NXP Semiconductors
UM10301
User Manual for PCF85x3, PCF85x63, PCA8565, PCF2123, and
PCA21125
UM10301
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© NXP Semiconductors N.V. 2015. All rights reserved.
User manual
Rev. 2.1 — 23 July 2015
16 of 54
pin directly or via a small capacitor of e.g. 22 pF - 100 pF. Alternatively a capacitive
divider circuit can be used to reduce the signal amplitude.
Either square or sine wave is ok.
6.2.2 PCF2123
As other RTCs, but the required amplitude should be somewhat smaller, around 300 mV.
6.2.3 PCF8583 and PCF8593
When used with a crystal the signal would swing around a bias about 100 mV below
VDD. If these RTCs are fed with an external signal, it should be either AC coupled, or
swinging with amplitude of around 1 V below VDD, where the lower value may be lower
than 1 V below VDD.
7. Crystal and crystal selection
Select a crystal of the tuning fork type with a nominal frequency of 2
15
Hz = 32768 Hz.
The allowed tolerance depends on the requirements for the application and on whether a
trimming capacitor will be used. If a trimming capacitor will be used even a tolerance of
±100 ppm is ok since it can be compensated. Either through hole or surface mount
crystals can be used where the latter provide the smallest dimensions which makes the
circuit less susceptible to noise pick up.
As previously pointed out crystals used for RTCs come in three versions, optimized for
three standard values for C
L
with 12.5 pF the most common. Generally, an RTC using a
12.5 pF crystal has a timekeeping current of about 1.6x more than an RTC using a 7 pF
crystal. If lowest power consumption is a key consideration, a 7 pF crystal (some
manufacturers use 6 pF) should be selected. The PCF2123 has been optimized for use
with such a crystal. The other RTCs include load capacitance optimized for a 12.5 pF
crystal. Using a 7 pF crystal would require an external capacitor of about 9.7 pF and thus
the capacitances at OSCI and OSCO would not be balanced. In general this may have a
detrimental influence on start-up behavior but no problems are expected when a 7 pF
crystal is used in combination with the PCF8563 because it uses an AGC in its oscillator.
An oscillator using a 12.5 pF crystal will be more stable and less susceptible to noise and
parasitic capacitances. One reason for this is that the capacitors on the input and output
will have higher values and therefore create a higher load for noise. Further these higher
values make the parasitic capacitance relatively smaller for the same PCB.
Besides technical considerations there are also procurement issues. Crystals designed
for a 12.5 pF load capacitance are readily available through many distributors. Crystals
designed for a load capacitance of 7 pF or 9 pF are not as readily available and may
have longer lead times or require a minimum quantity to be purchased.
The series resistance R
1
should ideally remain below 50 kΩ. If higher values are used
(up to 100 kΩ is ok) the current consumption of the oscillator will increase a bit. If the
value is really too high startup problems may occur, but up to 100 kΩ no startup
problems are expected. See 6.1”Oscillation allowance”.
The frequency accuracy of the oscillator depends mainly on the accuracy of the crystal
and on how well the crystal is matched to the oscillator capacitive load (C
L
). A too small
capacitive load results in the oscillator running fast, if the capacitive load is greater than
what the crystal was designed for the oscillator and thus clock runs slow. This initial error
