DS1925 datasheet(13/46 Pages) MAXIM

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RTC Control

To minimize the DS1925’s power consumption, the RTC

oscillator should be turned off when device is not in use.

The oscillator on/off bit is located in the RTC Control reg-

ister (0212h). Turning the oscillator on when the device

is in clear memory state incurs an additional tSRTC delay.

This register also includes the EHSS bit, which deter-

mines whether the sample rate is specified in seconds or

minutes. Table 9 shows the register bitmap. See Table 10

for register descriptions. During a mission, there is only

read access to this register. Bits 7:2 have no function;

they always read 0 and cannot be written to 1.

Mission Control

The DS1925 is set up for its operation by writing appropri-

ate data to its special function registers, which are located

in the two register pages. The settings in the Mission

Control register determine which format (8 or 16 bits) is

to be used and whether old data can be overwritten by

new data, once the data-log memory is full. An additional

control bit can be set to tell the DS1925 to wait with log-

ging data until a temperature alarm is encountered. See

Table 11 for the register bitmap. During a mission, there is

only read access to this register. Bits 7:6 have no function;

they always read 1 and cannot be written to 0. Bits 1 and

3 control functions that are not available with the DS1925.

Bit 1 must be set to 0. Under this condition the setting of

bit 3 becomes a “don’t care.”

Table 9. RTC Control Register Bitmap

Table 10. RTC Control Register Bit Descriptions

Table 11. Mission Control Register Bitmap

Table 12. Mission Control Register Bit Descriptions

DS1925

iButton High-Capacity Temperature Logger

with 122KB Data-Log Memory

www.maximintegrated.com

Maxim Integrated │ 13

ADDR

0212h

EHSS

EOSC

BIT

NAME

FUNCTION

EHSS

Enable High Speed Sample. This bit controls the speed of the sample rate counter. When set to logic 0,

the sample rate is specified in minutes. When set to logic 1, the sample rate is specified in seconds.

EOSC

Enable Oscillator. This bit controls the crystal oscillator of the RTC. When set to logic 1, the oscillator

starts operation. When written to logic 0, the oscillator stops and the device is in a low-power data-

retention mode. This bit must be 1 for normal operation. XPC Start Mission command automatically

starts the RTC by changing the EOSC bit to logic 1.

ADDR

0213h

SUTA

(X)

TLFS

ETL

BIT

NAME

FUNCTION

SUTA

Start Mission Upon Temperature Alarm. This bit specifies whether a mission begins

immediately (includes delayed start) or if a temperature alarm is required to start the

mission. If this bit is 1, the device performs an 8-bit temperature conversion at the

selected sample rate and begins with data logging only if an alarming temperature

(high alarm or low alarm) was found. The first logged temperature is when the alarm

occurred. However, the Mission Sample Counter does not increment. This functionality

is guaranteed by design and not production tested.

TLFS

Temperature Logging Format Selection. This bit specifies the format used to store

temperature readings in the data-log memory. If this bit is 0, the data is stored in 8-bit

format. If this bit is 1, the 16-bit format is used (higher resolution). With 16-bit format,

the most significant byte is stored at the lower address.

ETL

Enable Temperature Logging. To set up the device for a temperature-logging mission,

this bit must be set to logic 1. The recorded temperature values start at address 1000h.

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