NXP Semiconductors | UM10301 |
User Manual PCF85x3, PCA8565 and PCF2123, PCA2125
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 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 as well. For example, swinging from (VDD – 1 V) to VDD would be ok, but also swinging from VSS to VDD.
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 215 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 CL with 12.5 pF the most common. Generally, an RTC using a
12.5pF 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
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 R1 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
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User manual | Rev. 01 — 23 December 2008 | 16 of 52 |