How come my DS18S20 code still works?

[Wilson]

I have been trying for 2 weeks to get my project working properly and now I am at my wits end.
Part of my project is to read a DS18S20 and display the correct termperature on a 2x16 lcd display. I have searched the forum and all the examples for reading the device are the same so I just used it and it worked great.
Problem is I cannot understand what the count_remain variable actually does. I am also confused about the count_per_c variable. So I did what any older person would do and just remarked them out of the code to see what would happen. But nothing did. Everything worked the same.

the code is as follows:

Hope someone can help

Wilson

@ DEVICE pic16F628a, INTRC_OSC_NOCLKOUT,LVP_OFF,WDT_OFF,MCLR_OFF,PROTEC T_OFF
@ DEVICE pic16F628a, CPD_OFF
trisa =%00100000
trisb =%00000000
cmcon =%00000111 'Comparators Off
Define LCD_BITS 4
Define LCD_DREG PORTB
Define LCD_DBIT 0
Define LCD_RWREG PORTA
Define LCD_RWBIT 4
Define LCD_RSREG PORTA
Define LCD_RSBIT 3
Define LCD_EREG PORTA
Define LCD_EBIT 2
Define LCD_COMMANDUS 5000
Define LCD_DATAUS 100
Define LCD_INITMS 2

pause 100

DQ VAR PORTB.5 ' One-wire data pin
temperature VAR WORD ' Temperature storage
'I removed the next two lines
'count_remain VAR BYTE ' Count remaining
'count_per_c VAR BYTE ' Count per degree C

loop:
LCDOUT $FE, 1
LCDOut $FE, $C0, "Temp ", dec(temperature / 100)," C "

OWOut DQ, 1, [$CC, $44] ' Start temperature conversion
pause 750 'NEEDED FOR TEMP STABILIZING

'and the following three lines
'waitloop:
'OWIn DQ, 4, [count_remain] ' Check for still busy converting
'IF count_remain = 0 Then waitloop

OWOut DQ, 1, [$CC, $BE] ' Read the temperature

'as well as the last bits of the next two lines
OWIn DQ, 0, [temperature.LOWBYTE, temperature.HIGHBYTE]', Skip 4, count_remain, count_per_c]

temperature = (((temperature >> 1) * 100) - 25)' + (((count_per_c - count_remain) * 100) / count_per_c)

goto loop

[Wilson]

I have studied (but not fully understood) the 18S20 data sheet and concluded that all that needs to happen is...

1: Initiate the device (start the conversion)
2: Wait until the conversion is complete (pause for at least 750ms)
3: Read what is in the register.

I have it running okay. The LCD displays the raw temp i.e. the register value in decimal as well as hex and whether bit 0 is set or not.

It seems that the value in the 18S20 register is actually 0.5 degrees centigrade units
so I figured out why not just divide it by 2 to get the temperature.

@ DEVICE pic16F628a, INTRC_OSC_NOCLKOUT,LVP_OFF,WDT_OFF,MCLR_OFF,PROTECT_OFF
@ DEVICE pic16F628a, CPD_OFF
trisa =%00100000
trisb =%00000000
cmcon =%00000111 'Comparators Off
Define LCD_BITS 4
Define LCD_DREG PORTB
Define LCD_DBIT 0
Define LCD_RWREG PORTA
Define LCD_RWBIT 4
Define LCD_RSREG PORTA
Define LCD_RSBIT 3
Define LCD_EREG PORTA
Define LCD_EBIT 2
Define LCD_COMMANDUS 5000
Define LCD_DATAUS 100
Define LCD_INITMS 2

pause 100

DQ VAR PORTB.5 ' One-wire data pin
temperature VAR WORD ' Temperature storage
decimal var byte
loop:
	 
OWOut DQ, 1, [$CC, $44] ' Start temperature conversion
pause 750 'Needed to complete conversion
OWOut DQ, 1, [$CC, $BE] ' Read the temperature
OWIn DQ, 0, [temperature.LOWBYTE, temperature.HIGHBYTE]

'just check if bit0 is set
decimal = temperature mod 2
if decimal = 1 then 'if set, it must be .5
decimal =5
else
decimal = 0
endif

' raw value is the register value
lcdout $FE,1, "Raw ", dec(temperature)," ", hex(temperature), " ", dec(decimal)

LCDOut $FE, $C0, "Temp ", dec(temperature /2), ".", dec(decimal), " C"

goto loop

My LCD displays
Raw 44 2C 0
Temp 22.0 C

When I touch the 18S20 for five seconds I get

Raw 55 37 5
Temp 27.5 C

I am not sure how accurate it is because I do not have a reference temperature.
I tried putting the 18S20 under my tongue but I got a shock.
It seems to work okay but I am not convinced it is the correct way to go about it.

Am I on the right track or just plain lucky that it works?

Wilson

[Acetronics2]

Hi,

At this time, there's another thread running about the 18S20 or 18B20 ...

just have a look to the methods used ...

http://www.picbasic.co.uk/forum/showthread.php?t=7079

Alain

[Wilson]

Thanks Alain

As I mentioned when I started the thread, I have already searched the forum read all the posts about the 18S20.
The only thing I learned was that someone wrote a piece of code a long time ago and it seems to be the defacto standard- everyone uses it. The samples are the same, comments and all. Thats okay. Cutting and pasting is easy but it doesn't help me learn.

Wilson

[Acetronics2]

Hi, Wilson

Learning is on your own whatever the book ...

so just trying to understand what happens is ... enough !!!

Why not cut and paste ... if you know what you're pasting ...

LOL ...

Following you, ... you also can make it work without Owin ansd Owout ... just toggling pins on the adequate moment ...

Alain

[b1arrk5]

Have you looked at Bruce's website? His Rentron Electronics site has some tutorials on using one wire devices, and explains it very well. He used the 18b20 in his examples, but I was able to get an 18s20 working after reading his explanations. I'm not where I can look it up, but I think the count remaining is a way of checking to see if the temperature conversion is completed or not, instead of just waiting a set amount of time like the 'pause' command does. So just pausing 750ms eliminates the need to check if the conversion is completed. Bruce also has an example that uses DIV32 to get better precision, but for my application anything within three degrees was close enough. Here is the link to the first example.

http://www.rentron.com/PicBasic/PBP1-wire.htm

Jerry.