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230 lines
6.9 KiB
C++
230 lines
6.9 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 <stdlib.h>
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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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/*!
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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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*/
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Flywheel(unsigned int standard_period, unsigned int retrace_time) :
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_standard_period(standard_period),
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_retrace_time(retrace_time),
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_sync_error_window(standard_period >> 7),
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_counter(0),
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_expected_next_sync(standard_period),
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_counter_before_retrace(standard_period - retrace_time),
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_number_of_surprises(0) {}
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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, unsigned int cycles_to_run_for, unsigned int *cycles_advanced)
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{
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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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{
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if(_counter < _sync_error_window || _counter > _expected_next_sync - _sync_error_window)
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{
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unsigned 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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}
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else
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{
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_number_of_surprises++;
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if(_counter < _retrace_time + (_expected_next_sync >> 1))
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{
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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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else
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{
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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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unsigned int proposed_sync_time = cycles_to_run_for;
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// will we end an ongoing retrace?
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if(_counter < _retrace_time && _counter + proposed_sync_time >= _retrace_time)
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{
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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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// will we start a retrace?
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if(_counter + proposed_sync_time >= _expected_next_sync)
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{
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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(unsigned int cycles_advanced, SyncEvent event)
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{
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_counter += cycles_advanced;
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switch(event)
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{
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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 unsigned int get_current_output_position()
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{
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if(_counter < _retrace_time)
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{
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unsigned int retrace_distance = (_counter * _standard_period) / _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 unsigned int get_current_time()
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{
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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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{
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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 unsigned int get_scan_period()
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{
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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 unsigned int get_standard_period()
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{
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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 unsigned int get_and_reset_number_of_surprises()
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{
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unsigned 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 the time at which it was expected was recent.
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*/
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inline bool is_near_expected_sync()
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{
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return abs((int)_counter - (int)_expected_next_sync) < (int)_standard_period / 50;
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}
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private:
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unsigned int _standard_period; // the normal length of time between syncs
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const unsigned int _retrace_time; // a constant indicating the amount of time it takes to perform a retrace
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const unsigned 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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unsigned int _counter; // time since the _start_ of the last sync
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unsigned int _counter_before_retrace; // the value of _counter immediately before retrace began
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unsigned int _expected_next_sync; // our current expection of when the next sync will be encountered (which implies velocity)
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unsigned int _number_of_surprises; // 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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