LOW = Receive = B -> A); /OE gates both directions, HIGH disabling every
// buffer to High-Z regardless of T//R.
//
// Truth table (per channel):
//   OE_n=L, TR=L -> B drives A (A = B);
//   OE_n=L, TR=H -> A drives B (B = A);
//   OE_n=H        -> both sides High-Z.
//
// Unlike 'F242/'F243, there is no independent-enable hazard here: T//R is a
// single select bit, so exactly one direction (or neither, under /OE HIGH)
// can ever be active. See src/f242.v's and src/f243.v's header comments for
// the full backstory on why the T//R-style transceivers are structurally
// immune to the both-directions-enabled contention 'F242/'F243 carry.
//
// Timing values from the data sheet AC Characteristics table, T_A = +25 C,
// V_CC = +5.0 V, C_L = 50 pF column, min:typ:max ns. The sheet's second
// column, 74F over the commercial T_A/V_CC range, gives min/max only:
//
//   tPLH/tPHL (An to Bn or Bn to An)  2.5 / 7.0 ns  /  2.5 / 7.0 ns
//   tPZH (Output Enable Time)         3.0 / 8.0 ns
//   tPZL (Output Enable Time)         3.5 / 9.0 ns
//   tPHZ (Output Disable Time)        3.0 / 7.5 ns
//   tPLZ (Output Disable Time)        2.0 / 7.5 ns
//
// A single Prop Delay row in the data sheet covers both directions (An to Bn
// and Bn to An share the same figures) — the buffers are symmetric.
//
// DC CHARACTERISTICS gives ICCH/ICCL/ICCZ power-supply-current figures only;
// this project's models don't simulate current draw.
//
// Ports are scalar and named after the data sheet pin names: Icarus Verilog
// does not fully support multi-bit (parallel) specify path connections, so
// vector ports would get incorrect per-bit delays. Each bus pin is a path
// DESTINATION in one direction and a SOURCE in the other, so the specify
// block carries one statement per scalar pin per direction.
// ============================================================================
`timescale 1ns/100ps

module f245 (
    input  wire oe_n,                       // output enable (active LOW)
    input  wire tr,                         // direction select: H = A->B (Transmit), L = B->A (Receive)
    inout  wire a0, a1, a2, a3, a4, a5, a6, a7, // bus A (bidirectional)
    inout  wire b0, b1, b2, b3, b4, b5, b6, b7  // bus B (bidirectional)
);

    // A -> B (Transmit, non-inverting), active when OE_n LOW and TR HIGH
    wire drive_ab = ~oe_n & tr;
    // B -> A (Receive, non-inverting), active when OE_n LOW and TR LOW
    wire drive_ba = ~oe_n & ~tr;

    assign b0 = drive_ab ? a0 : 1'bz;
    assign b1 = drive_ab ? a1 : 1'bz;
    assign b2 = drive_ab ? a2 : 1'bz;
    assign b3 = drive_ab ? a3 : 1'bz;
    assign b4 = drive_ab ? a4 : 1'bz;
    assign b5 = drive_ab ? a5 : 1'bz;
    assign b6 = drive_ab ? a6 : 1'bz;
    assign b7 = drive_ab ? a7 : 1'bz;

    assign a0 = drive_ba ? b0 : 1'bz;
    assign a1 = drive_ba ? b1 : 1'bz;
    assign a2 = drive_ba ? b2 : 1'bz;
    assign a3 = drive_ba ? b3 : 1'bz;
    assign a4 = drive_ba ? b4 : 1'bz;
    assign a5 = drive_ba ? b5 : 1'bz;
    assign a6 = drive_ba ? b6 : 1'bz;
    assign a7 = drive_ba ? b7 : 1'bz;

    specify
        // T_A = +25 C, V_CC = +5.0 V, C_L = 50 pF, min:typ:max ns.

        // Prop Delay An to Bn or Bn to An (data sheet: tPLH 2.5/4.2/6.0,
        // tPHL 2.5/4.6/6.0) — one row covers both directions.
        specparam tlh = 2.5:4.2:6.0;
        specparam thl = 2.5:4.6:6.0;

        // Output Enable Time (data sheet: tPZH 3.0/5.3/7.0, tPZL 3.5/6.0/8.0)
        specparam tzh = 3.0:5.3:7.0;
        specparam tzl = 3.5:6.0:8.0;

        // Output Disable Time (data sheet: tPHZ 3.0/5.0/6.5, tPLZ 2.0/5.0/6.5)
        specparam thz = 3.0:5.0:6.5;
        specparam tlz = 2.0:5.0:6.5;

        // A -> B direction: bus B pins are the path destinations
        (a0, tr, oe_n => b0) = (tlh, thl, tlz, tzh, thz, tzl);
        (a1, tr, oe_n => b1) = (tlh, thl, tlz, tzh, thz, tzl);
        (a2, tr, oe_n => b2) = (tlh, thl, tlz, tzh, thz, tzl);
        (a3, tr, oe_n => b3) = (tlh, thl, tlz, tzh, thz, tzl);
        (a4, tr, oe_n => b4) = (tlh, thl, tlz, tzh, thz, tzl);
        (a5, tr, oe_n => b5) = (tlh, thl, tlz, tzh, thz, tzl);
        (a6, tr, oe_n => b6) = (tlh, thl, tlz, tzh, thz, tzl);
        (a7, tr, oe_n => b7) = (tlh, thl, tlz, tzh, thz, tzl);

        // B -> A direction: the same physical pins, opposite roles —
        // bus A pins are the path destinations
        (b0, tr, oe_n => a0) = (tlh, thl, tlz, tzh, thz, tzl);
        (b1, tr, oe_n => a1) = (tlh, thl, tlz, tzh, thz, tzl);
        (b2, tr, oe_n => a2) = (tlh, thl, tlz, tzh, thz, tzl);
        (b3, tr, oe_n => a3) = (tlh, thl, tlz, tzh, thz, tzl);
        (b4, tr, oe_n => a4) = (tlh, thl, tlz, tzh, thz, tzl);
        (b5, tr, oe_n => a5) = (tlh, thl, tlz, tzh, thz, tzl);
        (b6, tr, oe_n => a6) = (tlh, thl, tlz, tzh, thz, tzl);
        (b7, tr, oe_n => a7) = (tlh, thl, tlz, tzh, thz, tzl);
    endspecify

endmodule