Detail of my sketch, Mk2_fasterControl_1a.ino
 *
 *  (initially released as Mk2_bothDisplays_1 in March 2014)
 * This sketch is for diverting suplus PV power to a dump load using a triac or  
 * Solid State Relay. It is based on the Mk2i PV Router code that I have posted in on  
 * the OpenEnergyMonitor forum.  The original version, and other related material, 
 * can be found on my Summary Page at www.openenergymonitor.org/emon/node/1757
 *
 * In this latest version, the pin-allocations have been changed to suit my 
 * PCB-based hardware for the Mk2 PV Router.  The integral voltage sensor is 
 * fed from one of the secondary coils of the transformer.  Current is measured 
 * via Current Transformers at the CT1 and CT1 ports.  
 * 
 * CT1 is for 'grid' current, to be measured at the grid supply point.
 * CT2 is for the load current, so that diverted energy can be recorded
 *
 * A persistence-based 4-digit display is supported. This can be driven in two
 * different ways, one with an extra pair of logic chips, and one without.  The 
 * appropriate version of the sketch must be selected by including or commenting 
 * out the "#define PIN_SAVING_HARDWARE" statement near the top of the code.
 *
 * September 2014: renamed as Mk2_bothDisplays_2, with these changes:
 * - cycleCount removed (was not actually used in this sketch, but could have overflowed);
 * - removal of unhelpful comments in the IO pin section;
 * - tidier initialisation of display logic in setup();
 * - addition of REQUIRED_EXPORT_IN_WATTS logic (useful as a built-in PV simulation facility);
 *
 * December 2014: renamed as Mk2_bothDisplays_3, with these changes:
 * - persistence check added for zero-crossing detection (polarityConfirmed)
 * - lowestNoOfSampleSetsPerMainsCycle added, to check for any disturbances
 *
 * December 2014: renamed as Mk2_bothDisplays_3a, with some typographical errors fixed.
 *
 * January 2016: renamed as Mk2_bothDisplays_3b, with a minor change in the ISR to 
 *   remove a timing uncertainty.
 *
 * January 2016: updated to Mk2_bothDisplays_3c:
 *   The variables to store the ADC results are now declared as "volatile" to remove 
 *   any possibility of incorrect operation due to optimisation by the compiler.
 *
 * February 2016: updated to Mk2_bothDisplays_4, with these changes:
 * - improvements to the start-up logic.  The start of normal operation is now 
 *    synchronised with the start of a new mains cycle.
 * - reduce the amount of feedback in the Low Pass Filter for removing the DC content
 *     from the Vsample stream. This resolves an anomaly which has been present since 
 *     the start of this project.  Although the amount of feedback has previously been 
 *     excessive, this anomaly has had minimal effect on the system's overall behaviour.
 * - removal of the unhelpful "triggerNeedsToBeArmed" mechanism
 * - tidying of the "confirmPolarity" logic to make its behaviour more clear
 * - SWEETZONE_IN_JOULES changed to WORKING_RANGE_IN_JOULES 
 * - change "triac" to "load" wherever appropriate
 *
 * January 2020: updated to Mk2_fasterControl_1 with these changes:
 * - Half way through each mains cycle, a prediction is made of the likely energy level at the
 *   end of the cycle.  That predicted value allows the triac to be switched at the +ve going 
 *   zero-crossing point rather than waiting for a further 10 ms.  These changes allow for 
 *   faster switching of the load.
 * - The range of the energy bucket has been reduced to one tenth of its former value. This
 *   allows the unit's operation to commence more rapidly whenever surplus power is available.
 * - controlMode is no longer selectable, the unit's operation being effectively hard-coded 
 *   as "Normal" rather than Anti-flicker. 
 * - Port D3 now supports an indicator which shows when the level in the energy bucket
 *   reaches either end of its range.  While the unit is actively diverting surplus power,
 *   it is vital that the level in the reduced capacity energy bucket remains within its 
 *   permitted range, hence the addition of this indicator.
 *   
 * February 2020: updated to Mk2_fasterControl_1a with these changes:
 * - removal of some redundant code in the logic for determining the next load state.
 * 
 *      Robin Emley
 *      www.Mk2PVrouter.co.uk
 */