2313temper8.txt

temp cont with assmbly code

10MHZ 2313Temper8  version date 20090402

2313Temper8 is an AT90S2313 AVR assembly language program that reads the extended temperature data (good to about 1/16 degrees C) from up to 8 Dallas DS18x20 (DS1820 or DS18S20) temperature sensors.

The DS18x20 is a simple 3-pin device that returns its temperature as a binary number. 
When commanded, 2313Temper8 reads the indicated DS18S20, converts its temperature data to degrees C and returns this temperature (as an ASCII string) via the '2313's serial port. 

Even though the software provides two significant digits past the decimal point, the DS18S20 isn't quite capable of this accuracy since the extended temperature calculation is limited to 1/16C (0.0625C).

2313Temper8 was originally written for an 8MHZ AT90S2313. The code has been upgraded to run at 10MHZ. It communicates via its serial port at 9600,8,N,1. A TTL <-> RS-232 level converter is required to communicate with a PC.

Commands (not case sensitive):
R returns the number of devices and their ROM IDs:
4                                                                       
0800080007CC0010                                                        
C000080007728A10                                                        
0D00080007455E10                                                        
3300080007B60510                                                        
0000000000000000                                                        
0000000000000000                                                        
0000000000000000                                                        
0000000000000000  

Tn (where n is a single digit from 0 to 7) returns the device temperature ie:
t2 returns:

 +023.52C

Vn (where n is a single digit from 0 to 7) returns a verbose reply, with the register contents of the device and the temperature ie:

v1 returns:

3300A199FFFF0810C4
+25.25C
 
The ROM ID codes are read at startup (or reset). If new devices are added after powerup, they will not be recognized. 
On reset, the program will look for DS18S20s, and if it finds them, reads their ID codes. 
It'll then send the ROM codes through the serial port. Devices are numbered and listed from 0 to 7.
                                                      
The DS18S20s are connected to Port B bit 2 (PB2), which is set as a bi-directional I/O pin. 
This setup is often referred to as the 1-Wire bus.

GND -------- DS18S20 pin 1 (GND)
PB2 -------- DS18S20 pin 2 (DQ)
+5V -------- DS18S20 pin 3 (VDD)

In most cases, a 4.7K pullup resistor must be connected between the 1-Wire bus (PB2) and +5V. 
In the code, I enable the internal PB2 pullup resistor every time I read or write to the DS18S20s, so this external pullup resistor is not needed. The code runs fine with the 4.7K resistor installed, though.

Reading multiple devices requires considerable code space and ram storage, so this version of the program will only read up to eight DS18S20s.

The extended temperature (as listed in the DS 1820 data sheet) is calculated using this equation:

Extended Temperature = Temperature_read - 0.25 + ((count_per_c - Count_remain)/count_per_c)

Where Temperature_read is the value read from the device, trucated to 8 bits (shift right by 1 or divide by 2)

The equation can be simplified to:

Temperature = Temperature_read - 0.25 + ((16 - Count_remain)/16)

since the DS1820 family always (double check, for new devices, though) uses 16 for the count_per_c value, this equation can be further simplified by multiplying everything by 16:

16 * Temperature = 16 * Temperature_read + 12 - Count_Remain

Multiplying the result by 25 then dividing by 4 eliminates fixed point values; 25.75 becomes 2575 an easy to deal with integer.




Acknowledgments
The AVR version of this code was developed on an Atmel STK500 board, using AVRStudio 4.16. 
The AT902313 code was ported from my AT90S8515 program, 8515Temp8, which is in turn a port from my 68HC11 program, LCDTemper8.                      

The ROMSrch (ROM Search) routine was ported from ML6805,a Public Domain assembly language example from Dallas Semiconductor. 
It was originally written for the 6805. 
I also used the code in The Dallas/Maxim Application Note 162: 
INTERFACING THE DS18X20/DS1822 1-WIRE TEMPERATURE SENSOR IN A MICRO-CONTROLLER ENVIRONMENT as a guide. 
Several Atmel AVR application notes donated code, and the AVR Freaks web page:
(http://www.avrfreaks.com) was a great source of good AVR assembly programming styles and methods.
Randy Ott (K5HJ) has a great collection of AVR code that was very helpful (http://www.qsl.net/k5hj/).

Thanks to Stephan Stadler for pointing out an arithmetic error that was producing incorrect temperature readings and to Michael Minns
for pointing out a bug that was not always getting rid of the 0.5C bit!

April 2, 2009

This program was edited and assembled in AVR Studio 4.16
Files in this zip archive:
2313Temp8.asm
2313Temp8.hex
2313Temp8.txt (this file)

Frank Henriquez <[e-mail]> 

The latest version of this code is at http://frank.bol.ucla.edu/2313Temper8.htm
<http://frank.bol.ucla.edu/index.htm>