Ultrasonic Flowmeter Using TDC-GP22 and Arduino Uno

Question

I'm working on a project of Ultrasonic Flow Meter with TDC-GP22 using Arduino UNO. I used the library of leokoppel

In my code,the configurations of the registers are copied from the Application Note of GP22, from page 22 to page 25. I didn't use the temperature sensor so I jumped those steps related to that. No calibration neither. I just want to get some results of time of flight to be able to calculate the distance between two sensors for now.

My code for test:

#include "GP22.h"
#include <SPI.h>
#include <avr/io.h>

/*
 *  
 * Test setup (Arduino Uno, through 5->3.3V level shift)

 *     10 -> SS 
 *     11 -> MOSI
 *     12 -> MISO
 *     13 -> SCK
 *     2  -> INT
 *     5V -> VCC
 *     GND -> GND
 *     4 -> DS18B20 temperature sensor (optional)
 */

const bool DEBUG = 1;
const int FAST_MODE = 1;
const int PIN_INT = 2;

GP22 tdc(PIN_INT, DEBUG);

float cycleFactor_ns = 0; 

void setup() {

  Serial.begin(115200);
  Serial.println("Go !");

  tdc.init();

  tdc.writeRegister(0, 0x430BE800);
  tdc.writeRegister(1, 0x21444000);
  tdc.writeRegister(2, 0xA0138800);
  tdc.writeRegister(3, 0xD0A24800);
  tdc.writeRegister(4, 0x10004000);
  tdc.writeRegister(5, 0x40000000);
  tdc.writeRegister(6, 0xC0C06100);

  tdc.printConfigRegisters(); 

  byte a = tdc.readRegister(READ_REG_1);             

  Serial.println(a,HEX);
  Serial.println("Test communication: ");
  Serial.print(tdc.testCommunication());
 }  

void loop() {

  tdc.sendOpcode(OPCODE_INIT);
  tdc.sendOpcode(OPCODE_START_TOF_RESTART);
  tdc.waitForInterrupt(10000000);

  delay(20);

  Serial.println(tdc.readRegister(READ_RES_3),HEX);
}

From oscilloscope I got the right number of fire pulses(20) with right frequency(1MHz): Signal (the blue one is the signal of transmitter before amplification, green one is after amplification, yellow one is the signal of receiver)

But what I got from the code:

Go !

attached pinChangeISR_debug

Register 0: 0x430be800

Register 1: 0x21444000

Register 2: 0xa0138800

Register 3: 0xd0a24800

Register 4: 0x10004000

Register 5: 0x40000000

Register 6: 0xc0c06100



21

Test communication: 

\/Test took 180 us

1

69978000

69978000

69978000

69978000

69978000

69978000

69978000

...

21 is the first byte of register 1, and 1 is the result of testcommunication(). These two means the test of communication is successful. But the results of reading RES_3 are always the same, and the waitForInterrupt() doesn't work(maybe the code shouldn't be written like this) -- Timeout all the time.

What I expect, of course, is the result can change each time it measures.(impossible to get exactly the same measurement of ToF all the time)

Any help appreciated!