NXP Semiconductors PCA2125, PCF85x3, PCF2123 Oscillation allowance, Using an external oscillator

NXP Semiconductors	UM10301
	User Manual PCF85x3, PCA8565 and PCF2123, PCA2125

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 R1 (sometimes called RS or ESR). If R1 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 RX in series with the crystal. The value of the resistor is changed (a trimmer is useful here) to see at which values of RX oscillation starts and stops. Starting from a large value of RX the resistance is lowered until oscillation starts. This value of RX is called RX-start. Now the value is increased again until oscillation stops, RX is called RX-stop.

The oscillation allowance OA is defined as:

OA = RX-start+ R1

As a rule of thumb, the motional resistance of the crystal chosen should be

	R ≤	OA
	1	5
	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 R1 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 VSS to VDD. 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 VSS to OSCI and from OSCI to VDD (not on PCF2123). However,
	performance will be better if the oscillator amplitude is brought within the range from 0 V
	to the actual VDD 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
UM10301_1			© NXP B.V. 2008. All rights reserved.
User manual			Rev. 01 — 23 December 2008	15 of 52