NXP Semiconductors UM10301 PCF2123 - Manual (Page 15)

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 15 of 52
6.1 Oscillation allowance
Fig 4 shows the Pierce oscillator schematic with the external crystal. For an oscillation to
take place the real component of the oscillator impedance has to be larger than the
motional resistance R
1
(sometimes called R
S
or ESR). If R
1
is too large no oscillation will
take place since no operating point can be reached.
Similarly, if the supply voltage is too low or the temperature is too low, no oscillation can
build up.
A method to test how much margin the design has is to include a resistor R
X
in series
with the crystal. The value of the resistor is changed (a trimmer is useful here) to see at
which values of R
X
oscillation starts and stops. Starting from a large value of R
X
the
resistance is lowered until oscillation starts. This value of R
X
is called R
X-start
. Now the
value is increased again until oscillation stops, R
X
is called R
X-stop
.
The oscillation allowance OA is defined as:
OA = R
X-start
+ R
1
As a rule of thumb, the motional resistance of the crystal chosen should be
5
1
OA
R
≤
This test can be done in the lab under room temperature. This should give enough safety
margins to allow for production spread of IC and crystal and to deal with the increasing
value of R
1
under influence of increased temperature.
6.2 Using an external oscillator
It is possible to supply a clock signal from an external oscillator instead of using the
internal oscillator if for some reason it is desired to not use the internal oscillator. In this
case no crystal will be connected to the OSCI and OSCO pins. Instead the external
oscillator must be connected to OSCI while OSCO must be left floating.
The signal may swing from V
SS
to V
DD
. However, with a crystal attached the signal
amplitude at the oscillator input pin would be about 500 mV, swinging around a 250 mV
bias i.e. never going negative (not for PCF8583 and PCF8593, see below). For the
PCF85x3 supplying a signal with amplitude between 500 mV and 1000 mV is a good
starting point, with the bias such that the signal doesn’t go negative and operates in the
same region as would have been the case with a crystal. Square or sine wave is both ok.
For the PCF2123 the amplitude should be somewhat smaller. If the oscillator amplitude
is larger than the supply voltage to the RTC it is advisable to use a resistive divider for
the oscillator signal to bring its amplitude within the supply voltage of the RTC. Without
such a divider it will work too and nothing will be damaged (as long as the currents via
the clamping diodes don’t exceed the maximum limits) because the device has internal
clamping diodes from V
SS
to OSCI and from OSCI to V
DD
(not on PCF2123). However,
performance will be better if the oscillator amplitude is brought within the range from 0 V
to the actual V
DD
used for the RTC. This will first prevent periodic currents flowing via the
upper clamping diode to the decoupling capacitor on the supply pin. Secondly the signal