NXP Semiconductors UM10301 PCF2123 - Manual (Page 24)

NXP Semiconductors
UM10301
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
UM10301_1 © NXP B.V. 2008. All rights reserved.
User manual Rev. 01 — 23 December 2008 24 of 52
• In the datasheets of PCF8583 and PCF8593 the following method is described:
Using the alarm function (via the I
2
C-bus) a signal faster than 1 Hz can be generated
at the interrupt output for fast setting of a trimmer. Procedure:
ƒ Power-on;
ƒ Initialization (alarm functions).
Routine:
ƒ Set clock to time T and set alarm to time T + ∆T
ƒ At time (T + ∆T) (Interrupt) repeat the routine.
However, this only works well when ∆T is an integer number of seconds. The 1/10 s
and the 1/100 s are derived from a combination of 1 Hz, 2 Hz, 4 Hz, 8 Hz, 16 Hz,
32 Hz etc. signals. The accuracy is therefore only < ± 5 ms. Generating an alarm
after T +
∆T with ∆T = 20 ms will show a jitter of plus or minus 5 ms which makes
automatic tuning very complicated.
10.1 PCF2123 Offset register
The PCF2123 incorporates an offset register which can be used to implement several
functions, e.g.:
• Ageing adjustment
• Temperature compensation
• Accuracy tuning
The offset is made once every two hours in the normal mode, or once every hour in the
course mode. Each LSB will introduce an offset of 2.17 ppm for normal mode and 4.34
ppm for course mode. These values are based on a nominal 32.768 kHz clock. The
offset value is coded in two’s complement giving a range of +63 LSB’s to -64 LSB’s. The
correction is made by adding or subtracting 64 Hz clock correction pulses, thereby
changing the period of a single second.
In normal mode, the correction is triggered once every two hours and then correction
pulses are applied once per minuted until the programmed correction value has been
implemented.
In course mode, the correction is triggered once per hour and then correction pulses are
applied once per minute up to a maximum of 60. When absolute correction values of
greater than 60 are used, additional correction pulses are made in the 59
th
minute.
It is possible to monitor when correction pulses are applied. The correction interrupt
enable (CIE) mode will generate a 1/128 second pulse on
INT for every correction
applied. In the case where multiple correction pulses are applied, a 1/128 second
interrupt pulse will be generated for each correction pulse applied. Correction is applied
to the 1 Hz clock. Any timer or clock output using a frequency of 1 Hz or below will also
be affected by the correction pulses. For more details, refer to the PCF2123 datasheet.