Resetting ADC control flags after continuous read (ATMega2560)/Arduino - c++

Question

I am using an ATMega2560 with a resistance touch screen (uses the ADC with analogRead commands) – I also use other ADC pins with analogRead (NTC temp sensor). This all works fine.
I have recently added a function that requires me to get ADC readings at known intervals and decided that using the automatic sampling was best – I found the code to do this online and it works well for the function it is required for.
My issue is, after I do the continuous sampling, I cannot get the ADC analogRead values to return the same as they where before the continuous sample.
I have checked the datasheet and it says the initial value for the ADCSRA, ADCSRB & ADMUX = 0 – I have tried this and quite a few other things with no success.
I have attached some code that does the same as my program – i.e. analogRead returns a different value after the continuous sample.
byte samplesADC[100];
int samplesADCCount = 0;
bool ADCEnd = false;
void startADCAcquire(){
int dacChan = 14;
Serial.println("Start ADC Aquire: ");
cli();//diable interrupts
//set up continuous sampling of analog pin 0
//clear ADCSRA and ADCSRB registers
ADCSRA = 0;
ADCSRB = 0;
ADMUX |= dacChan&0x07;
if (dacChan >= 8)
ADCSRB |= _BV(MUX5);
else
ADCSRB &= ~_BV(MUX5);
ADMUX |= (1 << REFS0); //set reference voltage
ADMUX |= (1 << ADLAR); //left align the ADC value- so we can read highest 8 bits from ADCH register only
ADCSRA |= (1 << ADPS2) | (1 << ADPS0); //set ADC clock with 32 prescaler- 16mHz/32=500kHz
ADCSRA |= (1 << ADATE); //enabble auto trigger
ADCSRA |= (1 << ADIE); //enable interrupts when measurement complete
ADCSRA |= (1 << ADEN); //enable ADC
ADCSRA |= (1 << ADSC); //start ADC measurements
sei();//enable interrupts
}
void stopADCAcquire(){
Serial.println("");
Serial.println("Stop ADC Acquire");
cli();//diable interrupts
//set up continuous sampling of analog pin 0
//clear ADCSRA and ADCSRB registers
ADCSRA = 0;
ADCSRB = 0;
ADMUX = 0;
// ADCSRB &= ~_BV(MUX5);
// ADMUX &= ~(1 << ADLAR);
// ADCSRA &= ~(1 << ADSC); // disable adc interrupt
sei();//enable interrupts
}
void setup(){
Serial.begin(9600);
Serial.println("");
Serial.println("START");
Serial.print("Pin A2 (1): ");
Serial.println(analogRead(A2));
startADCAcquire();
int i;
while(ADCEnd == false){
i++;
}
while ((UCSR0A & _BV (TXC0)) == 0)
{}
Serial.print("Pin A2 (2): ");
Serial.println(analogRead(A2));
Serial.println("END");
}
ISR(ADC_vect) {//when new ADC value ready
samplesADC[samplesADCCount] = ADCH;
Serial.print(samplesADC[samplesADCCount]);
Serial.print(", ");
samplesADCCount++;
if(samplesADCCount == 100){
stopADCAcquire();
ADCEnd = true;
}
}
void loop(){
}
Output:
START
Pin A2 (1): 0
Start ADC Aquire:
127, 127, 127...... (x100)
Stop ADC Acquire
Pin A2 (2): 510
END

Answer 1

Thanks #hcheung...
So I'm not sure if you pointed me to the interrupt page, with reference to the code issues around my interrupt handlers - I think I have sorted these by using volatile variables and removing excess code/print statements from the ISR handler. So thanks for the link - it was a helpful reminder.
However this did nothing to resolve my issue and the output remained the same! On the interrupt page I noticed some code where registers are copied to a buffer and then reassigned later. I decided to try this and copied the ADCSRA, ADCSRB & ADMUX registers on program load and reassigned them after the AD scan.
This also failed - Then as last ditch I copied the registers just before the call to start the AD scan (i.e afer analogRead had been called) and it worked. I am now able to use the analogRead command after my continuous read.
I attach updated code that works - i.e analogRead works after being in continuous mode.
volatile byte samplesADC[100];
volatile int samplesADCCount = 0;
volatile bool ADCEnd = false;
uint8_t ADCSRASave;
uint8_t ADCSRBSave;
uint8_t ADMUXSave;
void startADCAcquire(){
int dacChan = 14;
ADCSRASave = ADCSRA;
ADCSRBSave = ADCSRB;
ADMUXSave = ADMUX;
cli();//diable interrupts
//set up continuous sampling of analog pin 0
//clear ADCSRA and ADCSRB registers
ADCSRA = 0;
ADCSRB = 0;
ADMUX |= dacChan&0x07;
if (dacChan >= 8)
ADCSRB |= _BV(MUX5);
else
ADCSRB &= ~_BV(MUX5);
ADMUX |= (1 << REFS0); //set reference voltage
ADMUX |= (1 << ADLAR); //left align the ADC value- so we can read highest 8 bits from ADCH register only
ADCSRA |= (1 << ADPS2) | (1 << ADPS0); //set ADC clock with 32 prescaler- 16mHz/32=500kHz
ADCSRA |= (1 << ADATE); //enabble auto trigger
ADCSRA |= (1 << ADIE); //enable interrupts when measurement complete
ADCSRA |= (1 << ADEN); //enable ADC
ADCSRA |= (1 << ADSC); //start ADC measurements
sei();//enable interrupts
}
void stopADCAcquire(){
cli();//diable interrupts
ADCSRA = ADCSRASave; //0;
ADCSRB = ADCSRBSave; //0;
ADMUX = ADMUXSave; //;
sei();//enable interrupts
}
void setup(){
Serial.begin(9600);
Serial.println("");
Serial.println("START");
Serial.print("Pin A2 (1): ");
Serial.println(analogRead(A14));
Serial.println("Start ADC Aquire: ");
while ((UCSR0A & _BV (TXC0)) == 0){}
startADCAcquire();
while(ADCEnd == false){ }
stopADCAcquire();
for(int x = 0; x < 100; x++){
Serial.print(samplesADC[x]);
Serial.print(", ");
}
Serial.println("");
Serial.println("Stop ADC Acquire");
Serial.print("Pin A2 (2): ");
Serial.println(analogRead(A14));
Serial.println("END");
}
ISR(ADC_vect) {//when new ADC value ready
samplesADC[samplesADCCount] = ADCH;
// Serial.print("p");
samplesADCCount++;
if(samplesADCCount == 100){
ADCEnd = true;
}
}
void loop(){
}
Output:
START
Pin A2 (1): 511
Start ADC Aquire:
127, 127, 127, 127 <<< x 100
Stop ADC Acquire
Pin A2 (2): 511 <<< value now same as first
END