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80 lines
3.7 KiB
C++
80 lines
3.7 KiB
C++
//
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// DriveSpeedAccumulator.cpp
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// Clock Signal
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//
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// Created by Thomas Harte on 01/06/2019.
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// Copyright © 2019 Thomas Harte. All rights reserved.
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//
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#include "DriveSpeedAccumulator.hpp"
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namespace {
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/*
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For knowledge encapsulate below, all credit goes to the MAME team. No original research here.
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Per their investigation, the bytes collected for PWM output feed a 6-bit LFSR, which then keeps
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output high until it eventually reaches a state of 0x20. The LFSR shifts rightward and taps bits
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0 and 1 as the new input into bit 5.
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I've therefore implemented the LFSR as below, feeding into a lookup table to calculate actual
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pulse widths from the values stored into the PWM buffer.
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*/
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template<uint8_t value> constexpr uint8_t lfsr() {
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if constexpr (value == 0x20 || !value) return 0;
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return 1+lfsr<(((value ^ (value >> 1))&1) << 5) | (value >> 1)>();
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}
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constexpr uint8_t pwm_lookup[] = {
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lfsr<0>(), lfsr<1>(), lfsr<2>(), lfsr<3>(), lfsr<4>(), lfsr<5>(), lfsr<6>(), lfsr<7>(),
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lfsr<8>(), lfsr<9>(), lfsr<10>(), lfsr<11>(), lfsr<12>(), lfsr<13>(), lfsr<14>(), lfsr<15>(),
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lfsr<16>(), lfsr<17>(), lfsr<18>(), lfsr<19>(), lfsr<20>(), lfsr<21>(), lfsr<22>(), lfsr<23>(),
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lfsr<24>(), lfsr<25>(), lfsr<26>(), lfsr<27>(), lfsr<28>(), lfsr<29>(), lfsr<30>(), lfsr<31>(),
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lfsr<32>(), lfsr<33>(), lfsr<34>(), lfsr<35>(), lfsr<36>(), lfsr<37>(), lfsr<38>(), lfsr<39>(),
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lfsr<40>(), lfsr<41>(), lfsr<42>(), lfsr<43>(), lfsr<44>(), lfsr<45>(), lfsr<46>(), lfsr<47>(),
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lfsr<48>(), lfsr<49>(), lfsr<50>(), lfsr<51>(), lfsr<52>(), lfsr<53>(), lfsr<54>(), lfsr<55>(),
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lfsr<56>(), lfsr<57>(), lfsr<58>(), lfsr<59>(), lfsr<60>(), lfsr<61>(), lfsr<62>(), lfsr<63>(),
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};
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}
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using namespace Apple::Macintosh;
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void DriveSpeedAccumulator::post_sample(uint8_t sample) {
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if(!delegate_) return;
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// An Euler-esque approximation is used here: just collect all
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// the samples until there is a certain small quantity of them,
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// then produce a new estimate of rotation speed and start the
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// buffer afresh.
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//
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// Note the table lookup here; see text above.
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sample_total_ += pwm_lookup[sample & 0x3f];
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++sample_count_;
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if(sample_count_ == samples_per_bucket) {
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// The below fits for a function like `a + bc`; it encapsultes the following
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// beliefs:
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//
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// (i) motor speed is proportional to voltage supplied;
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// (ii) with pulse-width modulation it's therefore proportional to the duty cycle;
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// (iii) the Mac pulse-width modulates whatever it reads from the disk speed buffer, as per the LFSR rules above;
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// (iv) ... subject to software pulse-width modulation of that pulse-width modulation.
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//
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// So, I believe current motor speed is proportional to a low-pass filtering of
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// the speed buffer. Which I've implemented very coarsely via 'large' bucketed-averages,
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// noting also that exact disk motor speed is always a little approximate.
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// The formula below was derived from observing values the Mac wrote into its
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// disk-speed buffer. Given that it runs a calibration loop before doing so,
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// I cannot guarantee the accuracy of these numbers beyond being within the
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// range that the computer would accept.
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const float normalised_sum = float(sample_total_) / float(samples_per_bucket);
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const float rotation_speed = (normalised_sum - 3.7f) * 17.6f;
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delegate_->drive_speed_accumulator_set_drive_speed(this, rotation_speed);
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sample_count_ = 0;
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sample_total_ = 0;
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}
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}
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