Arduino very fast read and very cheap scope!

Arduino analog read at 77 kHz, no interrupts needed. Graphical representation of data on Serial Monitor.

Sep 21, 2020

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Devices & Components

Arduino Uno Rev3

Capacitor 100 nF

Resistor 220 ohm

Project description

Code

very_fast_analog_read

c_cpp

reads analog data at 77 kHz or faster

1/*  This small program shows that Arduino UNO is capable of recording   analog readings at a rate of at least 77 kHz (maybe even 154 kHz).
2 *  First,   the analog digital converter setup registers (ADSCRA and ADSCRB) are set in such   a way that analog values will be read and put in the ADCH register continuously   at a rate of 77 kHz. 
3 *  This ADCH register may then be read at any desired   frequency. If the reading frequency is faster than 77kHz, the same value will be   reported multiple times.
4 *  Advantage of this approach is that no interrupt   is needed (most sketches that I found so far wait for the analog read to get a new   fresh result and will then read that value).
5 *  
6 *  Instead of 10-bit, only   the 8 most significant bits are read. This reduces noise and allows for much more   compact storage. An 8 bit number can be stored in a byte, a 10 bit number 
7 *   will take an integer (2 bytes). At a sampling rate of 77 kHz, the memory of arduino   will be full very rapidly.... 
8 *  
9 *  Timer2 is used to read the values   at regular intervals. First the timer overflow flag is set to zero. Then actions   are performed. 
10 *  Then the controller waits for the overflow flag to be set.   As long as the execution time of all actions fits in the period for the flag to   be set, this will yield a very regular sampling.
11 *  
12 *  The result is shown   as a graph in the Serial Monitor. I guess a graphical display will allow for a much   better view of the data (work to be done).
13 *  
14 *  Author: Koen Meesters
15   *  Date:   16/09/2020
16 *  
17 *  Electrical scheme used for testing:
18 *  
19   *  
20 *  Pin 8----[ 220 OHM ]----+----[ 220 OHM ]----+-----[ 220 OHM ]----+--------   Pin A0
21 *                          |                   |                    |   
22 *                          |                   |                    |
23   *                         === 100 nF          ===  100 nF          === 100 nF    
24   *                          |                   |                    |  
25 *                          |                    |                    |
26 *  GND --------------------+-------------------+--------------------+
27   *  
28 *  
29 */
30
31void setup() {
32  Serial.begin(9600);
33  delay(1000);
34   Serial.println("start setup");
35  delay(1000);
36
37  /*  The bit of code   below was taken from:
38   *  http://yaab-arduino.blogspot.it/p/oscope.html
39    */
40  ADCSRA = 0;             // clear ADCSRA register
41  ADCSRB = 0;             //   clear ADCSRB register
42  ADMUX |= (0 & 0x07);    // set A0 analog input pin
43   ADMUX |= (1 << REFS0);  // set reference voltage
44  ADMUX |= (1 << ADLAR);  //   left align ADC value to 8 bits from ADCH register
45
46  // sampling rate is [ADC   clock] / [prescaler] / [conversion clock cycles]
47  // for Arduino Uno ADC clock   is 16 MHz and a conversion takes 13 clock cycles
48  //ADCSRA |= (1 << ADPS2) |   (1 << ADPS0);    // 32 prescaler for 38.5 KHz
49  ADCSRA |= (1 << ADPS2);                     //   16 prescaler for 76.9 KHz
50  //ADCSRA |= (1 << ADPS1) | (1 << ADPS0);    // 8   prescaler for 153.8 KHz
51
52  ADCSRA |= (1 << ADATE); // enable auto trigger
53//   ADCSRA |= (1 << ADIE);  // enable interrupts when measurement complete --> KM:   disabled here, because no interrupts will be used
54  ADCSRA |= (1 << ADEN);  //   enable ADC
55  ADCSRA |= (1 << ADSC);  // start ADC measurements
56
57// set   Timer2  to fast PWM, no prescaling, inverted pin 3
58// in this sketch only the   overflow flag of Timer2 will be used to trigger the reading frequency.
59  TCCR2A   = 0;
60  TCCR2B = 0;
61  TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(COM2B0) | _BV(WGM21)   | _BV(WGM20); 
62  TCCR2B = _BV(CS20);
63  
64  delay(1000);
65  pinMode(8,   OUTPUT);  //for testing a square wave signal will be produced on pin 8
66  Serial.println("end   setup");
67  delay(1000);
68}
69
70const byte maxData = 255;   //this value   could be chosen larger if desired
71byte data[maxData];         //here the data   will be stored
72unsigned long int t0 = 0;   //(t1-t0)/255 should be equal to 1/77000*1000000   (otherwise the execution time of the actions is to large).
73unsigned long int   t1 = 0;
74
75bool state = 0; //state of output at pin 8
76
77void loop() {
78   t0 = micros();
79  
80  TIFR2 = TIFR2 | _BV(TOV2); //reset overflow flag
81
82   for (int i=0; i<maxData; i++) {
83    data[i] = ADCH;        //read the analog   value and store in memory
84    PORTB = byte(state);   //this statement puts the   square wave at pin 8 (pin 8 is the least significant bit of PORTB)
85    if((i&B00000111)   == 0) {state = !state;} /* prescaler to set frequency of square wave at pin 8: 
86                                               * B00111111 = 64 B00011111=32 B00001111=16   B00000111=8, frequency will be: Timer2Freq/(prescaler*2)  
87                                              */
88     
89    /* More statements could be put here. Perhaps data could be written   to SD card, but I did not test if writing of one character to SD card 
90     *   can be performed in the little time available here
91     */
92    
93    while((TIFR2   & _BV(TOV2)) == 0) {
94      ; //wait for timer overflow flag
95    }
96    TIFR2   = TIFR2 | _BV(TOV2);  //reset overflow flag
97  }
98  
99  t1 = micros();
100
101   Serial.println(t0);
102  Serial.println(t1);
103  Serial.println(t1-t0);
104    
105   for (int i=0; i<maxData; i++) {
106    Serial.println(data[i]);
107  }
108
109   makeGraph(maxData);
110  delay(2000);
111}
112
113//make a Graph on the Serial   monitor
114void makeGraph(byte numData) {
115  
116  const byte horLineInt =  8;
117   const byte verLineInt = 16;
118  const byte y_offset   = 32;
119
120  const byte   screenHeight =  64;
121  const byte screenWidth  = 128;
122  if(numData<screenWidth)   {numData = screenWidth;}  //only data that fit on the screen will be shown
123
124     
125  for (byte y=0; y<=screenHeight; y++) { //each line, starting at bottom   of the graph
126    for (byte t=0; t<numData; t++) {     //each character, starting   from left to right
127      char character = ' ';    //put a space 
128      if   (t%verLineInt == 0) {
129        character = '|';       //put a vertical line for   the grid on regular intervals
130      }
131      if ((screenHeight-y-y_offset)%horLineInt   == 0) {
132        character = '-';     //put a horizontal line for the grid on   regular intervals
133        if (y == y_offset) {
134          character = 'o';      //put a horizontal line with zeros in the grid
135        }
136        if (t%verLineInt   == 0) {
137          character = '+';      //put a '+' where horizontal and vertical   grid lines meet
138        }
139      }
140      if (round(data[t]/4.0) == screenHeight-y)   {
141        character = 'x';        //if data point supposed to be here, then put   an 'x'    
142      }
143      Serial.print(character);  //put the character on   the Serial Monitor
144    }
145    Serial.println();           //next line
146  }
147   Serial.println("+---------------+ = 128 micro sec");  //add horizontal scale   dimenstion
148  //              01234567890123456
149  Serial.println("Full scale   on vertical axis is -5V to 0V to +5V"); //add vertical scale data
150}
151
152   
153

