Working with the DS2438 using the OW-SERVER v2 low level commands

There are wide range of 1-Wire sensors based off of Maxim Integrated's DS2438. The OW-SERVER v2 presents much of the DS2438 data within its XML tags along with a calculation of the humidity assuming the DS2438 is a TAI8540D.  This knowledge base article discusses how to work with the DS2438 sensor and an OW-SERVER v2 bus master using the low level commands.  Using the low level commands gives the programmer greater flexibility in how the sensor is read. 

The general process for interacting with DS2438 based sensors is as follows:

• Discover the Device
• Read Status/Configuration Register
• Parse the results
• Initiate the temperature conversion
• Wait for the conversion to complete
• Read the results
• Parse the results
• Configure AD Bit
• Wait for NV Write Cycle
• Initiate the second voltage conversion
• Wait for the conversion to complete
• Read the results
• Parse the result

The following is an annotated example of the specific commands to send and typical responses received from the OW-SERVER v2 to accomplish the steps above. In the examples below, TX= data transmitted to the OW-SERVER v2, and RX= data received from the HA7. This example is written for a DS2438 having ROMId 'DC0000006CC62126'. The manufacturer's datasheet on the DS2438 can be downloaded from http://short.eds.bz/ds2438-datasheet.

Discover the Devices

In this example, the ds2438 is the only device connected to port 1 so we simply use the 'S'earch Rom command to identify the first device:

Read Status/Configuration Register

Parse the results

Based on the DS2438 Memory Map on page 15 of the datasheet, we can parse the scratchpad bytes returned above to determine the following:

Bit 0 = IAD = Current A/D Control Bit
Bit 1 = CA = Current Accumulator Configuration
Bit 2 = EE = Current Accumulator Shadow Selector bit
Bit 3 = AD = Voltage A/D Input Select Bit
Bit 4 = TB = Temperature Busy Flag
Bit 5 = NVB = Nonvolatile Memory Busy Flag.
Bit 6 = ADB = A/D Converter Busy Flag.
Bit 7 = X = Don’t care

For most sensor applications, a combination of Temperature, VAD, VDD, and/or VSens are used.  Bit 0 (IAD) determines if the current readings are updating at a rate of 36.41Hz. The default "1" is enabled.  Writing the bit to "0" would disable the updating.  Bit 3 (AD) identifies whether a voltage conversion reads the VDD or VAD.  The default "1" selects the VDD.  Writing the bit to "0" reads the VAD.

Initiate Temperature ConversionThe temperature conversion process on the ds2438 is initiated by transmitting command byte '0x44' to the device, as follows:

Wait for conversion

You must now wait 10mS for the conversion to take place before you can read the results back from the device.  If the ds2438 is being powered parasitically, then no other activity can occur on the 1-Wire bus during the temperature conversion, or the ds2438 will be power starved.  Note that the HA7 automatically provides strong pull-up power on the 1-Wire bus during the conversion.

Initiate the first voltage conversion

Read the Results

The resulting temperature is stored in the ds2438's scratchpad memory.  To obtain the temperature data, simply read the scratchpad from the device and parse the temperature data out of the scratchpad.  For complete details on how the temperature data is encoded into the scratchpad, please refer to the ds2438 datasheet.

Reading the ds2438's scratchpad.

The read above returned the following scratchpad bytes: 0FD016DE01D80100FF.

Parse the Results

Based on the DS2438 Memory Map on page 6 of the datasheet, we can parse the scratchpad bytes returned above to determine the following:

Byte 0 = STATUS/CONFIGURATION = 0x0F
Byte 1 = TEMPERATURE LSB = 0xD0 = 208
Byte 2 = TEMPERATURE MSB  = 0x16 = 22
Byte 3 = VOLTAGE LSB = 0xDE = 222
Byte 4 = VOLTAGE MSB = 0x01 = 1
Byte 5 = CURRENT LSB = 0xD8 = 216
Byte 6 = CURRENT MSB = 0x01 = 1
Byte 7 = THRESHOLD = 0x00
Byte 8 = CRC = 0xFF

Temperature is signed and formatted such that Byte_2 holds the integer number of centigrade degrees in the least significant 7 bits.  Bit 8 is the sign bit and when the sign bit is set the value of the Byte_2 and Byte_1 must be inverted.  Byte_1 always describes the fractional part of the temperature in units of 0.00390625 = (2^-8).  The routine can be used to interpret the value.

if (Byte_2 & 0x80) {
  // Negative Temperature
  TempWhole = ~Byte_2
  TempFrac = ((~Byte_1) & 0xF8) * (2^-8)
  Temp = -1 * (TempWhole + TempFrac)
}
else {  
  // Positive Temperature
  TempWhole = Byte_2
  TempFrac = (Byte_1 & 0xF8) * (2^-8)
  Temp = Byte_2 + TempFrac
}

The algorithm applied

  // Positive Temperature
  TempWhole = 22
  TempFrac = (208 & 0xF8) * (2^-8) = 0.8125
  Temp = 22 + 0.8125 = 22.8125

Temperature = Temp and is expressed in C

The Voltage Bytes reflect the VDD as determined by the AD bit.    

Supply Voltage (VDD) = ((Byte_3 + (Byte_4 * 0xFF)) & 0x3FF) * 0.01
Supply Voltage (VDD) = ((222 + (1 * 256)) & 0x3FF) * 0.01 = 4.78

Configure AD Bit

The status configuration byte was 0x0F on the last read.  To change the AD bit (Bit 3) to "0".  The following command can be used to set the bit 3 to "0" while setting IAD, CA & EE to defaults.  Setting IAD bit to the default value of "1" would start the current readings updating. However, current readings were already updating in this example.   

Wait for NV Write Cycle

You must wait 10mS for NV write cycle to be to be completed.

Initiate the second voltage conversion

Read the results

Reading the ds2438's scratchpad.

Parse the Results

Based on the DS2438 Memory Map on page 6 of the datasheet, we can parse the scratchpad bytes returned above to determine the following:

Byte 0 = STATUS/CONFIGURATION = 0x07
Byte 1 = TEMPERATURE LSB = 0xD0 = 208
Byte 2 = TEMPERATURE MSB  = 0x16 = 22
Byte 3 = VOLTAGE LSB = 0xDE = 222
Byte 4 = VOLTAGE MSB = 0x01 = 1
Byte 5 = CURRENT LSB = 0xD8 = 216
Byte 6 = CURRENT MSB = 0x01 = 1
Byte 7 = THRESHOLD = 0x00
Byte 8 = CRC = 0xFF

The Voltage Bytes reflect the VAD as determined by the AD bit.

General Purpose A/D Voltage (VAD) = ((Byte_3 + (Byte_4 * 0xFF)) & 0x3FF) * 0.01
General Purpose A/D Voltage (VAD) = ((222 + (1 * 256)) & 0x3FF) * 0.01 = 4.78

When IAD bit is "1" the VSens readings are being calculated.

VSens (Current) = ((Byte_5 + (Byte_6 * 0xFF)) & 0x3FF) * 0.0002441
VSens (Current) = ((216 + (1 * 256)) & 0x3FF) * 0.0002441 = 0.1152

The VSens reads the voltage drop over a know resistance.  This data can be used to calculate the current.