DS1921 datasheet(23/44 Pages) DALLAS

23 / 44 page

DS1921H/Z

23 of 44

Mission Start and Logging Process

The DS1921H/Z does not use a special command to start a mission. Instead, a mission is started by

writing a non-zero value to the Sample Rate Register using the Copy Scratchpad command. As shown in

Figure 11, a new mission can only be started if the previous mission has been stopped (MIP = 0), the

memory is cleared (MEMCLR = 1) and the mission is enabled (EM = 0). If the new sample rate is

different from zero, the value will be copied to the sample rate register. At the same time the MIP bit will

be set and the MEMCLR bit will be cleared to indicate that the device is on a mission. Next the Mission

Start Delay counter will start decrementing every minute until it is down to 0. Now the DS1921H/Z will

wait until the next minute boundary and start the logging process, which as its first action copies the

applicable RTC registers to the Mission Time Stamp.

MISSION START AND LOGGING PROCESS Figure 11

The Mission Start Process is invoked when the Copy Scratchpad function is used to set a new sample rate

by writing to the Sample Rate Register at address 020Dh. One minute after the start delay countdown is

over, the Logging Process begins and the Mission Start Process ends.

Logging Process

DS1921 sets Datalog

Address = 1000h

DS1921 Measures

Temperature

DS1921 Updates His-

togram, Device Sam-

ples Counter, Mission

Samples Counter and

Alarm, if applicable

RO = 1?

DS1921 Stores Temp.

at Datalog Address

DS1921 Increments

lower 11 bits of

Datalog Address

DS1921 Stores Temp.

at Datalog Address

DS1921 Increments

Datalog Address

Datalog

Address =

1800h?

DS1921 Waits

One Sample

Period

MIP = 1?

End of Process

DS1921 Copies new

Sample Rate from

Scratchpad to Sample

Rate Register

DS1921 sets MIP = 1;

MEMCLR = 0

MIP = 1?

MEMCLR = 1

EM = 0?

New Sample

Rate = 0?

Start Delay

Counter = 0?

Mission Start Process

DS1921 Decrements

Start Delay Counter

DS1921 Waits Until

Next Minute Boundary

DS1921 Copies RTC to

Mission Time Stamp

End of Process

DS1921 Sets MIP = 0

MIP = 1?

DS1921

Logging

Process

DS1921 Waits

Until Next

Minute

Boundary

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