/*  This small program shows that Arduino UNO is capable of recording   analog readings at a rate of at least 77 kHz (maybe even 154 kHz).
 *  First,   the analog digital converter setup registers (ADSCRA and ADSCRB) are set in such   a way that analog values will be read and put in the ADCH register continuously   at a rate of 77 kHz. 
 *  This ADCH register may then be read at any desired   frequency. If the reading frequency is faster than 77kHz, the same value will be   reported multiple times.
 *  Advantage of this approach is that no interrupt   is needed (most sketches that I found so far wait for the analog read to get a new   fresh result and will then read that value).
 *  
 *  Instead of 10-bit, only   the 8 most significant bits are read. This reduces noise and allows for much more   compact storage. An 8 bit number can be stored in a byte, a 10 bit number 
 *   will take an integer (2 bytes). At a sampling rate of 77 kHz, the memory of arduino   will be full very rapidly.... 
 *  
 *  Timer2 is used to read the values   at regular intervals. First the timer overflow flag is set to zero. Then actions   are performed. 
 *  Then the controller waits for the overflow flag to be set.   As long as the execution time of all actions fits in the period for the flag to   be set, this will yield a very regular sampling.
 *  
 *  The result is shown   as a graph in the Serial Monitor. I guess a graphical display will allow for a much   better view of the data (work to be done).
 *  
 *  Author: Koen Meesters
   *  Date:   16/09/2020
 *  
 *  Electrical scheme used for testing:
 *  
   *  
 *  Pin 8----[ 220 OHM ]----+----[ 220 OHM ]----+-----[ 220 OHM ]----+--------   Pin A0
 *                          |                   |                    |   
 *                          |                   |                    |
   *                         === 100 nF          ===  100 nF          === 100 nF    
   *                          |                   |                    |  
 *                          |                    |                    |
 *  GND --------------------+-------------------+--------------------+
   *  
 *  
 */

