DS1678 Datasheet, PDF (7/25 Pages) Maxim Integrated Products – Real-Time Event Recorder

DS1678 Real-Time Event Recorder
DETAILED DESCRIPTION
The Event Counter Register contains the total number of events that have been logged in the current
event-logging mission. The Event Counter Register also allows the user to determine if the data in the
event-log memory has rolled over.
The 16-bit ETC can be incremented once per second, once per minute, or once per hour. Each event
transfers the current ETC value into the event-log memory, then clears and restarts the ETC. The three
increment periods allow users to maximize the resolution while providing an adequate maximum time
between events. The seconds resolution provides the time of an event down to the second, while allowing
up to 65,535 seconds (18.2 hours) between events without using additional event-log memory. The
minutes resolution provides the time of an event down to the minute, while allowing up to 45.5 days
between events without using extra memory locations. The hours resolution provides the time of an event
down to the hour in which it occurred, while allowing up to 7.5 years between events without using
additional event-log memory. Based on the expected frequency of events, an increment period can be
selected to maximize the resolution while minimizing use of the event-log memory.
The event can be triggered in three different ways depending on how the user programs the trigger select
(TRx) bits in the Control Register. The event can be triggered by a falling edge on the INT pin only, a
rising edge only, or it can be triggered by rising and falling edges. Triggering with both the rising and
falling edges allows for monitoring when something is turned on/off and how long it is in either state.
The RTC provides seconds, minutes, hours, day, date, month, and year information with leap-year
compensation, and year 2000 compliance. The RTC also provides an alarm interrupt. The I²C interface
allows the RTC to function as a stand-alone RTC in the system.
The programmable alarm trip points in the RTC allow a flag to be set in the Control Register when the
specified time in the Alarm Trip Point Register is reached. The flag is readable via the I²C interface
during an event-logging mission or, when the DS1678 is not in a mission, INT becomes an output and
generates an alarm interrupt if the value in the RTC equals the value in the RTC Alarm Register and the
duration interval select (DISx) bits are both set to zeros.
The DS1678 operates as a slave device on the I²C serial bus. Access is obtained by generating a START
condition and providing a device identification code. All data is transferred to and from the DS1678 most
significant bit (MSB) first. The address counter automatically increments so that subsequent registers can
be accessed sequentially until a STOP condition is executed. When V_CC falls below 1.25 x V_BAT, the
device automatically write protects itself by disabling the I²C interface, terminates any access in progress,
and resets the device address counter. Inputs to the device via the I²C bus are not recognized at this time
in order to prevent erroneous data from being written to the device from an out-of-tolerance system.
When V_CC falls below V_BAT, the device switches into a low-current battery-backup mode. Upon power-
up, the device switches from battery power to V_CC when V_CC is greater than V_BAT + 0.2V, and recognizes
inputs from the system when V_CC is greater than 1.25 x V_BAT by releasing control of the write protection
on the I²C bus.
The Block Diagram shows the main elements of the RTC event recorder. The device has four major
components: a 64-bit RTC and control block, 32-byte user NV RAM, 2048 bytes of event-log memory
(1024 events), and an I²C serial interface.
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