DATASHEET
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM
IDT1338B-31
General Description
The IDT1338B-31 is a serial real-time clock (RTC) device
that consumes ultra-low power and provides a full
binary-coded decimal (BCD) clock/calendar with 56 bytes
of battery backed Non-Volatile Static RAM. The
clock/calendar provides seconds, minutes, hours, day, date,
month, and year information. The clock operates in either
the 24-hour or 12-hour format with AM/PM indicator. The
end of the month date is automatically adjusted for months
with fewer than 31 days, including corrections for leap year.
Access to the clock/calendar registers is provided by an I
2
C
interface capable of operating in fast I
2
C mode. Built-in
Power-sense circuitry detects power failures and
automatically switches to the backup supply, maintaining
time and date operation.
•
Others (Thermostats, Vending Machines, Modems, Utility
Meters)
Features
•
Real-Time Clock (RTC) counts seconds, minutes, hours,
day, date, month, and year with leap-year compensation
valid up to 2100
•
56-Byte battery-backed Non Volatile RAM for data
storage
•
•
•
•
Fast mode I
2
C Serial interface
Automatic power-fail detect and switch circuitry
Programmable square-wave output
Packaged in 8-pin MSOP, 8-pin SOIC, or 16-pin SOIC
(surface-mount package with an integrated crystal)
Applications
•
Telecom (Routers, Switches, Servers)
•
Handheld (GPS, Point of Sale POS terminals)
•
Consumer Electronics (Set-Top Box, Digital Recording,
Network Applications, Digital photo frames)
•
Industrial temperature range (-40°C to +85°C)
•
Office (Fax/Printers, Copiers)
•
Medical (Glucometer, Medicine Dispensers)
Block Diagram
Crystal inside package
for 16-pin SOIC ONLY
1 Hz/4.096 kHz/
8.192 kHz/32.768 kHz
X1
32.768 kHz
Oscillator and
Divider
MUX/
Buffer
SQW/OUT
X2
VCC
GND
V
BAT
SCL
SDA
I
2
C
Interface
56 Byte
RAM
1 Byte
Control
7 Bytes
Buffer
Power
Control
Control
Logic
Clock, Calendar
Counter
IDT™
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM 1
IDT1338B-31
REV A 112309
IDT1338B-31
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM
RTC
Pin Assignment
(8-pin MSOP/8-pin SOIC)
X1
X2
V
BAT
GND
1
2
3
4
8
VCC
SQW/OUT
SCL
SDA
Pin Assignment
(16-pin SOIC)
SCL
SQW/OUT
VCC
NC
NC
NC
NC
NC
1
2
3
4
5
6
7
8
16
15
14
SDA
GND
V
BAT
NC
NC
NC
NC
NC
IDT
1338
7
6
5
IDT
1338C
13
12
11
10
9
Pin Descriptions
Pin
Number
8MSOP,
8SOIC
1
2
16SOIC
—
—
Pin
Name
X1
X2
Pin Description/Function
Connections for standard 32.768 kHz quartz crystal. The internal oscillator circuitry is designed
for operation with a crystal having a specified load capacitance (CL) of 12.5 pF. An external
32.768 kHz oscillator can also drive the IDT1338B-31. In this configuration, the X1 pin is
connected to the external oscillator signal and the X2 pin is left floating.
Backup Supply Input for Lithium Coin Cell or Other Energy Source. Battery voltage must be held
between the minimum and maximum limits for proper operation. Diodes placed in series
between the backup source and the V
BAT
pin may prevent proper operation. If a backup supply is
not required, V
BAT
must be connected to ground.
Connect to ground.
Serial data input/output. SDA is the input/output pin for the I
2
C serial interface. It is an open-drain
output and requires an external pull-up resistor (2 Kohm typical).
Serial clock input. SCL is used to synchronize data movement on the serial interface. It is an
open-drain output and requires an external pull-up resistor (2 Kohm typical)
3
14
V
BAT
4
5
6
7
15
16
1
2
GND
SDA
SCL
SQW/OUT Square-Wave/Output driver. When enabled and the SQWE bit set to 1, the SQW/OUT pin
outputs one of four square-wave frequencies (1 Hz, 4 kHz, 8 kHz, 32 kHz). It is an open drain
output and requires an external pull-up resistor (10K ohm typical). Operates when the device is
powered with VCC or V
BAT
.
V
CC
NC
Device power supply. When voltage is applied within specified limits, the device is fully
accessible by I
2
C and data can be written and read.
No connect. These pins are unused and must be connected to ground for proper operation.