void setup() {
  Serial.begin(9600);
  delay(1000);
   Serial.println("start setup");
  delay(1000);

  /*  The bit of code   below was taken from:
   *  http://yaab-arduino.blogspot.it/p/oscope.html
    */
  ADCSRA = 0;             // clear ADCSRA register
  ADCSRB = 0;             //   clear ADCSRB register
  ADMUX |= (0 & 0x07);    // set A0 analog input pin
   ADMUX |= (1 << REFS0);  // set reference voltage
  ADMUX |= (1 << ADLAR);  //   left align ADC value to 8 bits from ADCH register

  // sampling rate is [ADC   clock] / [prescaler] / [conversion clock cycles]
  // for Arduino Uno ADC clock   is 16 MHz and a conversion takes 13 clock cycles
  //ADCSRA |= (1 << ADPS2) |   (1 << ADPS0);    // 32 prescaler for 38.5 KHz
  ADCSRA |= (1 << ADPS2);                     //   16 prescaler for 76.9 KHz
  //ADCSRA |= (1 << ADPS1) | (1 << ADPS0);    // 8   prescaler for 153.8 KHz

  ADCSRA |= (1 << ADATE); // enable auto trigger
//   ADCSRA |= (1 << ADIE);  // enable interrupts when measurement complete --> KM:   disabled here, because no interrupts will be used
  ADCSRA |= (1 << ADEN);  //   enable ADC
  ADCSRA |= (1 << ADSC);  // start ADC measurements

// set   Timer2  to fast PWM, no prescaling, inverted pin 3
// in this sketch only the   overflow flag of Timer2 will be used to trigger the reading frequency.
  TCCR2A   = 0;
  TCCR2B = 0;
  TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(COM2B0) | _BV(WGM21)   | _BV(WGM20); 
  TCCR2B = _BV(CS20);
  
  delay(1000);
  pinMode(8,   OUTPUT);  //for testing a square wave signal will be produced on pin 8
  Serial.println("end   setup");
  delay(1000);
}

const byte maxData = 255;   //this value   could be chosen larger if desired
byte data[maxData];         //here the data   will be stored
unsigned long int t0 = 0;   //(t1-t0)/255 should be equal to 1/77000*1000000   (otherwise the execution time of the actions is to large).
unsigned long int   t1 = 0;

bool state = 0; //state of output at pin 8

void loop() {
   t0 = micros();
  
  TIFR2 = TIFR2 | _BV(TOV2); //reset overflow flag

   for (int i=0; i<maxData; i++) {
    data[i] = ADCH;        //read the analog   value and store in memory
    PORTB = byte(state);   //this statement puts the   square wave at pin 8 (pin 8 is the least significant bit of PORTB)
    if((i&B00000111)   == 0) {state = !state;} /* prescaler to set frequency of square wave at pin 8: 
                                               * B00111111 = 64 B00011111=32 B00001111=16   B00000111=8, frequency will be: Timer2Freq/(prescaler*2)  
                                              */
     
