DS1678S Maxim Integrated Products, DS1678S Datasheet - Page 6

• Manufacturer Part Number: DS1678S
• Description: IC RECORDER REALTIME EVENT 8SOIC
• Manufacturer: Maxim Integrated Products
• Type: Time Event Recorderr

Specifications of DS1678S

• Memory Size: 32B
• Time Format: HH:MM:SS (12/24 hr)
• Date Format: YY-MM-DD-dd
• Interface: I²C, 2-Wire Serial
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (5.3mm Width), 8-SOP, 8-SOEIAJ
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

DS1678
CLOCK ACCURACY
The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was
trimmed. Additional error will be added by crystal frequency drift caused by temperature shifts.
External circuit noise coupled into the oscillator circuit may result in the clock running fast. See
Application Note 58, “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.
MEMORY
The memory map in Figure 2a shows the general organization of the DS1678. As can be seen in the
figure, the device memory is in one contiguous segment with a data port to access the event log memory.
Since the 2-wire bus only has the capability to use one byte addressing, the DS1678 utilizes the data port
to access the 2048 bytes of event log memory. The address that the next data will be written to is stored
in the address pointer registers LSB (3Fh) and MSB (40h). These will be helpful in recovering all of the
data if a rollover occurs. The address pointer will point to the oldest event in the memory after a rollover.
This is the memory location in event log memory that would be over written by the next event. Read the
data from this point to the end of the memory and the start time stamp, including the 2-byte ETC from
last event. Working backward from the value in the start time stamp, subtract the value in the ETC from
last event to get the time the last event in the memory occurred. Then subtract the values in each of the
2-byte memory locations for elapsed time between events in order to recover the time the previous event
occurred.
The value in the ETC register LSB (3Dh) and MSB (3Eh) is the value in the actual ETC. This is the time
from the last event recorded until logging was stopped. Since a new event has not occurred, this data has
not been stored in the event memory yet.
The data port is made up of three bytes. The first byte (41h) is the event log memory address LSB, the
second byte (42h) is the event log memory address MSB, and the third byte (43h) is the event log
memory data byte. To access data via the data port, the LSB of the event log memory address is written
into 41h, the main memory address pointer will automatically increment to 42h where the event log
memory address MSB data will be written. The next transfer will indicate a read command with a 1 in
the R/
bit of the 2-wire address byte. The data from the event log memory location corresponding to
W
the address written into main memory locations 41h and 42h will be available in location 43h to be read.
Any address greater than 43h will read back 00h and will not be able to be written. For address above
43h, the address pointer will increment until it reaches FFh then rollover to 00h.
The event log memory address pointer in main memory locations 41h and 42h will be auto-incremented
to the next higher event log memory address, while the pointer for the main memory will remain at
location 43h. This will allow the event log memory to be read continuously without having to write the
next desired event log memory location prior to each data read. The even address locations in the event
log memory will correspond to the LSB of the elapsed time between events and the odd memory
locations will correspond to the MSB of the elapsed time between events. For more information about
how the data is stored in the event log memory, see Figure 2b.
When the event log memory address pointer gets to the last address location (07FFh), the automatic
incrementing will stop. A new starting address will then have to be written into the event log memory
pointer bytes (41h & 42h) in order to begin reading additional data. The event log memory addresses that
can be put into the pointer (41h & 42h) are 0000h to 07FFh. The five MSB of the address are ignored.
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