How to modify output voltage of a buck converter using PMBus?

[mhajihosseini]

Hello Everyone,

I am trying to modify the output voltage of a buck converter that is as a slave device via Arduino MEGA2560 as a Master device and using PMBus protocol.
I have written a code to write and read the output voltage, but I received garbage value (C0).
I would be grateful if someone can help me to solve the problem.
Here is the entire code.

#include <Wire.h>

void setup() {

  Wire.begin();                // join i2c bus (address optional for master)

  Serial.begin(9600);          // start serial communication at 9600bps
}

byte msb = 0;
byte lsb = 0;
byte V_out = 0;


void loop() {

  // step 1: Unlock PMBUS write protection

  Wire.beginTransmission(0x38);  // transmit to device(0x38)

  Wire.write(0x10);             // sets register pointer to the command register (0x10)

  Wire.write((byte)0x00);      // sends Data:0x00

  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(70);

  // step 2: VOUT setpoint
  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x21);      // sets register pointer to the command register(0x21)

  Wire.write((byte)0x00);      //sends  Byte#0:0x66 to the command register (0x21)

  Wire.write((byte)0xC6);       //sends  Byte#1:0xC6 to the command register (0x21)

  Wire.write(0x17);            // PEC

  Wire.endTransmission();      // stop transmitting

  delay(70);                   // datasheet suggests at least 65 milliseconds


  // step 3: Set VOUT_OV_WARN

  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x42);      // sets register pointer to to the command register(0x42)

  Wire.write((byte)0x33);      //sends  Byte#0:0x33 to the command register (0x42)

  Wire.write((byte)0xD3);       //sends  Byte#1:0xD3 to the command register (0x42)

  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(70);

  // step 4: Command 0xEC should be send to unlock the MCU which save the data.

  Wire.beginTransmission(0x38);     // transmit to device(0x38)

  Wire.write(0xEC);                 //sets register pointer to to the command register(0xEC)

  Wire.write(0x04);                 //sends  Byt count:N:4(0x04) to the command register (0xEC)

  Wire.write((byte)0x7E);           //sends  Byte#0:0x7E to the command register (0xEC)

  Wire.write((byte)0x15);           //sends  Byte#1:0x15 to the command register (0xEC)

  Wire.write((byte)0xDC);           //sends  Byte#2:0xDC to the command register (0xEC)

  Wire.write((byte)0x42);          //sends  Byte#3:0x42 to the command register (0xEC)

  Wire.write(0x45);                // PEC

  Wire.endTransmission();         // stop transmitting

  delay(70);

  //step 5: Command 0x11 should be send to save the Vout setpoint in the MCU


  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x11);            //sets register pointer to to the command register(0x11)


  Wire.endTransmission();      // stop transmitting


  delay(70);


  // step 6: request reading Vout setpoint  from the Buck Converter

  Wire.beginTransmission(0x38);       // transmit to device(0x38)

  Wire.write(0x21);                  //sets register pointer to to the command register(0x21)

  Wire.endTransmission(false);       // stop transmitting


  Wire.requestFrom((uint8_t) 0x38, (uint8_t) 2, (uint8_t) true);    //  Wire.requestFrom(38, 2);    // request 2 bytes from peripheral device #0x38


  // step 7: receive reading from the Buck Converter

  if (2 <= Wire.available()) {   // if two bytes were received

    msb = Wire.read();        // receive high byte (overwrites previous reading)
    
    msb =  msb << 8;       // shift high byte to be high 8 bits
    
    lsb= Wire.read();     // receive low byte as lower 8 bits

     V_out = msb | lsb;
     
    Serial.print("  V_out = ");
    Serial.print(V_out, HEX);   // print the reading
    Serial.println(" V");
  }

  delay(250);                  // wait a bit since people have to read the output :)
}

[b707]

@mhajihosseini

I am trying to modify the output voltage of a buck converter

Could you show a datasheet or at least a link to the buck converter?

[b707]

by the way...
Your msb defined as a byte, so this line is non-sense:

@mhajihosseini

msb = msb << 8;

Result of shifting byte left by 8 bits always will be zero.

[pylon]

@mhajihosseini

byte msb = 0;

...

@mhajihosseini

   msb =  msb << 8;       // shift high byte to be high 8 bits

This will always result in the MSB being ignored. A byte can only hold 8 bits. If you move these 8 bits by 8 bits you have no value left in the variable.

[mhajihosseini]

Hi
Thank you very much for reply.
Yes, I have datasheet.
DS_Q48SC12042 (3).pdf (1012.5 KB)

[b707]

@mhajihosseini

Thank you very much for reply.

Do you fix the error?
Is the problem solved?