    /* More statements could be put here. Perhaps data could be written   to SD card, but I did not test if writing of one character to SD card 
     *   can be performed in the little time available here
     */
    
    while((TIFR2   & _BV(TOV2)) == 0) {
      ; //wait for timer overflow flag
    }
    TIFR2   = TIFR2 | _BV(TOV2);  //reset overflow flag
  }
  
  t1 = micros();

   Serial.println(t0);
  Serial.println(t1);
  Serial.println(t1-t0);
    
   for (int i=0; i<maxData; i++) {
    Serial.println(data[i]);
  }

   makeGraph(maxData);
  delay(2000);
}

//make a Graph on the Serial   monitor
void makeGraph(byte numData) {
  
  const byte horLineInt =  8;
   const byte verLineInt = 16;
  const byte y_offset   = 32;

  const byte   screenHeight =  64;
  const byte screenWidth  = 128;
  if(numData<screenWidth)   {numData = screenWidth;}  //only data that fit on the screen will be shown

     
  for (byte y=0; y<=screenHeight; y++) { //each line, starting at bottom   of the graph
    for (byte t=0; t<numData; t++) {     //each character, starting   from left to right
      char character = ' ';    //put a space 
      if   (t%verLineInt == 0) {
        character = '|';       //put a vertical line for   the grid on regular intervals
      }
      if ((screenHeight-y-y_offset)%horLineInt   == 0) {
        character = '-';     //put a horizontal line for the grid on   regular intervals
        if (y == y_offset) {
          character = 'o';      //put a horizontal line with zeros in the grid
        }
        if (t%verLineInt   == 0) {
          character = '+';      //put a '+' where horizontal and vertical   grid lines meet
        }
      }
      if (round(data[t]/4.0) == screenHeight-y)   {
        character = 'x';        //if data point supposed to be here, then put   an 'x'    
      }
      Serial.print(character);  //put the character on   the Serial Monitor
    }
    Serial.println();           //next line
  }
   Serial.println("+---------------+ = 128 micro sec");  //add horizontal scale   dimenstion
  //              01234567890123456
  Serial.println("Full scale   on vertical axis is -5V to 0V to +5V"); //add vertical scale data
}

