DS1921H/Z Maxim Integrated Products, DS1921H/Z Datasheet - Page 8

• Manufacturer Part Number: DS1921H/Z
• Description: High Resolution Thermochron Ibutton Range H
• Manufacturer: Maxim Integrated Products

If a temperature conversion yields a temperature that is out-of-range, it will be recorded as 00h (if too
low) or FFh (if too high). Since out-of-range results are accumulated in histogram bins 0 and 63 the data
in these bins is of limited value (see the Temperature Logging and Histogram section). For this reason the
specified temperature range of the DS1921H and DS1921Z is considered to begin at code 04h and end at
code FBh, which corresponds to histogram bins 1 to 62.
With T[7..0] representing the decimal equivalent of a temperature reading, the temperature value is
calculated as
J (°C) = T[7…0] / 8 + 14.500 (DS1921H)
J (°C) = T[7…0] / 8 - 5.500 (DS1921Z)
This equation is valid for converting temperature readings stored in the datalog memory as well as for
data read from the forced temperature conversion readout Register (address 211h).
To specify the high or low temperature alarm thresholds, this equation needs to be resolved to
T[7…0] = 8 * J (°C) -116 (DS1921H)
T[7…0] = 8 * J (°C) + 44 (DS1921Z)
A value of 23°C, for example, thus translates into 68 decimal or 44h for the DS1921H, and 228 decimal
or E4h for the DS1921Z. This corresponds to the binary patterns 0100 0100 and 1110 0100 respectively,
which could be written to a Temperature Alarm Register (address 020Bh and 020Ch, respectively).
Temperature Alarm Register Map
SAMPLE RATE
The content of the Sample Rate Register (address 020Dh) determines how many minutes the temperature
conversions are apart from each other during a mission. The sample rate may be any value from 1 to 255,
coded as an unsigned 8-bit binary number. If the memory has been cleared (Status Register bit MEMCLR
= 1) and a mission is enabled (Status Register bit EM = 0), writing a non-zero value to the Sample Rate
Register will start a mission. For a full description of the correct sequence of steps to start a temperature-
logging mission see sections Missioning or Missioning Example.
Sample Rate Register Map
CONTROL REGISTER
The DS1921H/Z is set up for its operation by writing appropriate data to its special function registers that
are located in the register page. Several functions that are controlled by a single bit only are combined
into a single byte called Control Register (address 20Eh). This register can be read and written. If the
device is programmed for a mission, writing to the Control Register will, at the first attempt, end the
mission, but not overwrite any settings. Every subsequent write attempt, however, will change the register
contents.
020Bh
020Ch
020Dh
ADDR
ADDR
b7
b7
b6
b6
Temperature High Alarm Threshold
Temperature Low Alarm Threshold
b5
b5
Sample Rate
b4
b4
b3
b3
8 of 40
b2
b2
b1
b1
b0
b0