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MCP4725 problem

 
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hemnath



Joined: 03 Oct 2012
Posts: 197
Location: chennai

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MCP4725 problem
PostPosted: Sat Jan 27, 2018 5:15 am     Reply with quote

I have interfaced 18F2620 with MCP4725 12 bit DAC. Below is the test program.
Code:
#include "18F2620.h"
#fuses INTRC_IO
#use delay(clock=4000000)
#use I2C(MASTER, scl=PIN_C3, sda=PIN_C4)

void dac_i2c(unsigned int16 sample)
{     
   i2c_start();
   i2c_write(0b11000000);                // Device address
   i2c_write(0b1000000);                 // Internal Device address   
   i2c_write((sample & 0xFF0) >> 4);     // Upper data bits          (D11.D10.D9.D8.D7.D6.D5.D4)
   i2c_write((sample & 0xF) << 4);       //lower bits
   i2c_stop();                           // Stop
}   

void main()
{
   unsigned int16 i;
   
   setup_adc_ports(NO_ANALOGS|VSS_VDD);   
   setup_adc(ADC_OFF|ADC_TAD_MUL_0);
   setup_comparator (NC_NC_NC_NC);      // Disable comparator
   enable_interrupts(GLOBAL);         // Enable Global Interrupt

   while(1)
   {
      i=200;
      if(i>=4096)
         i=0;
      dac_i2c(4000);   
      delay_ms(200);   
   }
}



But the output voltage is +2.5V.. Why is it so? Please help.
temtronic



Joined: 01 Jul 2010
Posts: 6020
Location: Greensville,Ontario

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PostPosted: Sat Jan 27, 2018 5:51 am     Reply with quote

WIth every I2C device you should run PCM P 'I2C Scanner' program located in the code library. It will locate every I2C device attached to the PIC.
While I don't use that device I did download it's data sheet and saw that the address can be factory set at customers request. Running the scanner program will confirm it's true address.
I have to ask what value are the I2C bus pullup resistors as well as the DAC output load resistor?

I haven't looked deep at your code as I suspect an addressing problem and you do need to confirm what it is first.
'Play PIC' and confirm the data being sent to the device is correct ! You've got some 'maths' involved in selcting the device and DAC value so simply send that data to a PC terminal program, and confirm bit for bit , is correct! Sometimes 'casting' with alter the data
Try 3 or 4 fixed values. For a DAC I'd use 0, 1/4 scale,1/2 and full values and compare the results to expected. Simply copy your DAC function several times,with names like DAC_zero, DAC_half,DAC_full. Easy to remmeber and understand what the function is supposed to do !

EDIT !!!!
you've got global interrupts enabled ! NOT a good idea. Not too sure what happens, though it's never good to enable ANY interrupt without a 'handler'.

2.5 is 1/2 way of FS, so I suspect that might be a factory test value,as the DAC value is the last one saved.

Your 'internal device address' is only 7 bits long. When dealing with data in 0b form, it's best to show ALL bits. Bit 7 in this case is a 'command' bit.I'd hate to assume bit 7 is a '0' when it's 'unknown'. Best to have 0b01000000 instead of 0b1000000. Maybe the compiler doesn't care but for debugging by a 3rd person who hasn't had enough coffee( or too much ) it does.


Jay


Last edited by temtronic on Sat Jan 27, 2018 7:22 am; edited 1 time in total
Ttelmah



Joined: 11 Mar 2010
Posts: 12951

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PostPosted: Sat Jan 27, 2018 6:58 am     Reply with quote

Obvious question. What is connected to the A0 pin?.

The default address is:

0b1100 00P0

Where 'P' is the value on the A0 pin.

If this is not pulled down to 0v, the address will be wrong.

The I2C scanner program is always a useful starting point.

Code:

#define MCP4725ADDR 0xC0 //Check this is what the scanner says
#define WRITE_REG 0x40 //Write and select normal power

void dac_i2c(unsigned int16 sample)
{     
   i2c_start();
   i2c_write(MCP4725ADDR);       // Device address
   i2c_write(WRITE_REG);            // Internal Device address & command
   i2c_write((sample & 0x0FF0) >> 4); //upper 2 nibbles
   i2c_write((sample & 0x0F) << 4);    //low nibble
   i2c_stop();                           // Stop
}

//Then test with something like:

   while(1)
   {
      for (i=0; i<4095); i+=16)
      {
         dac_i2c(i);
         delay_ms(200);
       }
   }



I'd be most suspicious the A0 pin is high or floating, so the address is wrong. The scanner program will tell you where it is.
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