very_fast_analog_read

c_cpp

reads analog data at 77 kHz or faster

1/*  This small program shows that Arduino UNO is capable of recording  analog readings at a rate of at least 77 kHz (maybe even 154 kHz).
2 *  First,  the analog digital converter setup registers (ADSCRA and ADSCRB) are set in such  a way that analog values will be read and put in the ADCH register continuously  at a rate of 77 kHz. 
3 *  This ADCH register may then be read at any desired  frequency. If the reading frequency is faster than 77kHz, the same value will be  reported multiple times.
4 *  Advantage of this approach is that no interrupt  is needed (most sketches that I found so far wait for the analog read to get a new  fresh result and will then read that value).
5 *  
6 *  Instead of 10-bit, only  the 8 most significant bits are read. This reduces noise and allows for much more  compact storage. An 8 bit number can be stored in a byte, a 10 bit number 
7 *  will take an integer (2 bytes). At a sampling rate of 77 kHz, the memory of arduino  will be full very rapidly.... 
8 *  
9 *  Timer2 is used to read the values  at regular intervals. First the timer overflow flag is set to zero. Then actions  are performed. 
10 *  Then the controller waits for the overflow flag to be set.  As long as the execution time of all actions fits in the period for the flag to  be set, this will yield a very regular sampling.
11 *  
12 *  The result is shown  as a graph in the Serial Monitor. I guess a graphical display will allow for a much  better view of the data (work to be done).
13 *  
14 *  Author: Koen Meesters
15  *  Date:   16/09/2020
16 *  
17 *  Electrical scheme used for testing:
18 *  
19  *  
20 *  Pin 8----[ 220 OHM ]----+----[ 220 OHM ]----+-----[ 220 OHM ]----+--------  Pin A0
21 *                          |                   |                    |  
22 *                          |                   |                    |
23  *                         === 100 nF          ===  100 nF          === 100 nF    
24  *                          |                   |                    |  
25 *                          |                   |                    |
26 *  GND --------------------+-------------------+--------------------+
27  *  
28 *  
29 */
30
31void setup() {
32  Serial.begin(9600);
33  delay(1000);
34  Serial.println("start setup");
35  delay(1000);
36
37  /*  The bit of code  below was taken from:
38   *  http://yaab-arduino.blogspot.it/p/oscope.html
39   */
40  ADCSRA = 0;             // clear ADCSRA register
41  ADCSRB = 0;             //  clear ADCSRB register
42  ADMUX |= (0 & 0x07);    // set A0 analog input pin
43  ADMUX |= (1 << REFS0);  // set reference voltage
44  ADMUX |= (1 << ADLAR);  //  left align ADC value to 8 bits from ADCH register
45
46  // sampling rate is [ADC  clock] / [prescaler] / [conversion clock cycles]
47  // for Arduino Uno ADC clock  is 16 MHz and a conversion takes 13 clock cycles
48  //ADCSRA |= (1 << ADPS2) |  (1 << ADPS0);    // 32 prescaler for 38.5 KHz
49  ADCSRA |= (1 << ADPS2);                     //  16 prescaler for 76.9 KHz
50  //ADCSRA |= (1 << ADPS1) | (1 << ADPS0);    // 8  prescaler for 153.8 KHz
51
52  ADCSRA |= (1 << ADATE); // enable auto trigger
53//  ADCSRA |= (1 << ADIE);  // enable interrupts when measurement complete --> KM:  disabled here, because no interrupts will be used
54  ADCSRA |= (1 << ADEN);  //  enable ADC
55  ADCSRA |= (1 << ADSC);  // start ADC measurements
56
57// set  Timer2  to fast PWM, no prescaling, inverted pin 3
58// in this sketch only the  overflow flag of Timer2 will be used to trigger the reading frequency.
59  TCCR2A  = 0;
60  TCCR2B = 0;
61  TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(COM2B0) | _BV(WGM21)  | _BV(WGM20); 
62  TCCR2B = _BV(CS20);
63  
64  delay(1000);
65  pinMode(8,  OUTPUT);  //for testing a square wave signal will be produced on pin 8
66  Serial.println("end  setup");
67  delay(1000);
68}
69
70const byte maxData = 255;   //this value  could be chosen larger if desired
71byte data[maxData];         //here the data  will be stored
72unsigned long int t0 = 0;   //(t1-t0)/255 should be equal to 1/77000*1000000  (otherwise the execution time of the actions is to large).
73unsigned long int  t1 = 0;
74
75bool state = 0; //state of output at pin 8
76
77void loop() {
78  t0 = micros();
79  
80  TIFR2 = TIFR2 | _BV(TOV2); //reset overflow flag
81
82  for (int i=0; i<maxData; i++) {
83    data[i] = ADCH;        //read the analog  value and store in memory
84    PORTB = byte(state);   //this statement puts the  square wave at pin 8 (pin 8 is the least significant bit of PORTB)
85    if((i&B00000111)  == 0) {state = !state;} /* prescaler to set frequency of square wave at pin 8: 
86                                              * B00111111 = 64 B00011111=32 B00001111=16  B00000111=8, frequency will be: Timer2Freq/(prescaler*2)  
87                                              */
88    
89    /* More statements could be put here. Perhaps data could be written  to SD card, but I did not test if writing of one character to SD card 
90     *  can be performed in the little time available here
91     */
92    
93    while((TIFR2  & _BV(TOV2)) == 0) {
94      ; //wait for timer overflow flag
95    }
96    TIFR2  = TIFR2 | _BV(TOV2);  //reset overflow flag
97  }
98  
99  t1 = micros();
100
101  Serial.println(t0);
102  Serial.println(t1);
103  Serial.println(t1-t0);
104    
105  for (int i=0; i<maxData; i++) {
106    Serial.println(data[i]);
107  }
108
109  makeGraph(maxData);
110  delay(2000);
111}
112
113//make a Graph on the Serial  monitor
114void makeGraph(byte numData) {
115  
116  const byte horLineInt =  8;
117  const byte verLineInt = 16;
118  const byte y_offset   = 32;
119
120  const byte  screenHeight =  64;
121  const byte screenWidth  = 128;
122  if(numData<screenWidth)  {numData = screenWidth;}  //only data that fit on the screen will be shown
123
124    
125  for (byte y=0; y<=screenHeight; y++) { //each line, starting at bottom  of the graph
126    for (byte t=0; t<numData; t++) {     //each character, starting  from left to right
127      char character = ' ';    //put a space 
128      if  (t%verLineInt == 0) {
129        character = '|';       //put a vertical line for  the grid on regular intervals
130      }
131      if ((screenHeight-y-y_offset)%horLineInt  == 0) {
132        character = '-';     //put a horizontal line for the grid on  regular intervals
133        if (y == y_offset) {
134          character = 'o';     //put a horizontal line with zeros in the grid
135        }
136        if (t%verLineInt  == 0) {
137          character = '+';      //put a '+' where horizontal and vertical  grid lines meet
138        }
139      }
140      if (round(data[t]/4.0) == screenHeight-y)  {
141        character = 'x';        //if data point supposed to be here, then put  an 'x'    
142      }
143      Serial.print(character);  //put the character on  the Serial Monitor
144    }
145    Serial.println();           //next line
146  }
147  Serial.println("+---------------+ = 128 micro sec");  //add horizontal scale  dimenstion
148  //              01234567890123456
149  Serial.println("Full scale  on vertical axis is -5V to 0V to +5V"); //add vertical scale data
150}
151
152  
153

