#!/usr/bin/env python3
"""Exhaustive HC11 SCI rate scan vs 16550-standard rates, per candidate
oscillator can. Evidence behind the "Standard-rate (16550) feasibility"
section of RIOv4_2-ANALYSIS.md.

HC11 SCI baud = E / (16 * SCP * 2^SCR), E = can/4,
SCP (prescale) in {1,3,4,13}, SCR (divider) 2^0..2^7.
16550 standard = 1.8432MHz / (16 * divisor), divisor integer >= 1.
A pairing is workable if |error| <= 2.0% (conservative per-link UART
budget; >3% is hopeless, 2-3% marginal). From the stock 8 MHz can the
only in-tolerance pairings are 9615<->9600 (stock, the /13 prescale)
and 10417<->115200/11 (+8.3% throughput, pointless) -- every faster
rate carries the +8.51% residue of 125000/115200 = 625/576.
"""
STD = [9600, 14400, 19200, 28800, 38400, 57600, 115200]  # 16550-reachable
XTALS = {
    "8.000000 (stock)": 8_000_000,
    "7.372800": 7_372_800,
    "9.830400": 9_830_400,
    "11.059200": 11_059_200,
    "12.288000": 12_288_000,
    "14.745600": 14_745_600,
}
for name, x in XTALS.items():
    e = x / 4
    print(f"\n=== can {name} MHz -> E = {e/1e6:.4f} MHz ===")
    rows = []
    for scp, scpbits in [(1, 0b00), (3, 0b01), (4, 0b10), (13, 0b11)]:
        for scr in range(8):
            rate = e / (16 * scp * (1 << scr))
            if rate < 9000:
                continue
            baudreg = (scpbits << 4) | scr
            best = min(STD, key=lambda s: abs(rate - s) / s)
            err = (rate - best) / best * 100
            cyc = e / (rate / 10)  # E-cycles per 10-bit byte
            rows.append((rate, baudreg, best, err, cyc))
    for rate, baudreg, best, err, cyc in sorted(rows, reverse=True):
        flag = "  <== EXACT" if abs(err) < 0.01 else (
               "  <-- ok" if abs(err) <= 2.0 else "")
        print(f"  BAUD=${baudreg:02X}  {rate:9.1f}  vs {best:6d}  "
              f"err {err:+6.2f}%  {cyc:6.0f} E-cyc/byte{flag}")