DS1921H/Z
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.
Mission Start Process
Logging Process
Y
DS1921 Copies RTC to
Mission Time Stamp
MIP = 1?
N
DS1921 Sets MIP = 0
DS1921 sets Datalog
Address = 1000h
N
EM = 0?
Y
DS1921 Measures
Temperature
N
MEMCLR = 1
Y
DS1921 Updates His-
togram, Device Sam-
ples Counter, Mission
Samples Counter and
Alarm, if applicable
Y
New Sample
Rate = 0?
N
N
Y
RO = 1?
DS1921 Copies new
Sample Rate from
Scratchpad to Sample
Rate Register
Datalog
Address =
1800h?
Y
DS1921 sets MIP = 1;
MEMCLR = 0
N
DS1921 Stores Temp.
at Datalog Address
DS1921 Stores Temp.
at Datalog Address
Y
N
Start Delay
Counter = 0?
DS1921 Increments
lower 11 bits of
Datalog Address
DS1921 Increments
Datalog Address
DS1921 Waits
Until Next
Minute
N
Boundary
DS1921 Waits
One Sample
Period
Y
MIP = 1?
Y
MIP = 1?
DS1921
Logging
Process
N
DS1921 Waits Until
Next Minute Boundary
End of Process
DS1921 Decrements
Start Delay Counter
End of Process
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DALLAS [ DALLAS SEMICONDUCTOR ]
