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This allows the Macintosh to ensure that the IWM is kept up just-in-time with drive speed changes.
57 lines
2.0 KiB
C++
57 lines
2.0 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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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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samples_[sample_pointer_] = sample;
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++sample_pointer_;
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if(sample_pointer_ == samples_.size()) {
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sample_pointer_ = 0;
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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;
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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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// Sum all samples.
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// TODO: if the above is the correct test, do it on sample receipt rather than
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// bothering with an intermediate buffer.
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int sum = 0;
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for(auto s: samples_) {
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sum += s;
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}
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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(sum) / float(samples_.size());
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const float rotation_speed = (normalised_sum * 27.08f) - 259.0f;
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delegate_->drive_speed_accumulator_set_drive_speed(this, rotation_speed);
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}
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}
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