DS1925 datasheet(11/46 Pages) MAXIM

11 / 46 page

Detailed Register Descriptions

Timekeeping and Calendar

The real-time clock (RTC) is a second counter accessed

by reading/writing the appropriate bytes in the register

page, address 200h–203h. For readings to be valid,

all RTC registers must be read sequentially starting at

address 0200h. The number representation of the RTC

registers is in seconds format. Typically, this is the number

of seconds since January 1, 1970 12:00a.m. See Table 2

for the bitmap.

Sample Rate

The content of the Sample Rate register (addresses

0206h, 0207h) specifies the time elapse between tem-

perature-logging events. The sample rate is coded as an

unsigned 14-bit binary number with a maximum value

of 16,383. If EHSS = 1, the sample rate is in seconds.

If EHSS = 0, the sample rate is in minutes. The fatest

recommended sample rate is 5 minutes. Setting a sample

rate less than 3 minutes (180 seconds ESHSS = 1 or

3 minutes EHSS = 0) results in a failure on the XPC

Start Mission command with repeat code 77h Invalid

Parameter. The EHSS bit is located in the RTC Control

sets the EHSS bit accordingly while setting the Sample

Rate register. During a mission, there is only read access

to these registers. Bits cells marked 0 always read 0 and

cannot be written to 1. See Table 3 for the bitmap.

Temperature Conversion

The DS1925 measures temperatures in the -40°C to

+85°C range. Temperature values are represented as an

8-bit 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 16-bit

mode, only the three highest bits of the lower byte TRL

are valid. The five lower bits all read 0. 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. See Table 4 for

the bitmap.

Table 2. Real-Time Clock and RTC Alarm Registers Bitmap

Table 3. Sample Rate Register Bitmap

Table 4. Latest Temperature Conversion Result Register Bitmap

Table 5. Temperature Conversion Examples

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 11

ADDR

0200h

LSByte Seconds

0201h

. . .

0202h

. . .

0203h

MSByte Seconds

ADDR

0206h

Sample Rate Low

0207h

Sample Rate High

ADDR

020Ch

TRL

020Dh

T10

TRH

MODE

TRH

TRL

= (°C)

DS1925

HEX

DECIMAL

HEX

DECIMAL

8-bit

54h

—

1.0

8-bit

17h

—

-29.5

16-bit

54h

00h

1.000

16-bit

17h

60h

-29.3125

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