ADC 16bits programming problem - ACD161s626

Projects Programming
1

Hi everyone,

I am trying to program the ADC161s626 (datasheet http://www.ti.com/lit/ds/symlink/adc161s626.pdf with timing diagram p.7) on basis of this code : https://www.allaboutcircuits.com/projects/arduino-spi-library-ltc1286-dac714/ (exemple 2).

Communication SPI and Arduino Mega.

My code is the following :

#include <SPI.h>

const int spi_ss = 48;      // set SPI SS Pin
uint8_t byte_0, byte_1, byte_2;// First and second, third bytes read
uint16_t spi_bytes;       // final 15 bit shited value
float v_out;             // decimal voltage
float vref = 2.5;       // voltage on Vref pin


void setup() {
    // put your setup code here, to run once:
  Serial.begin(9600);          // begin serial and set speed
  pinMode(spi_ss, OUTPUT);     // Set SPI slave select pin as output
  digitalWrite(spi_ss, HIGH);  // Make sure spi_ss is held high 
  SPI.begin();                 // begin SPI
}

void loop() {
    // put your main code here, to run repeatedly:
  SPI.beginTransaction(SPISettings(2000000000, MSBFIRST, SPI_MODE3)); 
    // set speed bit format and clock/data polarity while starting SPI transaction 
  digitalWrite(spi_ss, LOW);
    // write the LTC CS pin low to initiate ADC sample and data transmit 
  byte_0 = SPI.transfer(0); // read firt 8 bits
  byte_1 = SPI.transfer(0); // read second 8 bits
  byte_2 = SPI.transfer(0); // read third 8 bits
  
  digitalWrite(spi_ss, HIGH);
    // wite LTC CS pin high to stop LTC from transmitting zeros. 
  SPI.endTransaction();
    // close SPI transaction
  spi_bytes = ( ( (byte_0 & B00111111) <<10) + (byte_1 <<2) + ( (byte_2 & B10000000) >>6));
    // & B00 inital 1 bit offset + one for null bit by & and shift into spi_bytes
    // then we add the remaining byte and shift right to remove bit 12 = à refaire pour expliquer 
  v_out = vref * (float(spi_bytes) / 32767.0);
    // finaly we recover the true value in volts. 1LSB = vref/32767
    // 15bits ADC = 2^15 - 1 = 32767
  Serial.println(v_out, 3);
  delay(250);
    // Delay that is fast but easy to read.
//  delayMicroseconds(83);
    // Delay that matches 12 khz delay time.
}

But it does not work, for 4.7V analog voltage, I obtain 3V digital voltage.

Currently, I read 3 bytes but when I look at the timing diagram, I know that is wrong, I have to read 18bits

I think that I have to do :

  1. start cs
  2. 2bits not read
  3. read 2 bytes
  4. stop cs

Problem, I do not know how to do this...

If someone can help me, it would be nice

Laurent

2

maybe this would work...

#include <SPI.h>

const int spi_ss = 48;      // set SPI SS Pin
float v_out;             // decimal voltage
float vref = 2.5;       // voltage on Vref pin

union {
  long Dout;
  byte bytes[4];
} spi_bytes;


void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);          // begin serial and set speed
  pinMode(spi_ss, OUTPUT);     // Set SPI slave select pin as output
  digitalWrite(spi_ss, HIGH);  // Make sure spi_ss is held high
  SPI.beginTransaction(SPISettings(5000000, MSBFIRST, SPI_MODE3));  // set speed bit format and clock/data polarity while starting SPI transaction
  SPI.begin();                 // begin SPI
}

void loop() {

  digitalWrite(spi_ss, LOW);

  spi_bytes.Dout = 0;

  for (int i = 2; i >=0; --i) {
    spi_bytes.bytes[i] = SPI.transfer(0);
  }

  digitalWrite(spi_ss, HIGH); // wite LTC CS pin high to stop LTC from transmitting zeros.

  spi_bytes.Dout >>= 2;

  v_out = vref * (int(spi_bytes.Dout) / 32767.0);
  // finaly we recover the true value in volts. 1LSB = vref/32767
  // 15bits ADC = 2^15 - 1 = 32767 (signed 16 bit ADC)
  Serial.println(v_out, 3);
  delay(250);
}
3

Thank you very much for your answer. Results are better but not good enough.

It seems that it works when I use

spi_bytes = ( ( (byte_0 & B000111111) <<10) + (byte_1 <<2) + (byte_2 >>6));

instead of spi_bytes = ( ( (byte_0 & B00111111) <<10) + (byte_1 <<2) + (byte_2 >>6));

So when I put D15 to zero. But regarding the timing diagram I really do not understand why...

I will retry your solution with an oscio

Thank you

1 Like