DS1678 Datasheet, PDF (18/25 Pages) Maxim Integrated Products – Real-Time Event Recorder

DS1678 Real-Time Event Recorder
represents the total elapsed time since the previous event. The event counter is not incremented when the
ETC rolls over because a new event has not occurred.
The ETC is incremented as the selected seconds, minutes, or hours register of the RTC increments.
Because the RTC continues to run, even when the ETC is cleared to prepare for a new event, and the ETC
is incremented every time the selected byte in the RTC increments, the actual time resolution is not lost
even when events occur more frequently than the minimum time resolution selected. If an event occurs
half way between increments of the ETC, the first increment of the next event occurs when the seconds,
minutes, or hours register increments next, thus preserving the correct time to the resolution selected in
the DISx bits.
When the alarm interrupt output is used, the DISx bits should be set to zeros. An event-logging mission
cannot be started if the DISx bits are set to zero. This enables the alarm flag to generate an alarm interrupt
via the INT output pin.
With the DIS0 bit set to one and the DIS1 bit set to zero, the ETC increments every time the seconds
register in the RTC is incremented. This gives the maximum resolution between events, but the counter
rolls over to the next two memory bytes when it reaches the maximum value. The largest interval between
events that can be accurately measured without using additional memory space and reducing the total
number of events able to be logged is 65,535 seconds, or about 18.2 hours. If the maximum time between
events could be greater than 18.2 hours, consider using one of the courser resolutions to conserve
memory.
With the DIS0 bit set to zero and the DIS1 bit set to one, the ETC increments every time the minutes
register in the RTC is incremented. This gives a medium resolution between events, but increases the
largest possible interval between events that can be accurately measured without using additional memory
space and reduces the total number of events able to be logged to 65,535 minutes, or about 45.5 days.
With both DISx bits set to one, the ETC increments every time the single hours byte in the RTC is
incremented. This gives the lowest resolution between events, but increases the largest possible interval
between events that can be accurately measured without using additional memory space and reduces the
total number of events able to be logged to 65,535 hours, or about 7.5 years.
If a second event occurs before the ETC is able to increment for the first time, all zeros are logged in the
event-log memory and the ETC resets. If this occurs, the time base remains correct as it is based on the
separate RTC incrementing, but the exact time of the event is no more accurate than the size of the time
increment that is chosen. For this reason, it is recommended to use the finest resolution possible for your
logging to minimize the errors. If the normal duration between events is several days or months, then a
few minutes or an hour may not be significant to your data accuracy.
Table 5. Duration Interval Select Bits
DIS1
0
0
1
1
DIS0
0
1
0
1
ELAPSED TIME COUNT
RESOLUTION
Alarm Interrupt Output Enabled
Counter Increments Every Second
Counter Increments Every Minute
Counter Increments Every Hour
MAX TIME
BETWEEN EVENTS
—
18.2 Hours
45.5 Days
7.5 Years
18 of 25