NXP Semiconductors UM10301 PCF2123 - Manual (Page 16)

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 16 of 52
levels can be tuned such that they are similar to those when the internal oscillator is
used.
Suppose that the RTC is supplied with 3.3 V and that the amplitude of the external CLK
is 5 V (from 0 V to 5 V). Using 1 M and 220 k resistors the signal could be reduced to
(220 / 1220) x 5 V = 0.9 V. This is better in line with the signals that the internal circuitry
handles when an external crystal is used as is the case in the standard application. This
reduced signal can then be applied to the OSCI pin directly or via a small capacitor of
e.g. 22 pF - 100 pF. This is a lower power option, where bias from the resistive devider
and oscillator will be lost and will be determined by the oscillator input. This option is also
more susceptible to noise.
If PCF8583 and PCF8593 are used together with a crystal, the signal would swing
around a bias of some 100 mV below V
DD
. If these RTCs are fed with an external signal,
it should be either AC coupled, or swinging with amplitude of around 1 V below V
DD
,
where the lower value may be lower than 1 V below V
DD
as well. For example, swinging
from (V
DD
– 1 V) to V
DD
would be ok, but also swinging from V
SS
to V
DD
.
Remark: Values mentioned here are guidelines only. For every application correct
operation must be verified.
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 behaviour 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 k Ω. If higher values are used
(up to 100 k Ω is ok) the current consumption of the oscillator will increase a bit. If the