Events/2020/AVR: Difference between revisions

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 Have you ever wondered what a little Arduino is really capable of? Ever wanted to ''really'' understand a chip? Learn lots of nitty-gritty details about the Microchip AVR architecture, the processor architecture that powers the Arduino and many other products.
+Slides: [[Media:AVR-v0.pptx]]
+Video: https://video.hacksburg.org/videos/watch/8c9e3a8d-41d5-4c56-8eef-814f9966ce3f
 == Code Samples ==
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 // --- setup() and loop()
+void setup() {
+  doLowLevelConfig();
+  pinMode(2, INPUT_PULLUP); // Changing this pin requires changes in doLowLevelConfig()
+  pinMode(LED_BUILTIN, OUTPUT);
+  digitalWrite(LED_BUILTIN, LOW);
+  // Initialize communication with PC.
+  Serial.begin(115200);
+  Serial.println(getHeader());
+}
+void loop() {
+  if (readComplete) {
+    readComplete = false;
+	  // TODO - format output using C-style strings instead
+    sample = sampleTime;
+    for (uint8_t i = 0; i <= maxIndex; i++) {
+      sample += "," + String(rawADC[i]);
+    }
+    Serial.println(sample); // Blocking? Technically, but this is usually fast.
+  }
+  // Handle button presses (or releases in our case).
+  if (btnReleased) {
+    btnReleased = false;
+    if (recording) {
+      // If we're recording, stop.
+      bitClear(ADCSRA, ADEN);
+      recording = false;
+    }
+    else {
+      // If we're not recording, start.
+      recording = true;
+      bitSet(ADCSRA, ADEN);
+    }
+  }
+}
+// --- Subsidiary functions
+String getHeader() {
+  // TODO - re-implement using C-style strings
+  String header = "microseconds";
+  String colName;
+  for (byte i = 0; i <= maxIndex; i++) {
+    switch (channelSequence[i]) {
+      case A_0 | ADCref: colName = ",CH_0"; break;
+      case A_1 | ADCref: colName = ",CH_1"; break;
+      case A_2 | ADCref: colName = ",CH_2"; break;
+      case A_3 | ADCref: colName = ",CH_3"; break;
+      case A_4 | ADCref: colName = ",CH_4"; break;
+      case A_5 | ADCref: colName = ",CH_5"; break;
+      default: colName = ",unknown"; break;
+    }
+    header += colName;
+  }
+  return header;
+}
+void doLowLevelConfig() {
+  noInterrupts();
+  // Combine the ADC reference bits with the channel selection bits. Each value
+  // in channelSequence should be the complete value of ADMUX for recording the
+  // desired channel. Tables 23-3 and 23-4.
+  // TODO - make this separate from the user-specified channel selection array?
+  for (byte i = 0; i <= maxIndex; i++){
+    channelSequence[i] = channelSequence[i] | ADCref;
+  }
+  // Configure the Timer 0 Compare Match A interrupt.
+  OCR0A = 64;             // Keep away from the overflow (0) so micros() can update.
+  bitSet(TIMSK0, OCIE0A); // Enable the interrupt.
+  // Configure Timer 1 Compare Match A interrupt to trigger @ freq Hz
+  TCCR1A = TCCR1B = TCNT1 = 0;      // Clear Timer 1 settings.
+  TCCR1B = bit(WGM12) | bit(CS12);  // Count @ 62500 Hz: CTC mode, prescaler = 256. Tables 15-5 and 15-5.
+  OCR1A = CTCmatch;                 // Count up to CTCmatch. Section 15.11.6.
+  bitSet(TIMSK1, OCIE1A);           // Enable Output Compare Match A interrupt. Section 15.11.8.
+  // Enable the ADC-conversion-complete interrupt, configure ADC for
+  // single-conversion mode, and disable the ADC. Section 23.9.2.
+  ADCSRA |=  bit(ADIE) | bit(ADIF);     // sets these bits
+  ADCSRA &= ~(bit(ADATE) | bit(ADEN));  // clears these bits
+  ADMUX = channelSequence[0];
+  interrupts();
+}
+// --- Interrupt service routines
+ISR(TIMER0_COMPA_vect) {
+  // Debounce the button. See comments in 'AccelCalib.ino'.
+  static volatile uint16_t btnHistory = 0xFFFF;
+  btnHistory = (btnHistory << 1) | bitRead(PIND, PIND2);
+  if (btnHistory =/= 0x7FFF) btnReleased = true; //MediaWiki won't let me post the equality operator
+}
+ISR(TIMER1_COMPA_vect) {
+  // Begin a conversion on the first channel in the sequence and record the time.
+  readComplete = false;
+  index = 0;
+  ADMUX = channelSequence[index];
+  bitSet(ADCSRA, ADSC); // ADSC =/= "Analog-Digital converter Start //MediaWiki won't let me post the equality operatorConversion"
+  sampleTime = micros();
+}
+ISR(ADC_vect) {
+  // Store the conversion result.
+  rawADC[index] = ADC;
+  // Check if all channels have been read. If not, start the next one.
+  if (index =/= maxIndex) readComplete = true; //MediaWiki won't let me post the equality operator
+  else {
+    index++;
+    ADMUX = channelSequence[index];
+    bitSet(ADCSRA, ADSC);
+  }
+}
 </pre>