/*  This small program shows that Arduino UNO is capable of recording  analog readings at a rate of at least 77 kHz (maybe even 154 kHz).
 *  First,  the analog digital converter setup registers (ADSCRA and ADSCRB) are set in such  a way that analog values will be read and put in the ADCH register continuously  at a rate of 77 kHz. 
 *  This ADCH register may then be read at any desired  frequency. If the reading frequency is faster than 77kHz, the same value will be  reported multiple times.
 *  Advantage of this approach is that no interrupt  is needed (most sketches that I found so far wait for the analog read to get a new  fresh result and will then read that value).
 *  
 *  Instead of 10-bit, only  the 8 most significant bits are read. This reduces noise and allows for much more  compact storage. An 8 bit number can be stored in a byte, a 10 bit number 
 *  will take an integer (2 bytes). At a sampling rate of 77 kHz, the memory of arduino  will be full very rapidly.... 
 *  
 *  Timer2 is used to read the values  at regular intervals. First the timer overflow flag is set to zero. Then actions  are performed. 
 *  Then the controller waits for the overflow flag to be set.  As long as the execution time of all actions fits in the period for the flag to  be set, this will yield a very regular sampling.
 *  
 *  The result is shown  as a graph in the Serial Monitor. I guess a graphical display will allow for a much  better view of the data (work to be done).
 *  
 *  Author: Koen Meesters
  *  Date:   16/09/2020
 *  
 *  Electrical scheme used for testing:
 *  
  *  
 *  Pin 8----[ 220 OHM ]----+----[ 220 OHM ]----+-----[ 220 OHM ]----+--------  Pin A0
 *                          |                   |                    |  
 *                          |                   |                    |
  *                         === 100 nF          ===  100 nF          === 100 nF    
  *                          |                   |                    |  
 *                          |                   |                    |
 *  GND --------------------+-------------------+--------------------+
  *  
 *  
 */

void setup() {
  Serial.begin(9600);
  delay(1000);
  Serial.println("start setup");
  delay(1000);

  /*  The bit of code  below was taken from:
   *  http://yaab-arduino.blogspot.it/p/oscope.html
   */
  ADCSRA = 0;             // clear ADCSRA register
  ADCSRB = 0;             //  clear ADCSRB register
  ADMUX |= (0 & 0x07);    // set A0 analog input pin
  ADMUX |= (1 << REFS0);  // set reference voltage
  ADMUX |= (1 << ADLAR);  //  left align ADC value to 8 bits from ADCH register

  // sampling rate is [ADC  clock] / [prescaler] / [conversion clock cycles]
  // for Arduino Uno ADC clock  is 16 MHz and a conversion takes 13 clock cycles
  //ADCSRA |= (1 << ADPS2) |  (1 << ADPS0);    // 32 prescaler for 38.5 KHz
  ADCSRA |= (1 << ADPS2);                     //  16 prescaler for 76.9 KHz
  //ADCSRA |= (1 << ADPS1) | (1 << ADPS0);    // 8  prescaler for 153.8 KHz

