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MCP9800 setup [SOLVED]

 
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-Terppa-



Joined: 08 Jan 2018
Posts: 25
Location: Finland

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MCP9800 setup [SOLVED]
PostPosted: Fri Jan 31, 2020 3:34 am     Reply with quote

Hello for everyone!

I have small question about MCP9800 temperature sensor which is very similar to LM75.

I try to set that sensor higher resolution but i always get only 0.5c resolution.

MCP9800 init:
Code:

i2c_start(I2CTEMPSENSOR);
i2c_write(I2CTEMPSENSOR,mcp9800_i2c_write);                 
i2c_write(I2CTEMPSENSOR,MCP9800_CONFIG_REG_ADDR); //0x01
i2c_write(I2CTEMPSENSOR,0b01100000);
i2c_stop();


MCP9800 read temperature (test code)
Code:

signed long datah, datal;
signed long data;

i2c_start();
i2c_write(mcp9800_i2c_write);
i2c_write(MCP9800_TEMP_REG_ADDR); //0x00
i2c_start();
i2c_write(mcp9800_i2c_write+1);
datah=i2c_read();
datal=i2c_read(0);
i2c_stop();

data=(signed long)datah*10;

   if(bit_test(datal,7))
   {
      if(data<0)data-=5;
      else data+=5;
   }
//fahrenheit:
data=(data)*90;
data=(data/50)+320;

//celcius:
data*=10;
data=(data-3200)/18;


What silly mistake i have here? Thank you very much for your help!


Last edited by -Terppa- on Tue Feb 11, 2020 6:39 am; edited 1 time in total
Ttelmah



Joined: 11 Mar 2010
Posts: 16935

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PostPosted: Fri Jan 31, 2020 4:50 am     Reply with quote

Problem is your conversion code is only using the top bit of the datal
return, so any extra resolution is lost....

Code:

union {
    signed int16 data;
    unsigned int8 bytes[2];
} value;


i2c_start();
i2c_write(mcp9800_i2c_write);
i2c_write(MCP9800_TEMP_REG_ADDR); //0x00
i2c_start();
i2c_write(mcp9800_i2c_write+1);
value.bytes[1]=i2c_read();
value.bytes[0]=i2c_read(0);
i2c_stop();
value.data/=16;



value.data, is now the signed integer data, in 1/16th degree steps.
-Terppa-



Joined: 08 Jan 2018
Posts: 25
Location: Finland

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PostPosted: Fri Jan 31, 2020 8:47 am     Reply with quote

Thank you very much Mr.Ttelmah! I just not noticed it at all.
-Terppa-



Joined: 08 Jan 2018
Posts: 25
Location: Finland

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PostPosted: Thu Feb 04, 2021 4:23 am     Reply with quote

This seems works okay but if temperature gets negative i can get say: 6543.8?

and if temperature gets positive everything is okay..

How i tune this function?

Thank you for your help!

Code:

signed long MCP9800_read_temp(void)
{

union
{
signed int16 data;
unsigned int8 bytes[2];
} value;

i2c_start();
i2c_write(mcp9800_i2c_write);
i2c_write(MCP9800_TEMP_REG_ADDR);
i2c_start();
i2c_write(mcp9800_i2c_write+1);
value.bytes[1]=i2c_read();
value.bytes[0]=i2c_read(0);
i2c_stop();
value.data/=16;

value.data*=10;
return value.data=(value.data-320)/18;

Ttelmah



Joined: 11 Mar 2010
Posts: 16935

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PostPosted: Thu Feb 04, 2021 4:37 am     Reply with quote

Use the union as I show. Casting does not carry the sign bit.
-Terppa-



Joined: 08 Jan 2018
Posts: 25
Location: Finland

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PostPosted: Thu Feb 04, 2021 5:27 am     Reply with quote

Thank you! It works. Too much haste here Embarassed
I just put sensor outside and now it shows -17.3 Hmm.. very cold in here.
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