NXP Semiconductors UM10301 PCF2123 - Manual (Page 22)

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 22 of 52
5. Tuning C
T
will increase the initial accuracy, indicated by reducing the range covered
to the spread of C
IN
. Depending on how well C
T
is tuned, the initial spread can be
well compensated for.
6. Here the influence on accuracy due to aging of the crystal is given. This shift occurs
mainly during the first year of the crystal’s life and in the graph a range of about 420
ms/day to 850 ms/day is indicated (±10 ppm). So if the RTC were running correctly
initially, after a year it could be fast or slow by about 0.4 s/day.
7. As already mentioned and illustrated in Fig 7, the largest impact on the accuracy is
due to the temperature dependence of the crystal. The parabolic nature of this
dependence is indicated here as well and covers about 40 ppm over the temperature
range -10 °C to +60 °C.
8. The impact of a change in V
DD
is small; a ∆V
DD
of 1.5 V will change the clock speed
by about 17 ms/day.
From this an important recommendation follows: If possible place the crystal and IC-
circuit at the spot with the least temperature variations.
10. Oscillator tuning
The PCF8563, PCF2123, PCA8565 and PCA2125 all have a CLKOUT pin which is an
open drain output. It provides the option to output the buffered crystal frequency (or a
lower frequency derived from the crystal frequency using division by a power of 2) which
can be achieved by enabling CLKOUT in the appropriate control register and choosing
the desired frequency. Refer to the datasheet of the actual device used for details on
how to enable CLKOUT and set the frequency at this pin. Possible output frequencies for
the PCF8563 and PCA8565 are 1 Hz, 32 Hz, 1024 Hz and 32768 Hz. The PCF2123 and
PCA2125 offer some additional choices and the possible frequencies at the CLKOUT pin
are 1 Hz, 1024 Hz, 2048 Hz, 4096 Hz, 8192 Hz, 16384 Hz and 32768 Hz.
PCF8573, PCF8583 and PCF8593 do not include a CLKOUT pin.
Having the CLKOUT pin enables easy tuning of the crystal frequency. A designers’ initial
impulse may be to connect an oscilloscope probe to the OSCO pin, but this is not a good
idea. It may cause the oscillator to stop, but even if the oscillator keeps running the
added capacitance of the probe will cause a drift in oscillator frequency. By connecting a
pull-up resistor to the CLKOUT pin and measuring the frequency there, a much more
accurate result can be achieved. The frequency can now be tuned by adjusting the
variable capacitor C
T
.
Remark: Touching the adjustment screw often causes the capacitance to shift. The setup
is shown in Fig 9.