- Blog entry posted in 'Electronics and Other Ramblings...', November 22, 2012.
I’ve been meaning to work with the DS18B20 1Wire device from Maxim for a while now. During an information exchange with a fellow user on the forum on reading multiple DS18B20s I decided to take the plunge. I ordered 10 devices from some “reputable” far east vendor on eBay. The parts (along with some much needed relays) came in a short 10days.
Parts came in right before the Thanksgiving holiday (what a break). What are we supposed to do but give them a try. I checked each of them and all seem to operate ok. To address multiple 1Wire DS18B20 devices using an MCU a few options are available:
• connect each device to individual pins on the MCU,
• connect all devices to one pin through a multiplexer (two or more extra MCU pins are required for addressing),
• connect all devices to one pin an address each through 1Wire commands.
To address all DS18B20 using the same MCU pin, I followed the guidelines from Reynolds Electronics to read the ROM from each device so each device could be addressed on the same bus (credit also to Heckler@PBP). After reading the ROM code for each of the individual DS18B20, each can then be addressed through the 1Wire commands.
Three sensors were connected and their ROM codes read.
sensor1
FC = 28h, CRC = 89h
S/N = 2A 83 4E 04 00 00
ROM CODE: 28 2A 83 4E 04 00 00 89
sensor2
FC = 28h, CRC = FFh
S/N = 64 50 4E 04 00 00
ROM CODE: 28 64 50 4E 04 00 00 FF
sensor3
FC = 28h, CRC = 7Ch
S/N = 6B 0A 4F 04 00 00
ROM CODE: 28 6B 0A 4F 04 00 00 7C
The code to measure the temperature was updated to address each sensor, and the temperature read.
T = +26.2
T = +26.1
T = +26.1
Below is the code used to read the ROM code from each device (using PIC18F1320 and Oshonsoft).
'Port I/O definition
Symbol io_rs232 = RB7
'variable definition
Dim temp_w As Word
Dim temp_b As Byte
Dim romdata(8) As Byte
Dim cnt As Byte
'constant definition
Const readrom = $33 'read ID, serial num, CRC
Const matchrom = $55 'look for specific device
Const skiprom = $cc 'skip rom (one device)
Const converttemp = $44 'do temperature conversion
Const readscratch = $be 'read DS1820 scratchpad
main:
AllDigital
'setup port B for 1-wire use
Define 1WIRE_REG = PORTB
Define 1WIRE_BIT = 0
'setup rs232 output
Config io_rs232 = Output
WaitMs 5000
Serout io_rs232, 9600, "Start...", CrLf
'acquire ROM data
Serout io_rs232, 9600, "1Wire Init...", CrLf
1wireInit
1wireSendByte readrom
For cnt = 0 To 7
1wireGetByte romdata(cnt)
Next cnt
'display on rs232
Serout io_rs232, 9600, "FC = "
temp_b = romdata(0)
temp_w = byte2hex(temp_b)
Serout io_rs232, 9600, temp_w.HB, temp_w.LB, "h, CRC = "
temp_b = romdata(7)
temp_w = byte2hex(temp_b)
Serout io_rs232, 9600, temp_w.HB, temp_w.LB, "h", CrLf
Serout io_rs232, 9600, "S/N = "
For cnt = 1 To 6 Step 1
temp_b = romdata(cnt)
temp_w = byte2hex(temp_b)
Serout io_rs232, 9600, temp_w.HB, temp_w.LB, " "
Next cnt
Serout PORTB.1, 9600, "h", CrLf
End
'Convert byte to HEX for ASCII display
Function byte2hex(num As Byte) As Word
Dim hexh As Byte
Dim hexl As Byte
hexh = num And 0xf0
hexh = ShiftRight(hexh, 4)
byte2hex.HB = nibble2hex(hexh)
hexl = num And 0x0f
byte2hex.LB = nibble2hex(hexl)
End Function
'Convert HEX nibble to ASCII and display
Function nibble2hex(num As Byte) As Byte
If num <= 9 Then
nibble2hex = num + 30h
Else
nibble2hex = num + 37h
Endif
End Function
Following is the code used to read the temperature from each of the three DS18B20s connected to the same MCU pin (using PIC18F1320 and Oshonsoft).
