DS1925 datasheet(18/46 Pages) MAXIM

18 / 46 page

Data-Log Memory Usage

Once set up for a mission, the DS1925 logs the tempera-

ture measurements at equidistant time points entry after

entry in its data-log memory. The data-log memory is able

to store 125,440 entries in 8-bit format or 62,720 entries in

16-bit format (Figure 7). In 16-bit format, the higher 8 bits

of an entry are stored at the lower address. Knowing the

starting time point (Mission Timestamp) and the interval

between temperature measurements, one can recon-

struct the time and date of each measurement.

The contents of the Mission Samples Counter in conjunc-

tion with the sample rate and the Mission Timestamp then

allows reconstructing the time points of all values stored

in the data-log memory. This gives the exact history over

time for the most recent measurements taken.

Missioning

The DS1925’s typical task is recording temperature.

Before the device can perform this function, it needs to

be set up properly. This procedure is called missioning.

First, the DS1925 needs to have its RTC set to valid time

and date. This reference time can be the local time, or,

when used inside of a mobile unit, UTC/GMT (Coordinated

Universal Time or 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 Timestamp, Mission Samples Counter,

and Alarm Flags must be cleared using the memory clear

command. To enable the device for a mission, the ETL bit

must 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 low

and high thresholds must be defined. See the Temperature

Conversion section on how to convert a temperature value

into the binary code to be written to the threshold regis-

ters. In addition, the temperature alarm must be enabled

for the low and/or high threshold. This makes the device

respond to a Conditional Search ROM command (see the

1-Wire ROM Function Commands section), provided that

an alarming condition has been encountered.

The setting of the sample rate depends on the duration of

the mission and the monitoring requirements. One should

estimate the duration of the mission in minutes and divide

the number by 125,440 (8-bit format) or 62,720 (16-bit

format) to calculate the value of the sample rate (number

of minutes between conversions). If the estimated dura-

tion of a mission is 300 days (= 432,000 minutes), for

example, then the 122KB capacity of the data-log mem-

ory would be sufficient to store a new 11-bit value every

6.9 minutes (413 seconds). If the data-log memory of the

DS1925 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.

After the Mission Start Delay is set, the sample rate needs

to be written to the Sample Rate register. The sample rate

may be any value from 1 to 16,383, coded as an unsigned

14-bit binary number. The fastest sample rate is one sam-

ple per 3 minutes (EHSS = 1, sample rate = 0B4h); how-

ever, the fasted recommended sample rate is 5 minutes

and the slowest is one sample every 273.05 hours (EHSS

= 0, Sample Rate = 3FFFh). 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).

If there is a risk of unauthorized access to the DS1925

or manipulation of data, one should define passwords

for read access and full access. Before the passwords

become effective, their use needs to be enabled. See the

Security by Password section for more details.

The last step to begin a mission is to issue the XPC

Start Mission command. As soon as it has received

this command, the DS1925 sets the MIP flag and

clears the MEMCLR flag. With the immediate/delayed

start mode (SUTA = 0), after as many minutes as

specified by the Mission Start Delay are over, the

device wakes up, copies the current date and time

to the Mission Timestamp register, and logs the first

entry of the mission. This increments both the Mission

Samples Counter and Device Samples Counter. All

subsequent log entries are made as specified by the value

in the Sample Rate register and the EHSS bit.

Figure 7. Temperature Logging

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 18

ETL = 1

TLFS = 0

01000h

125440

8-BIT ENTRIES

TEMPERATURE

1F9FFh

ETL = 1

TLFS = 1

01000h

1F9FFh

16-BIT ENTRIES

TEMPERATURE

1F9FFh

WITH 16-BIT

FORMAT, THE

MOST-SIGNIFICANT

BYTE IS STORED AT

THE LOWER

ADDRESS.

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