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216 lines
7.0 KiB
C++
216 lines
7.0 KiB
C++
//
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// Flywheel.hpp
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// Clock Signal
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//
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// Created by Thomas Harte on 11/02/2016.
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// Copyright 2016 Thomas Harte. All rights reserved.
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//
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#ifndef Flywheel_hpp
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#define Flywheel_hpp
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#include <cassert>
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#include <cstdlib>
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#include <cstdint>
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namespace Outputs {
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namespace CRT {
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/*!
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Provides timing for a two-phase signal consisting of a retrace phase followed by a scan phase,
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announcing the start and end of retrace and providing the abiliy to read the current
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scanning position.
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The @c Flywheel will attempt to converge with timing implied by synchronisation pulses.
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*/
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struct Flywheel {
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/*!
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Constructs an instance of @c Flywheel.
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@param standard_period The expected amount of time between one synchronisation and the next.
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@param retrace_time The amount of time it takes to complete a retrace.
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@param sync_error_window The permitted deviation of sync timings from the norm.
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*/
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Flywheel(int standard_period, int retrace_time, int sync_error_window) :
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standard_period_(standard_period),
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retrace_time_(retrace_time),
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sync_error_window_(sync_error_window),
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counter_before_retrace_(standard_period - retrace_time),
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expected_next_sync_(standard_period) {}
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enum SyncEvent {
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/// Indicates that no synchronisation events will occur in the queried window.
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None,
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/// Indicates that the next synchronisation event will be a transition into retrce.
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StartRetrace,
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/// Indicates that the next synchronisation event will be a transition out of retrace.
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EndRetrace
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};
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/*!
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Asks the flywheel for the first synchronisation event that will occur in a given time period,
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indicating whether a synchronisation request occurred at the start of the query window.
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@param sync_is_requested @c true indicates that the flywheel should act as though having
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received a synchronisation request now; @c false indicates no such event was detected.
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@param cycles_to_run_for The number of cycles to look ahead.
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@param cycles_advanced After this method has completed, contains the amount of time until
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the returned synchronisation event.
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@returns The next synchronisation event.
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*/
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inline SyncEvent get_next_event_in_period(bool sync_is_requested, int cycles_to_run_for, int *cycles_advanced) {
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// do we recognise this hsync, thereby adjusting future time expectations?
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if(sync_is_requested) {
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if(counter_ < sync_error_window_ || counter_ > expected_next_sync_ - sync_error_window_) {
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const int time_now = (counter_ < sync_error_window_) ? expected_next_sync_ + counter_ : counter_;
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expected_next_sync_ = (3*expected_next_sync_ + time_now) >> 2;
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} else {
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++number_of_surprises_;
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if(counter_ < retrace_time_ + (expected_next_sync_ >> 1)) {
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expected_next_sync_ = (3*expected_next_sync_ + standard_period_ + sync_error_window_) >> 2;
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} else {
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expected_next_sync_ = (3*expected_next_sync_ + standard_period_ - sync_error_window_) >> 2;
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}
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}
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}
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SyncEvent proposed_event = SyncEvent::None;
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int proposed_sync_time = cycles_to_run_for;
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// End an ongoing retrace?
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if(counter_ < retrace_time_ && counter_ + proposed_sync_time >= retrace_time_) {
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proposed_sync_time = retrace_time_ - counter_;
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proposed_event = SyncEvent::EndRetrace;
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}
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// Start a retrace?
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if(counter_ + proposed_sync_time >= expected_next_sync_) {
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proposed_sync_time = expected_next_sync_ - counter_;
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proposed_event = SyncEvent::StartRetrace;
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}
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*cycles_advanced = proposed_sync_time;
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return proposed_event;
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}
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/*!
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Advances a nominated amount of time, applying a previously returned synchronisation event
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at the end of that period.
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@param cycles_advanced The amount of time to run for.
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@param event The synchronisation event to apply after that period.
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*/
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inline void apply_event(int cycles_advanced, SyncEvent event) {
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// In debug builds, perform a sanity check for counter overflow.
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#ifndef NDEBUG
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const int old_counter = counter_;
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#endif
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counter_ += cycles_advanced;
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assert(old_counter <= counter_);
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switch(event) {
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default: return;
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case StartRetrace:
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counter_before_retrace_ = counter_ - retrace_time_;
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counter_ = 0;
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return;
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}
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}
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/*!
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Returns the current output position; while in retrace this will go down towards 0, while in scan
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it will go upward.
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@returns The current output position.
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*/
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inline int get_current_output_position() {
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if(counter_ < retrace_time_) {
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const int retrace_distance = int((int64_t(counter_) * int64_t(standard_period_)) / int64_t(retrace_time_));
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if(retrace_distance > counter_before_retrace_) return 0;
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return counter_before_retrace_ - retrace_distance;
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}
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return counter_ - retrace_time_;
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}
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/*!
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@returns the amount of time since retrace last began. Time then counts monotonically up from zero.
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*/
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inline int get_current_time() {
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return counter_;
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}
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/*!
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@returns whether the output is currently retracing.
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*/
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inline bool is_in_retrace() {
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return counter_ < retrace_time_;
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}
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/*!
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@returns the expected length of the scan period (excluding retrace).
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*/
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inline int get_scan_period() {
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return standard_period_ - retrace_time_;
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}
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/*!
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@returns the expected length of a complete scan and retrace cycle.
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*/
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inline int get_standard_period() {
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return standard_period_;
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}
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/*!
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@returns the number of synchronisation events that have seemed surprising since the last time this method was called;
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a low number indicates good synchronisation.
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*/
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inline int get_and_reset_number_of_surprises() {
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const int result = number_of_surprises_;
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number_of_surprises_ = 0;
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return result;
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}
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/*!
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@returns `true` if a sync is expected soon or if the time at which it was expected (or received) was recent.
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*/
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inline bool is_near_expected_sync() {
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return
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(counter_ < (standard_period_ / 100)) ||
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(counter_ >= expected_next_sync_ - (standard_period_ / 100));
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}
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private:
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const int standard_period_; // the normal length of time between syncs
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const int retrace_time_; // a constant indicating the amount of time it takes to perform a retrace
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const int sync_error_window_; // a constant indicating the window either side of the next expected sync in which we'll accept other syncs
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int counter_ = 0; // time since the _start_ of the last sync
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int counter_before_retrace_; // the value of _counter immediately before retrace began
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int expected_next_sync_; // our current expection of when the next sync will be encountered (which implies velocity)
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int number_of_surprises_ = 0; // a count of the surprising syncs
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/*
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Implementation notes:
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Retrace takes a fixed amount of time and runs during [0, _retrace_time).
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For the current line, scan then occurs from [_retrace_time, _expected_next_sync), at which point
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retrace begins and the internal counter is reset.
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All synchronisation events that occur within (-_sync_error_window, _sync_error_window) of the
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expected synchronisation time will cause a proportional adjustment in the expected time for the next
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synchronisation. Other synchronisation events are clamped as though they occurred in that range.
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*/
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};
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}
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}
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#endif /* Flywheel_hpp */
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