DS1678 Datasheet, PDF (14/25 Pages) Maxim Integrated Products – Real-Time Event Recorder

DS1678 Real-Time Event Recorder
ROLLOVER HANDLING
There are two options for dealing with the potential occurrence of a data overrun (i.e., more than 1024
total event logs in the event-log memory) (Figure 4). The first option is to enable rollovers. This is
accomplished by setting the rollover bit (bit 3 of the Control Register) to one. When rollover is enabled,
new data is written over previous data, starting with the Start Time Stamp Register, as if a new mission is
starting.
When a rollover occurs, the Event 0 Rollover Stamp has the elapsed time since event 1024 of the event-
log memory. This is to allow the user to recover the information recorded prior to the rollover. At the start
of a mission, the Event 0 Rollover Stamp data is zero, as there was no previous event from which to have
an elapsed time.
When 1024 events are recorded in the event-log memory, the next event causes a new time/date stamp to
be written to the Start Time Stamp Register and the elapsed time since event 1024 written to the event 0
rollover stamp. The new event is written to the first location in the event log, overwriting the old data, and
the address pointer is incremented. When the rollover occurs, the rollover flag (ROF) in the Control
Register is set to one to indicate that the memory has rolled over at least one time.
The second option for dealing with data overrun is to disable rollovers by setting the rollover bit to 0. The
DS1678 stops recording after event 1025, and the address pointer is incremented from 07FFh to 0000h.
The device continues monitoring INT and the event counter continues to increment when INT is
activated, even though the event-log memory has been filled.
A time stamp for the first event is recorded after a mission begins. The time of acquisition for subsequent
events is determined by considering the start time recorded by the time stamp; the value in the Event
Counter Register, ROF; and the address of the particular data sample in the event-log memory.
If no rollover has occurred in the event-log memory (ROF = 0), the sample time associated with any
particular data point can be calculated by multiplying the sum of the elapsed time between the events up
to that event by one second, minute, or hour depending on which resolution is selected in the DISx bits of
the Control Register, and adding this elapsed time to the value in the Start Time Stamp Register.
If rollover has been enabled, the user can determine if rollover has occurred by reading the value in the
Event Counter Register or the ROF. The Event Counter Register counts the total number of events that
have been acquired. If this value is greater than 0400h (decimal 1025), then the user knows that rollover
has occurred. If rollover has occurred, the user needs to determine how many times rollover occurred in
determining the sample time for any particular data sample. The address pointer points to the oldest data
in the event-log memory, and, if the memory has rolled over at least one time, the rollover flag is set to
one.
The DS1678 has been designed so the user cannot directly write to the event-log memory. This prevents
writing invalid data to the event-log registers.
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