DS12885 Maxim, DS12885 Datasheet - Page 19

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DS12885

Manufacturer Part Number
DS12885
Description
The DS12885, DS12887, and DS12C887 real-time clocks (RTCs) are designed to be direct replacements for the DS1285 and DS1287
Manufacturer
Maxim
Datasheet

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Table 3. Periodic Interrupt Rate and
Square-Wave Output Frequency
The device executes an update cycle once per second
regardless of the SET bit in Register B. When the SET
bit in Register B is set to 1, the user copy of the double-
buffered time, calendar, and alarm bytes is frozen and
does not update as the time increments. However, the
time countdown chain continues to update the internal
copy of the buffer. This feature allows time to maintain
accuracy independent of reading or writing the time,
calendar, and alarm buffers, and also guarantees that
time and calendar information is consistent. The update
cycle also compares each alarm byte with the corre-
Figure 3. UIP and Periodic Interrupt Timing
RS3
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
SELECT BITS
REGISTER A
RS2
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
UIP
UF
PF
t
CYCLE = 244μs
BUC
RS1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
= DELAY TIME BEFORE UPDATE
RS0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
t
PI
INTERRUPT
1.953125ms
976.5625µs
3.90625ms
3.90625ms
122.070µs
244.141µs
488.281µs
7.8125ms
7.8125ms
15.625ms
31.25ms
PERIODIC
62.5ms
125ms
250ms
500ms
RATE
None
Update Cycle
____________________________________________________________________
t
BUC
SQW OUTPUT
FREQUENCY
8.192kHz
4.096kHz
2.048kHz
1.024kHz
256Hz
128Hz
512Hz
256Hz
128Hz
None
64Hz
32Hz
16Hz
8Hz
4Hz
2Hz
t PI
1 SECOND
sponding time byte and issues an alarm if a match or if
a don’t-care code is present in all three positions.
There are three methods that can handle RTC access
that avoid any possibility of accessing inconsistent time
and calendar data. The first method uses the update-
ended interrupt. If enabled, an interrupt occurs after
every update cycle that indicates over 999ms is avail-
able to read valid time and date information. If this
interrupt is used, the IRQF bit in Register C should be
cleared before leaving the interrupt routine.
A second method uses the update-in-progress bit (UIP)
in Register A to determine if the update cycle is in
progress. The UIP bit pulses once per second. After
the UIP bit goes high, the update transfer occurs 244µs
later. If a low is read on the UIP bit, the user has at least
244µs before the time/calendar data is changed.
Therefore, the user should avoid interrupt service rou-
tines that would cause the time needed to read valid
time/calendar data to exceed 244µs.
The third method uses a periodic interrupt to determine if
an update cycle is in progress. The UIP bit in Register A
is set high between the setting of the PF bit in Register C
(Figure 3). Periodic interrupts that occur at a rate greater
than t
reached at each occurrence of the periodic interrupt.
The reads should be complete within one (t
to ensure that data is not read during the update cycle.
The EDIP module can be successfully processed
through conventional wave-soldering techniques so long
as temperature exposure to the lithium energy source
does not exceed +85°C. Post-solder cleaning with water-
washing techniques is acceptable, provided that ultra-
sonic vibration is not used. Such cleaning can damage
the crystal.
BUC
allow valid time and date information to be
Handling, PC Board Layout,
Real-Time Clocks
t
P1/2
t
P1/2
and Assembly
PI/2
+ t
BUC
19
)

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