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Table 2. Periodic Interrupt Rate and Square-Wave Output Frequency
EXT. REG. B
SELECT BITS REGISTER A
E32K
RS3
RS2
RS1
RS0
tPI PERIODIC
INTERRUPT RATE
SQW OUTPUT
FREQUENCY
0
None
0
1
3.90625ms
256Hz
0
1
0
7.8125ms
128Hz
0
1
122.070
s
8.192kHz
0
1
0
244.141
s
4.096kHz
0
1
0
1
488.281
s
2.048kHz
0
1
0
976.5625
s
1.024kHz
0
1
1.953125ms
512Hz
0
1
0
3.90625ms
256Hz
0
1
0
1
7.8125ms
128Hz
0
1
0
1
0
15.625ms
64Hz
0
1
0
1
31.25ms
32Hz
0
1
0
62.5ms
16Hz
0
1
0
1
125ms
8Hz
0
1
0
250ms
4Hz
0
1
500ms
2Hz
1
X
(See Note)
32.768kHz
Note: RS3–RS0 determine periodic interrupt rates as listed for E32K = 0.
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 (see Figure 3). Periodic interrupts
that occur at a rate of greater than tBUC allow valid time and date information to be reached at each
occurrence of the periodic interrupt. The reads should be complete within (tPI / 2 + tBUC) to ensure that
data is not read during the update cycle.
Figure 3. Update-Ended and Periodic Interrupt Relationship