'variable definition
Dim temperature As Word
Dim frac_d1 As Word
Dim cnt As Byte
Dim sign_char As Byte
Dim sign_bit As Bit
Dim finish As Bit
'Port I/O definition
Symbol io_rs232 = RB7
'constant definition
Const readrom = 0x33 'read ID, serial num, CRC
Const matchrom = 0x55 'look for specific device
Const skiprom = 0xcc 'skip rom (one device)
Const converttemp = 0x44 'do temperature conversion
Const readscratch = 0xbe 'read DS1820 scratchpad
main:
AllDigital
'setup port B for 1-wire use
Define 1WIRE_REG = PORTB
Define 1WIRE_BIT = 0
'setup rs232 output
Config io_rs232 = Output
WaitMs 5000
Serout io_rs232, 9600, "Start...", CrLf
read_temp:
'request temperature conversion
Serout io_rs232, 9600, "1Wire Init...", CrLf
Dim temperature_array(3) As Word
For cnt = 0 To 2
1wireInit
If cnt = 0 Then
1wireSendByte matchrom, 0x28, 0x2a, 0x83, 0x4e, 0x04, 0x00, 0x00, 0x89, converttemp
Endif
If cnt = 1 Then
1wireSendByte matchrom, 0x28, 0x64, 0x50, 0x4e, 0x04, 0x00, 0x00, 0xff, converttemp
Endif
If cnt = 2 Then
1wireSendByte matchrom, 0x28, 0x6b, 0x0a, 0x4f, 0x04, 0x00, 0x00, 0x7c, converttemp
Endif
finish = 0
wait:
1wireGetBit finish
If finish = 0 Then Goto wait
'request temperature byte
1wireInit
If cnt = 0 Then
1wireSendByte matchrom, 0x28, 0x2a, 0x83, 0x4e, 0x04, 0x00, 0x00, 0x89, readscratch
Endif
If cnt = 1 Then
1wireSendByte matchrom, 0x28, 0x64, 0x50, 0x4e, 0x04, 0x00, 0x00, 0xff, readscratch
Endif
If cnt = 2 Then
1wireSendByte matchrom, 0x28, 0x6b, 0x0a, 0x4f, 0x04, 0x00, 0x00, 0x7c, readscratch
Endif
1wireGetByte temperature.LB, temperature.HB
temperature_array(cnt) = temperature
Next cnt
'display temperature
For cnt = 0 To 2
temperature = temperature_array(cnt)
'sign bit
sign_bit = temperature.11
If sign_bit = True Then
temperature = Not temperature
frac_d1 = ((temperature Mod 16) * 10 + 8) / 16 'adds 8 to the mod result to round-up
temperature = temperature / 16
sign_char = "-"
Else
temperature = temperature And 0b0000011111111111
frac_d1 = ((temperature Mod 16) * 10 + 8) / 16 'adds 8 to the mod result to round-up
temperature = temperature / 16
sign_char = "+"
Endif
'display on rs232
Serout io_rs232, 9600, "T = ", sign_char, #temperature, ".", #frac_d1, CrLf
Next cnt
Goto read_temp
End
Connection diagram shown below for reference.
70216Comments
Micheal_Up, December 03, 2012
The [URL="http://www.hqew.net/product-data/DS18B20/"]DS18B20[/URL] digital thermometer provides 9-bit to 12-bit Celsius temperature measurements and has an alarm function with nonvolatile user-programmable upper and lower trigger points. More information about at [URL="http://www.hqew.net/product-data/DS18B20/DS18B20-DataSheet.html"]Datasheet[/URL]. More information about at [URL="http://www.hqew.net/product-data/DS18B20/DS18B20-CircuitDiagram.html"]Circuit Diagram[/URL].
