Temprature conversion --- C to F

[Byte_Butcher]

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I have a temperature sensor (MCP9800) that spits out a 12 bit - 2's compliment number that represents degrees C from - 55 C to + 125 C.



12 bits of resolution gives increments of 1/16 degree C and displays to 4 decimal places, like so: 26.1875
I've got it up and running and successfully displaying temperature in degrees C. With this:

'-----------Read and display temperature from MCP9800-----------

checktemp9800:
    i2cread sda9800,clk9800,adr_R,[T_MSB,T_LSB]           'read the 2 bytes of temperature data
    TempC = abs T_MSB                                      'all the stuff to the left of the decimal point is in the MSB.
    TempCdec = (T_LSB >>4) * 625                          'stuff to the right of the decimal point. Side shift out the unused bits and multiply by 625 for 1/16 degree C resolution.

    If T_MSB.7  then                                         'if the sign bit is "1", display a minus sign
        signind = "-"
        else
        signind = "+ "
    endif


    Lcdout $fe,2, "  ", signind, DEC TempC, "." ,DEC4 TempCdec, 223, " C   "   ' Display the info

return

By I can't for the life of me figure out how to do the conversion from degrees C to degrees F. !!

I have NO problem for temperatures ABOVE 32 F (0 C) You just multiply by 1.8 (or 18/10) and add 32. Simple...

But when the temperature goes BELOW 0 C then my life (and math) falls apart and I can't make it work right. And I'm not even sure what to do when the temp falls even further to less than 0 F.
I'm not sure why my brain won't do this. It seems like it should be simple enough.

I want to end up with degrees F available to at least 1 (preferably 2) decimal places, and it should work right over the whole range of -55 C to + 125C (-67 F to +257F)

Can someone please spin me around and pull the blindfold off?

Oh, and my "device" is a 16F887, so I'm stuck with 16 bit math.

Thanks...

[Demon]

I haven't touched a PIC in a while, but I vaguely remember a lot of problems with negative math on the 16F family.

Wasn't there a thread here somewhere about something like DT's interrupt stuff but about negative math?

(searching...)

Robert

EDIT: Like this thread:
http://www.picbasic.co.uk/forum/showthread.php?t=7777

"...PBP can't multiply or divide negative numbers. (unless you are using PBPL on an 18F) - DT"

[Heckler]

I feel your pain!! the math was WAY over my head also... luck was in my favor and I stumbled across an INCLUDE file by Darrel Taylor. His routine will convert between C, F, Kelvin and possibly more!!!

Here is my temperature conversion... after reading a DS18B20 one wire temp sensor.

bear in mind that this routine is for a One Wire DS18b20 temp sensor... and the value is a two BYTE value that can be programmed for 9-12 bit resolution and the value is initially read in deg C

'=======TEMPERATURE subroutine==============================================
ShowTemp:
    while PortA.3=0 : wend
    PAUSE 500
    OWOUT  Comm_Pin, 1, [$CC, $4E, 0, 0, DS18B20_9bit]  'set resolution of sensor
Start_Convert:
    OWOUT  Comm_Pin, 1, [$CC, $44]' Skip ROM search & do temp conversion

Wait_Up:
    OWIN   Comm_Pin, 4, [Busy]    ' Read busy-bit
    IF     Busy = 0 THEN Wait_Up  ' Still busy..?, Wait_Up..!
    OWOUT  Comm_Pin, 1, [$CC, $BE]' Skip ROM search & read scratchpad memory
    OWIN   Comm_Pin, 2, [Raw.LOWBYTE, Raw.HIGHBYTE]' Read two bytes / end comms
Convert_Temp:
    Sign="+"
    IF Cold_Bit = 1 THEN    'it's below zero Celsius
    C=(ABS Raw>>4)*-10      'so shift the ABS value right and mult X -10
    ELSE
    C=(Raw>>4)*10           'else shift value right and mult X 10
    ENDIF

@ CtoF _C, _F             ; Convert Celsius to Fahrenheit
        'converted value will be X 10, ie. 756=75.6 deg F
        'so devide by 10 to get whole degrees

    IF f.15=1 THEN Sign="-" 'if converted value is below zero F then sign is "-"
    TempF = (ABS f)/10      'take tha ABS value and /10
    IF f//10 >4 THEN TempF=TempF+1 'check remainder, if >4 then round up

    IF TempF <10 THEN         '1 digit value plus "deg F"
        len=3
    elseif TempF <100 THEN    '2 digit value plus "deg F"
        len=4
    ELSE                      '3 digit value plus "deg F"
        len=5
    ENDIF
    arraywrite msg,[Sign,DEC TempF,"*"] ' *= lookup for "deg F"
    GOSUB Message                       ' display current temp

if intrpt=1 then SleepNow  'if here by interrupt then return to SleepNow
IF mode=0 THEN Motto        'if here from motto return to motto 
GOTO Icon                   'else return to icon

Here are most of the variables associated with the temp conversion routine.

'-------------[variables and constants for temperature routine]--------------
DS18B20_9bit  CON 011111   ' set resolution on DS18B20 93.75ms, 0.5 C
Comm_Pin      VAR   PortA.4   ' One-wire Data-Pin "DQ" on PortA.4
Busy          VAR BIT         ' Busy Status-Bit
Raw           VAR   WORD      ' RAW Temperature readings
TempF         VAR WORD        ' Temp in deg F
Cold_Bit      VAR Raw.BIT11   ' Sign-Bit for +/- Temp. 1 = Below 0 deg C
Sign          VAR BYTE        ' +/- sign for temp display

Don't forget to add this line to your program...

INCLUDE "Temp_Convert_CtoFonly.pbp"

also you will need to delete the .txt extension off from the attached include file.

I hope this helps... ask questions if you need to.

possibly most if what you need is Darrels (as always excellent) include file.

[Heckler]

Here is a link to an article that I wrote that describes reading the ONE WIRE DS18b20 temp sensor...


http://www.picbasic.co.uk/forum/cont...r-nine-of-them

[Byte_Butcher]

Thank You Heckler!

That looks like exactly what I need!
I should have known that Darrel had already written something cool to cover it. (Thank You Darrel !!)

I'll have a read through that stuff this evening and report back tomorrow.

[Normnet]

F = C * 9 'DO LESS THE BIT.7 NEG/POS BIT
F = F / 5
IF T_MSB.7 = 1 THEN 'if the sign bit is "1", is minus C
F = 32 - F
else
F = 32 + F
ENDIF

Norm

[Normnet]

F = C * 9 'DO LESS THE BIT.7 NEG/POS BIT
F = F / 5
IF T_MSB.7 = 1 THEN 'if the sign bit is "1", is minus C
F = 32 - F
else
F = 32 + F
ENDIF

Norm

Try again:
F = C * 9 'DO LESS THE NEG/POS BIT
F = F / 5
IF T_MSB.7 = 1 THEN 'if the sign bit is "1", is minus C
F = F - 32
else
F = F + 32
ENDIF