8
—
3
4 - 13
IDT™
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM 2
IDT1338B-31
REV A 112309
IDT1338B-31
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
IDT1338B-31
REV A 112309
IDT1338B-31
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM
RTC
ESR (Effective Series Resistance)
Choose the crystal with lower ESR. A low ESR helps the
crystal to start up and stabilize to the correct output
frequency faster compared to high ESR crystals.
the oscillator circuit locally on this separated island. The
ground connections for the load capacitors and the
oscillator should be connected to this island.
PCB Layout
Frequency Tolerance
The frequency tolerance for 32 KHz crystals should be
specified at nominal temperature (+25°C) on the crystal
manufacturer datasheet. The crystals used with IDT1338
typically have a frequency tolerance of +/-20ppm at +25°C.
Specifications for a typical 32kHz crystal used with our
device are shown in the table below.
Parameter
Nominal Freq.
Series Resistance
Load Capacitance
Symbol
f
O
ESR
C
L
Min
Typ
32.768
Max Units
kHz
50
kΩ
pF
PCB Assembly, Soldering and Cleaning
Board-assembly production process and assembly quality
can affect the performance of the 32 KHz oscillator.
Depending on the flux material used, the soldering process
can leave critical residues on the PCB surface. High
humidity and fast temperature cycles that cause humidity
condensation on the printed circuit board can create
process residuals. These process residuals cause the
insulation of the sensitive oscillator signal lines towards
each other and neighboring signals on the PCB to decrease.
High humidity can lead to moisture condensation on the
surface of the PCB and, together with process residuals,
reduce the surface resistivity of the board. Flux residuals on
the board can cause leakage current paths, especially in
humid environments. Thorough PCB cleaning is therefore
highly recommended in order to achieve maximum
performance by removing flux residuals from the board after
assembly. In general, reduction of losses in the oscillator
circuit leads to better safety margin and reliability.
12.5
PCB Design Consideration
•
Signal traces between IDT device pins and the crystal
must be kept as short as possible. This minimizes
parasitic capacitance and sensitivity to crosstalk and
EMI. Note that the trace capacitances play a role in the
effective crystal load capacitance calculation.
•
Data lines and frequently switching signal lines should be
routed as far away from the crystal connections as
possible. Crosstalk from these signals may disturb the
oscillator signal.
•
Reduce the parasitic capacitance between X1 and X2
signals by routing them as far apart as possible.
•
The oscillation loop current flows between the crystal and
the load capacitors. This signal path (crystal to CL1 to
CL2 to crystal) should be kept as short as possible and
ideally be symmetric. The ground connections for both
capacitors should be as close together as possible.
Never route the ground connection between the
capacitors all around the crystal, because this long
ground trace is sensitive to crosstalk and EMI.
•
To reduce the radiation / coupling from oscillator circuit,
an isolated ground island on the GND layer could be
made. This ground island can be connected at one point
to the GND layer. This helps to keep noise generated by
IDT™
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM 4
IDT1338B-31
REV A 112309
IDT1338B-31
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM
RTC
Power Control
A precise, temperature-compensated voltage reference and
a comparator circuit provides power-control function that
monitors the
V
CC
level. The device is fully accessible and
data can be written and read when
V
CC
is greater than V
PF
.
However, when
V
CC
falls below V
PF
, the internal clock
registers are blocked from any access. If V
PF
is less than
V
BAT
, the device power is switched from
V
CC
to V
BAT
when
V
CC
drops below V
PF
. If V
PF
is greater than V
BAT
, the device
power is switched from
V
CC
to V
BAT
when
V
CC
drops below
V
BAT
. The registers are maintained from the V
BAT
source
until
V
CC
is returned to nominal levels (Table 1). After
V
CC
returns above V
PF
, read and write access is allowed after
t
REC
(see the “Power-Up/Down Timing” diagram).
Table 1. Power Control
Supply Condition
V
CC
< V
PF
, V
CC
<
V
BAT
V
CC
< V
PF
, V
CC
>
V
BAT
V
CC
> V
PF
, V
CC
<
V
BAT
V
CC
> V
PF
, V
CC
>
V
BAT
Read/Write
Access
No
No
Yes
Yes
Powered
By
V
BAT
V
CC
V
CC
V
CC
Power-up/down Timing
Table 2. Power-up/down Characteristics
Ambient Temperature -40 to +85° C
Parameter
Recovery at Power-up
V
CC
Fall Time; V
PF(MAX)
to V
PF(MIN)
V
CC
Rise Time; V
PF(MIN)
to V
PF(MAX)
Symbol
t
REC
t
VCCF
t
VCCR
Conditions
(see note below)
Min.
300
0
Typ.
Max.
2
Units
ms
µs
µs
Note:
This delay applies only if the oscillator is running. If the oscillator is disabled or stopped, no power-up delay
occurs.
IDT™
REAL-TIME CLOCK WITH BATTERY BACKED NON-VOLATILE RAM 5
IDT1338B-31
REV A 112309
stm32/stm8