[mhajihosseini]

Not yet. My major is not digital electronic, so it is difficult for me to understand the concept of these kinds of codes. I need to change the code, but should study more, and it is time-consuming for me.

[pylon]

I tried to tell you that you have to use a variable type for "msb" that is able to hold the result. Try uint16_t.

[mhajihosseini]

Thank you very much for helping me.
Sure, I will use it.

[6v6gt]

You should also test return codes (where available) from the Wire library functions you are using to help with troubleshooting, especially Wire.endTransmission(). See Wire - Arduino Reference

As well as msb, which you have already been told about, V_out must also be defined as uint16_t.

If you are getting "garbage values" that is already quite good since it shows that I2C is delivering "something" so, for example, the I2C address must be correct.

[mhajihosseini]

Great! Thanks.
Yes, it could be a good way to find the problem.

[mhajihosseini]

Hi
I did all suggestions, but it still returns C0.

[b707]

Standard answer - show your actual code.

[mhajihosseini]

Hi,
My code is below.

#include <Wire.h>

void setup() {

  Wire.begin();                // join i2c bus (address optional for master)

  Serial.begin(9600);          // start serial communication at 9600bps
}

uint16_t msb = 0;
uint8_t lsb = 0;
byte V_out = 0;


void loop() {


  // step 1: Unlock PMBUS write protection


  Wire.beginTransmission(0x38);  // transmit to device(0x38)

  Wire.write(0x10);             // sets register pointer to the command register (0x10)

  Wire.write((byte)0x00);      // sends Data:0x00

  //  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);


  // step 2: VOUT setpoint
  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x21);      // sets register pointer to the command register(0x21)

  Wire.write((byte)0x00);      //sends  Byte#0:0x66 to the command register (0x21)

  Wire.write((byte)0xC6);       //sends  Byte#1:0xC6 to the command register (0x21)

  //  Wire.write(0x17);            // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);                   // datasheet suggests at least 65 milliseconds


  // step 3: Set VOUT_OV_WARN

  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x42);      // sets register pointer to to the command register(0x42)

  Wire.write((byte)0x33);      //sends  Byte#0:0x33 to the command register (0x42)

  Wire.write((byte)0xD3);       //sends  Byte#1:0xD3 to the command register (0x42)

  //  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);

  // step 4: Command 0xEC should be send to unlock the MCU which save the data.

  Wire.beginTransmission(0x38);     // transmit to device(0x38)

  Wire.write(0xEC);                 //sets register pointer to to the command register(0xEC)

  Wire.write(0x04);                 //sends  Byt count:N:4(0x04) to the command register (0xEC)

  Wire.write((byte)0x7E);           //sends  Byte#0:0x7E to the command register (0xEC)

  Wire.write((byte)0x15);           //sends  Byte#1:0x15 to the command register (0xEC)

  Wire.write((byte)0xDC);           //sends  Byte#2:0xDC to the command register (0xEC)

  Wire.write((byte)0x42);          //sends  Byte#3:0x42 to the command register (0xEC)

  //  Wire.write(0x45);                // PEC

  Wire.endTransmission();         // stop transmitting

  delay(50);

  //step 5: Command 0x11 should be send to save the Vout setpoint in the MCU


  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x11);            //sets register pointer to to the command register(0x11)

  Wire.endTransmission();      // stop transmitting


  delay(50);


  // step 6: request reading Vout setpoint  from the Buck Converter

  Wire.beginTransmission(0x38);       // transmit to device(0x38)

  Wire.write(0x21);                  //sets register pointer to to the command register(0x21)

  Wire.endTransmission(false);       // stop transmitting


  //  Wire.requestFrom((uint8_t) 0x38, (uint8_t) 2, (uint8_t) true);    //  Wire.requestFrom(38, 2);    // request 2 bytes from peripheral device #0x38


  // step 7: receive reading from the Buck Converter


  if (Wire.requestFrom(0x38, 2) >= 2) {  // if two bytes were received

    uint16_t msb = Wire.read();        // receive high byte (overwrites previous reading)

    msb =  msb << 8;       // shift high byte to be high 8 bits

    uint8_t lsb = Wire.read();    // receive low byte as lower 8 bits

    V_out = msb | lsb;
    Serial.print("  msb = ");
    Serial.println(msb, HEX);   // print the reading
  
    Serial.println(" lsb = ");
    Serial.print(lsb, HEX);   // print the reading

    Serial.print("  V_out = ");
    Serial.print(V_out, HEX);   // print the reading
    Serial.println(" V");
 
  }

  delay(250);                  // wait a bit since people have to read the output :)
}

Note: Data return from MCU is in Vout Mode format : (Byte #1<<8) + (Byte #0)

[b707]

Please read post #10 carefully. You have to change V_out type too.

