IDT1338
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM
RTC
Typical Operating Circuit
V
CC
V
CC
CRYSTAL
V
CC
2k
CPU
2k
X1
SCL
SDA
X2
V
CC
SQW/OUT
10k
IDT1338
GND
V
BAT
+
-
Detailed Description
The following sections discuss in detail the Oscillator block,
Power Control block, Clock/Calendar Register Block and
Serial I
2
C block.
Oscillator Block
Selection of the right crystal, correct load capacitance and
careful PCB layout are important for a stable crystal
oscillator. Due to the optimization for the lowest possible
current in the design for these oscillators, losses caused by
parasitic currents can have a significant impact on the
overall oscillator performance. Extra care needs to be taken
to maintain a certain quality and cleanliness of the PCB.
Crystal Selection
The key parameters when selecting a 32 kHz crystal to work
with IDT1338 RTC are:
In the above figure, X1 and X2 are the crystal pins of our
device. Cin1 and Cin2 are the internal capacitors which
include the X1 and X2 pin capacitance. Cex1 and Cex2 are
the external capacitors that are needed to tune the crystal
frequency. Ct1 and Ct2 are the PCB trace capacitances
between the crystal and the device pins. CS is the shunt
capacitance of the crystal (as specified in the crystal
manufacturer's datasheet or measured using a network
analyzer).
Note:
IDT1338CSRI integrates a standard 32.768 kHz
crystal in the package and contributes an additional
frequency error of 10ppm at nominal
V
CC
(+3.3 V) and
T
A
=
+25°C.
•
Recommended Load Capacitance
•
Crystal Effective Series Resistance (ESR)
•
Frequency Tolerance
Effective Load Capacitance
Please see diagram below for effective load capacitance
calculation. The effective load capacitance (CL) should
match the recommended load capacitance of the crystal in
order for the crystal to oscillate at its specified parallel
resonant frequency with 0ppm frequency error.
IDT™
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM 3
IDT1338
REV J 111009
IDT [ INTEGRATED DEVICE TECHNOLOGY ]
