Rainbow Electronics DS2422 - Manual (Page 24)

DS2422
24 of 48
Figure 10C. Two-Channel Logging, Different Resolution
2E00h
2FFFh
Temperature
2560
8-bit entries
External Data
2560
16-bit entries
(not used)
External Data
2560
8-bit entries
(not used)
Temperature
2560
16-bit entries
19FFh
1A00h
1000h
2DFFh
2E00h
2FFFh
2DFFh
2400h
1000h
23FFh
ETL = EDL = 1
TLFS = 0; DLFS = 1
ETL = EDL = 1
TLFS = 1; DLFS = 0
With 16-bit format,
the most-significant
byte is stored at the
lower address.
MISSIONING
The typical task of the DS2422 is recording temperature and/or external data. Before the device can perform this
function, it needs to be set up properly. This procedure is called missioning.
First of all, DS2422 needs to have its RTC set to valid time and date. This reference time may be the local time, or,
when used inside of a mobile unit, UTC (also called GMT, Greenwich Mean Time) or any other time standard that
was agreed upon. The RTC oscillator must be running (EOSC = 1). The memory assigned to store the Mission
Time Stamp, Mission Samples Counter, Sample Rate, and Alarm Flags must be cleared using the Memory Clear
command. To enable the device for a mission, at least one of the enable logging bits needs to be set to 1. These
are general settings that have to be made in any case, regardless of the type of object to be monitored and the
duration of the mission.
If alarm signaling is desired, the temperature alarm and/or data alarm low and high thresholds must be defined.
How to convert a temperature value into the binary code to be written to the threshold registers is described under
Temperature Conversion earlier in this document. Determining the thresholds for the data alarm depends on the
hardware/converter that is connected to the DS2422’s serial input. In addition, the temperature and/or data alarm
must be enabled for the low- and/or high-threshold. This makes the device respond to a Conditional Search
command (see ROM Function Commands), provided that an alarming condition has been encountered.
The setting of the RO bit (rollover enable) and sample rate depends on the duration of the mission and the
monitoring requirements. If the most recently logged data is important, the rollover should be enabled (RO = 1).
Otherwise one should estimate the duration of the mission in minutes and divide the number by 8192 (single
channel 8-bit format) or 4096 (single channel 16-bit format, two channels 8-bit format) or 2048 (two channels 16-bit
format) or 2560 (two channels, one 8-bit format and one 16-bit format) to calculate the value of the sample rate
(number of minutes between temperature conversions). If the estimated duration of a mission is 10 days (= 14400
minutes), for example, then the 8192-byte capacity of the datalog memory would be sufficient to store a new 8-bit
value every 1.8 minutes (110 seconds). If the datalog memory of the DS2422 is not large enough to store all
readings, one can use several devices and set the Mission Start Delay to values that make the second device start
logging as soon as the memory of the first device is full, and so on. The RO-bit needs to be set to 0 to disable
rollover that would otherwise overwrite the logged data.
After the RO bit and the Mission Start Delay are set, the sample rate needs to be written to the Sample Rate
Register. The sample rate may be any value from 1 to 16383, coded as an unsigned 14-bit binary number. A
sample rate of all zeros 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. The fastest sample
rate is one sample per second (EHSS = 1, Sample Rate = 0001h) and the slowest is one sample every 273.05
hours (EHSS = 0, Sample Rate =3 FFFh). To get one sample every 6 minutes, for example, the sample rate value
needs to be set to 6 (EHSS = 0) or 360 decimal (equivalent to 0168h at EHSS = 1).