[mhajihosseini]

Thank you very much for your help.
Sure, I will do it.

[mhajihosseini]

EDIT: I changed int V_out to uint16_t V_out, but the result does not change.

[b707]

So insert your new code.
Every time you change something in the code and ask why it doesn't work - post the code again

[6v6gt]

State what output you got here an what you were expecting:

@mhajihosseini

    Serial.print("  msb = ");
    Serial.println(msb, HEX);   // print the reading
  
    Serial.println(" lsb = ");
    Serial.print(lsb, HEX);   // print the reading

    Serial.print("  V_out = ");
    Serial.print(V_out, HEX);   // print the reading
    Serial.println(" V");

Note here in step #2 there is a mismatch between the comment and command:

@mhajihosseini

Wire.write((byte)0x00); //sends Byte#0:0x66 to the command register

[mhajihosseini]

Hi,
I put the last version code in below.

#include <Wire.h>

void setup() {

  Wire.begin();                // join i2c bus (address optional for master)

  Serial.begin(9600);          // start serial communication at 9600bps
}

 uint16_t msb = 0;
 uint8_t lsb = 0;
 uint16_t V_out = 0;


void loop() {


  // step 1: Unlock PMBUS write protection


  Wire.beginTransmission(0x38);  // transmit to device(0x38)

  Wire.write(0x10);             // sets register pointer to the command register (0x10)

  Wire.write((byte)0x00);      // sends Data:0x00

  //  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);


  // step 2: VOUT setpoint
  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x21);      // sets register pointer to the command register(0x21)

  Wire.write((byte)0x66);      //sends  Byte#0:0x66 to the command register (0x21)

  Wire.write((byte)0xC6);       //sends  Byte#1:0xC6 to the command register (0x21)

  //  Wire.write(0x17);            // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);                   // datasheet suggests at least 65 milliseconds


  // step 3: Set VOUT_OV_WARN

  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x42);      // sets register pointer to to the command register(0x42)

  Wire.write((byte)0x33);      //sends  Byte#0:0x33 to the command register (0x42)

  Wire.write((byte)0xD3);       //sends  Byte#1:0xD3 to the command register (0x42)

  //  Wire.write(0x17);             // PEC

  Wire.endTransmission();      // stop transmitting

  delay(50);

  // step 4: Command 0xEC should be send to unlock the MCU which save the data.

  Wire.beginTransmission(0x38);     // transmit to device(0x38)

  Wire.write(0xEC);                 //sets register pointer to to the command register(0xEC)

  Wire.write(0x04);                 //sends  Byt count:N:4(0x04) to the command register (0xEC)

  Wire.write((byte)0x7E);           //sends  Byte#0:0x7E to the command register (0xEC)

  Wire.write((byte)0x15);           //sends  Byte#1:0x15 to the command register (0xEC)

  Wire.write((byte)0xDC);           //sends  Byte#2:0xDC to the command register (0xEC)

  Wire.write((byte)0x42);          //sends  Byte#3:0x42 to the command register (0xEC)

  //  Wire.write(0x45);                // PEC

  Wire.endTransmission();         // stop transmitting

  delay(50);

  //step 5: Command 0x11 should be send to save the Vout setpoint in the MCU


  Wire.beginTransmission(0x38); // transmit to device(0x38)

  Wire.write(0x11);            //sets register pointer to to the command register(0x11)

  Wire.endTransmission();      // stop transmitting


  delay(50);


  // step 6: request reading Vout setpoint  from the Buck Converter

  Wire.beginTransmission(0x38);       // transmit to device(0x38)

  Wire.write(0x21);                  //sets register pointer to to the command register(0x21)

  Wire.endTransmission(false);       // stop transmitting


  //  Wire.requestFrom((uint8_t) 0x38, (uint8_t) 2, (uint8_t) true);    //  Wire.requestFrom(38, 2);    // request 2 bytes from peripheral device #0x38


  // step 7: receive reading from the Buck Converter


  if (Wire.requestFrom(0x38, 2) >= 2) {  // if two bytes were received

    uint16_t msb = Wire.read();        // receive high byte (overwrites previous reading)

     msb =  msb << 8;       // shift high byte to be high 8 bits

    uint8_t lsb = Wire.read();    // receive low byte as lower 8 bits

    uint16_t V_out = msb | lsb;
    Serial.print("  msb = ");
    Serial.println(msb, HEX);   // print the reading
  
    Serial.println(" lsb = ");
    Serial.print(lsb, HEX);   // print the reading

    Serial.print("  V_out = ");
    Serial.print(V_out, HEX);   // print the reading
    Serial.println(" V");
 
  }

  delay(250);                  // wait a bit since people have to read the output :)
}