Datasheet

PCF2129 All information provided in this document is subject to legal disclaimers. © NXP Semiconductors N.V. 2014. All rights reserved.
Product data sheet Rev. 7 — 19 December 2014 49 of 86
NXP Semiconductors
PCF2129
Accurate RTC with integrated quartz crystal for industrial applications
The signal applied to pin CLKOUT should have a minimum pulse width of 300 ns and a
maximum period of 1000 ns. The internal clock, now sourced from CLKOUT, is divided
down by a 2
6
divider chain called prescaler (see Table 73). The prescaler can be set into a
known state by using bit STOP. When bit STOP is logic 1, the prescaler is reset to 0.
STOP must be cleared before the prescaler can operate again.
From a stop condition, the first 1 second increment will take place after 32 positive edges
on pin CLKOUT. Thereafter, every 64 positive edges cause a 1 second increment.
Remark: Entry into test mode is not synchronized to the internal 64 Hz clock. When
entering the test mode, no assumption as to the state of the prescaler can be made.
Operating example:
1. Set EXT_TEST test mode (register Control_1, EXT_TEST is logic 1).
2. Set bit STOP (register Control_1, STOP is logic 1).
3. Set time registers to desired value.
4. Clear STOP (register Control_1, STOP is logic 0).
5. Apply 32 clock pulses to CLKOUT.
6. Read time registers to see the first change.
7. Apply 64 clock pulses to CLKOUT.
8. Read time registers to see the second change.
Repeat 7 and 8 for additional increments.
8.14 STOP bit function
The function of the STOP bit is to allow for accurate starting of the time circuits. STOP
causes the upper part of the prescaler (F
9
to F
14
) to be held in reset and thus no 1 Hz ticks
are generated. The time circuits can then be set and will not increment until the STOP bit
is released. STOP doesn’t affect the CLKOUT signal but the output of the prescaler in the
range of 32 Hz to 1 Hz (see Figure 26
).
The lower stages of the prescaler, F
0
to F
8
, are not reset and because the I
2
C-bus and the
SPI-bus are asynchronous to the crystal oscillator, the accuracy of restarting the time
circuits is between 0 and one 64 Hz cycle (0.484375 s and 0.500000 s), see Table 73
and
Figure 27
.
Fig 26. STOP bit functional diagram
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