Make h/w SPI still better and some cosmetic fixes

Open_Theremin_V3/Open_Theremin_V3.ino

@@ -1,7 +1,7 @@
 /*
  *  Open.Theremin control software for Arduino UNO
- *  Version 3.0
- *  Copyright (C) 2010-2016 by Urs Gaudenz
+ *  Version 3.1
+ *  Copyright (C) 2010-2020 by Urs Gaudenz
  *
  *  Open.Theremin control software is free software: you can redistribute it and/or
  *  modify it under the terms of the GNU General Public License as published
@@ -19,6 +19,7 @@
  *  With important contributions by 
  *  David Harvey
  *  Michael Margolis
+ *  "Theremingenieur" Thierry Frenkel
  */
  
 /**
@@ -76,4 +77,3 @@ void setup() {
 void loop() {
   app.loop();
 }
-

Open_Theremin_V3/SPImcpDAC.h

@@ -45,18 +45,20 @@ static inline void SPImcpDACinit()
 	HW_SPI_DDR |= _BV(HW_SPI_MOSI_BIT);
 	// initialize the hardware SPI registers
 	SPCR = _BV(SPE) | _BV(MSTR);	// no interrupt, SPI enable, MSB first, SPI master, SPI mode 0, clock = f_osc/4 (maximum)
-	SPSR |= SPI2X;					// double the SPI clock, ideally we get 8 MHz, so that a 16bit word goes out in 2us plus only a small overhead
+	SPSR = _BV(SPI2X);  // double the SPI clock, ideally we get 8 MHz, so that a 16bit word goes out in 3.5us (5.6us when called from an interrupt) including CS asserting/deasserting
 }
 
 static inline void SPImcpDACtransmit(uint16_t data)
 {
 	// Send highbyte and wait for complete
 	SPDR = highByte(data);
-	while (!(SPSR && _BV(SPIF)))
+  asm("nop");
+	while (!(SPSR & _BV(SPIF)))
 		;
 	// Send lowbyte and wait for complete
 	SPDR = lowByte(data);
-	while (!(SPSR && _BV(SPIF)))
+  asm("nop");
+	while (!(SPSR & _BV(SPIF)))
 		;
 }
 
@@ -69,12 +71,12 @@ static inline void SPImcpDAClatch()
 static inline void SPImcpDACsend(uint16_t data)
 {
 	MCP_DAC_CS_PORT &= ~_BV(MCP_DAC_CS_BIT);
-	// Sanitize input data and add DAC config MSBs
-	data &= 0x0FFF;
-	data |= 0x7000;
+  // Sanitize input data and add DAC config MSBs
+  data &= 0x0FFF;
+  data |= 0x7000;
 	SPImcpDACtransmit(data);
 	MCP_DAC_CS_PORT |= _BV(MCP_DAC_CS_BIT);
-	// Do not latch immpediately, let's do it at the beginning of the next interrupt to get consistent timing
+	// Do not latch immpediately, let's do it at the very beginning of the next interrupt to get consistent timing
 }
 
 static inline void SPImcpDAC2Asend(uint16_t data)
@@ -99,4 +101,4 @@ static inline void SPImcpDAC2Bsend(uint16_t data)
 	SPImcpDAClatch();
 }
 
-#endif
+#endif

Open_Theremin_V3/ihandlers.cpp

@@ -106,14 +106,15 @@ void ihInitialiseVolumeMeasurement() //Measurement of variable frequency oscilla
 
 /* Externaly generated 31250 Hz Interrupt for WAVE generator (32us) */
 ISR (INT1_vect) {
-  // Interrupt takes up a total of 16us plus overhead when interrupted itself.
-// Added by ThF 20200419
-#ifdef TH_DEBUG
-	HW_LED2_ON;
-#endif
+  // Interrupt takes up normally 14us but can take up to 22us when interrupted by another interrupt.
+
+  // Added by ThF 20200419
+  #ifdef TH_DEBUG
+    HW_LED2_ON;
+  #endif
 
-	// Latch previously written DAC value:
-	SPImcpDAClatch();
+  // Latch previously written DAC value:
+  SPImcpDAClatch();
 
 	disableInt1(); // Disable External Interrupt INT1 to avoid recursive interrupts
 	// Enable Interrupts to allow counter 1 interrupts
@@ -126,18 +127,18 @@ ISR (INT1_vect) {
 #if CV_ENABLED                                 // Generator for CV output
 
  vPointerIncrement = min(vPointerIncrement, 4095);
- mcpDacSend(vPointerIncrement);        //Send result to Digital to Analogue Converter (audio out) (9.6 us)
+ mcpDacSend(vPointerIncrement);        //Send result to Digital to Analogue Converter (audio out) (5.5 us)
 
 #else   //Play sound
 
 	// Read next wave table value
 	waveSample = (int16_t)pgm_read_word_near(wavetables[vWavetableSelector] + offset);
 
-	scaledSample = ((int32_t)waveSample * (uint32_t)vScaledVolume) >> 16;
+	scaledSample = ((int32_t)waveSample * (uint32_t)vScaledVolume) >> 16; // The compiler optimizes this better than any assembly written by hand !!!
 
-	SPImcpDACsend(scaledSample + MCP_DAC_BASE); //Send result to Digital to Analogue Converter (audio out) (6 us)
+	SPImcpDACsend(scaledSample + MCP_DAC_BASE); //Send result to Digital to Analogue Converter (audio out) (5.5 us)
 
-	pointer += vPointerIncrement; // increment table pointer (ca. 2us)
+	pointer += vPointerIncrement; // increment table pointer
 
 #endif                          //CV play sound
   incrementTimer();               // update 32us timer