  ADCSRA |= (1 << ADATE); // enable auto trigger
//  ADCSRA |= (1 << ADIE);  // enable interrupts when measurement complete --> KM:  disabled here, because no interrupts will be used
  ADCSRA |= (1 << ADEN);  //  enable ADC
  ADCSRA |= (1 << ADSC);  // start ADC measurements

// set  Timer2  to fast PWM, no prescaling, inverted pin 3
// in this sketch only the  overflow flag of Timer2 will be used to trigger the reading frequency.
  TCCR2A  = 0;
  TCCR2B = 0;
  TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(COM2B0) | _BV(WGM21)  | _BV(WGM20); 
  TCCR2B = _BV(CS20);
  
  delay(1000);
  pinMode(8,  OUTPUT);  //for testing a square wave signal will be produced on pin 8
  Serial.println("end  setup");
  delay(1000);
}

const byte maxData = 255;   //this value  could be chosen larger if desired
byte data[maxData];         //here the data  will be stored
unsigned long int t0 = 0;   //(t1-t0)/255 should be equal to 1/77000*1000000  (otherwise the execution time of the actions is to large).
unsigned long int  t1 = 0;

bool state = 0; //state of output at pin 8

void loop() {
  t0 = micros();
  
  TIFR2 = TIFR2 | _BV(TOV2); //reset overflow flag

  for (int i=0; i<maxData; i++) {
    data[i] = ADCH;        //read the analog  value and store in memory
    PORTB = byte(state);   //this statement puts the  square wave at pin 8 (pin 8 is the least significant bit of PORTB)
    if((i&B00000111)  == 0) {state = !state;} /* prescaler to set frequency of square wave at pin 8: 
                                              * B00111111 = 64 B00011111=32 B00001111=16  B00000111=8, frequency will be: Timer2Freq/(prescaler*2)  
                                              */
    
    /* More statements could be put here. Perhaps data could be written  to SD card, but I did not test if writing of one character to SD card 
     *  can be performed in the little time available here
     */
    
    while((TIFR2  & _BV(TOV2)) == 0) {
      ; //wait for timer overflow flag
    }
    TIFR2  = TIFR2 | _BV(TOV2);  //reset overflow flag
  }
  
  t1 = micros();

  Serial.println(t0);
  Serial.println(t1);
  Serial.println(t1-t0);
    
  for (int i=0; i<maxData; i++) {
    Serial.println(data[i]);
  }

  makeGraph(maxData);
  delay(2000);
}

//make a Graph on the Serial  monitor
void makeGraph(byte numData) {
  
  const byte horLineInt =  8;
  const byte verLineInt = 16;
  const byte y_offset   = 32;

  const byte  screenHeight =  64;
  const byte screenWidth  = 128;
  if(numData<screenWidth)  {numData = screenWidth;}  //only data that fit on the screen will be shown

    
  for (byte y=0; y<=screenHeight; y++) { //each line, starting at bottom  of the graph
    for (byte t=0; t<numData; t++) {     //each character, starting  from left to right
      char character = ' ';    //put a space 
      if  (t%verLineInt == 0) {
        character = '|';       //put a vertical line for  the grid on regular intervals
      }
      if ((screenHeight-y-y_offset)%horLineInt  == 0) {
        character = '-';     //put a horizontal line for the grid on  regular intervals
        if (y == y_offset) {
          character = 'o';     //put a horizontal line with zeros in the grid
        }
        if (t%verLineInt  == 0) {
          character = '+';      //put a '+' where horizontal and vertical  grid lines meet
        }
      }
      if (round(data[t]/4.0) == screenHeight-y)  {
        character = 'x';        //if data point supposed to be here, then put  an 'x'    
      }
      Serial.print(character);  //put the character on  the Serial Monitor
    }
    Serial.println();           //next line
  }
  Serial.println("+---------------+ = 128 micro sec");  //add horizontal scale  dimenstion
  //              01234567890123456
  Serial.println("Full scale  on vertical axis is -5V to 0V to +5V"); //add vertical scale data
}

Downloadable files

RC filter used for demo.

Shapes square wave of pin 8. Resulting signal is fed into pin A0.

RC filter used for demo.

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