RTC PCF8523 evaluation board OM13511
The I
2
C-bus interface works up to 1 MHz. Supply voltage: 1.8 V to 5.5 V. The RTC,
excluding the I
2
C-bus interface, however is operating down to as low as 1.0 V. It is
recommended to have a decoupling capacitor of 100 nF on the V
DD
-V
SS
rails close by.
Due to the low power consumption of below 1 μW, no precautions for heat dissipations
are required, even in a sealed housing environment. Frequencies of 1 Hz to 32.768 kHz
on CLKOUT pin can be used to measure the frequency for calibration and/or for general
purpose, e.g. as reference for frequency generation with a PLL.
3.2 Battery back-up
To guarantee the autonomy of the clock, continuous supply of power is needed. The
battery backup assures it. Any type of battery can be used. The battery voltage might be
larger (Lithium) or smaller (Alkaline or silver oxide) than the regular supply voltage V
DD
.
To reduce the power consumption of the RTC the V
DD
voltage is sampled for taking the
decision when to switch over to the battery. To assure the supply voltage does not drop
below the minimal threshold, the falling slew rate of V
DD
must be max 0.5 V/ms. R1, C2
are selected therefore accordingly (10 kΩ, 1 µF).
3.3 Optimizing power consumption
There are a number of factors influencing the power consumption:
• Supply voltage: the lower the supply voltage the lower the power consumption
• Oscillator: the quartz crystal is specified with the series resistance R
S
and a load
capacitance C
L
. The PCF8523 can be set to accept quartz crystals with a C
L
of
12.5 pF or 7 pF. With lower capacitance, the power consumption is lower. 6 pF
quartz crystals can also be used but then the frequency must be adjusted slightly via
the offset register. The serial resistance of the quartz dissipates energy. The lower
R
S
, the less energy is used. General rule: the larger the mechanical size of the quartz
package, the lower is the impedance. Check the components’ parameters of the
preferred quartz crystal supplier.
UM10760 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2015. All rights reserved.
User manual Rev. 1 — 2 February 2015 5 of 13
