Rainbow Electronics DS2422 - Manual (Page 13)

DS2422
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SAMPLE RATE
The content of the Sample Rate Register (addresses 0206h, 0207h) specifies the time elapse (in seconds if EHSS
= 1, or minutes if EHSS = 0) between two temperature/data logging events. The sample rate may be any value
from 1 to 16383, coded as an unsigned 14-bit binary number. If EHSS = 1, the shortest time between logging
events is 1 second and the longest (sample rate = 3FFFh) is 4.55 hours. If EHSS = 0, the shortest is 1 minute and
the longest time is 273.05 hours (sample rate = 3FFFh). The EHSS bit is located in the RTC Control Register at
address 0212h. It is important that the user sets the EHSS bit accordingly while setting the Sample Rate register. A
sample rate of 0000h is not valid and must be avoided under all circumstances. This causes the device to enter
into an undefined state, requiring a power-on reset and restore of the trim settings to recover.
Sample Rate Register Bitmap
ADDR b7 b6 b5 b4 b3 b2 b1 b0
0206h Sample Rate Low
0207h 0 0 Sample Rate High
During a mission, there is only read access to these registers. Bits cells marked "0" always read 0 and cannot be
written to 1.
TEMPERATURE CONVERSION
The DS2422 can measure temperatures from -40°C to +85°C. Temperature values are represented as an 8- or 16-
bit unsigned binary number with a resolution of 0.5°C in the 8-bit mode and 0.0625°C in the 16-bit mode.
The higher temperature byte TRH is always valid. In the 16-bit mode only the three highest bits of the lower byte
TRL are valid. The five lower bits all read zero. TRL is undefined if the device is in 8-bit temperature mode. An out-
of-range temperature reading is indicated as 00h or 0000h when too cold and FFh or FFE0h when too hot.
Latest Temperature Conversion Result Register Bitmap
ADDR b7 b6 b5 b4 b3 b2 b1 b0
020Ch T2 T1 T0 0 0 0 0 0 TRL
020Dh T10 T9 T8 T7 T6 T5 T4 T3 TRH
With TRH and TRL representing the decimal equivalent of a temperature reading the temperature value is
calculated as
J(°C) = TRH/2 - 41 + TRL/512 (16 bit mode, TLFS = 1, see address 0213h)
J(°C) = TRH/2 - 41 (8 bit mode, TLFS = 0, see address 0213h)
This equation is valid for converting temperature readings stored in the datalog memory as well as for data read
from the Latest Temperature Conversion Result Register.
To specify the temperature alarm thresholds, the equation above needs to be resolved to
TALM = 2 * J (°C) + 82
Since the temperature alarm threshold is only one byte, the resolution or temperature increment is limited to 0.5°C.
The TALM value needs to be converted into hexadecimal format before it can be written to one of the temperature
alarm threshold registers (Low Alarm address 0208h; High Alarm address 0209h). Independent of the
conversion mode (8 or 16 bit) only the most significant byte of a temperature conversion is used to determine
whether an alarm will be generated.
Temperature Conversion Examples
Mode
TRH
hex decimal
TRL
hex decimal
J(°C)
8-bit 54h 84 — — 1.0
8-bit 17h 23 — — -29.5
16-bit 54h 84 00h 0 1.000
16-bit 17h 23 60h 96 -29.3125