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ISL12059 Datasheet(PDF) 10 Page - Renesas Technology Corp

Part No. ISL12059
Description  Low Cost and Low Power I2C Bus Real Time Clock/Calendar Low Power and Low Cost RTC
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Maker  RENESAS [Renesas Technology Corp]
Homepage  http://www.renesas.com
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ISL12059 Datasheet(HTML) 10 Page - Renesas Technology Corp

 
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ISL12059
FN6757 Rev 0.00
Page 10 of 11
Jun 15, 2009
Application Section
Oscillator Crystal Requirements
The ISL12059 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 6
lists some recommended surface mount crystals and the
parameters of each. This list is not exhaustive and other
surface mount devices can be used with the ISL12059 if their
specifications are very similar to the devices listed. The
crystal should have a required parallel load capacitance of
12.5pF and an equivalent series resistance of less than 50k.
The crystal’s temperature range specification should match the
application. Many crystals are rated for -10°C to +60°C
(especially through-hole and tuning fork types), so an
appropriate crystal should be selected if extended temperature
range is required.
Layout Considerations
The crystal input at X1 has a very high impedance, and oscillator
circuits operating at low frequencies such as 32.768kHz are
known to pick up noise very easily if layout precautions are not
followed. Most instances of erratic clocking or large accuracy
errors can be traced to the susceptibility of the oscillator circuit to
interference from adjacent high speed clock or data lines. Careful
layout of the RTC circuit will avoid noise pickup and insure
accurate clocking.
Figure 10 shows a suggested layout for the ISL12059 device
using a surface mount crystal. Two main precautions should be
followed:
Do not run the serial bus lines or any high speed logic lines in
the vicinity of the crystal. These logic level lines can induce
noise in the oscillator circuit to cause misclocking.
Add a ground trace around the crystal with one end terminated
at the chip ground. This will provide termination for emitted
noise in the vicinity of the RTC device.
In addition, it is a good idea to avoid a ground plane under the
X1 and X2 pins and the crystal, as this will affect the load
capacitance and therefore the oscillator accuracy of the circuit.
If the FT/OUT pin is used as a clock, it should be routed away
from the RTC device as well. The traces for the VDD pins can
be treated as a ground, and should be routed around the
crystal.
FIGURE 9. MULTIPLE BYTES READ SEQUENCE
SIGNALS
FROM THE
MASTER
SIGNALS FROM
THE SLAVE
SIGNAL AT
SDA
S
T
A
R
T
IDENTIFICATION
BYTE WITH
R/W = 0
ADDRESS
BYTE
A
C
K
A
C
K
0
S
T
O
P
A
C
K
1
IDENTIFICATION
BYTE WITH
R/W = 1
A
C
K
S
T
A
R
T
LAST READ
DATA BYTE
FIRST READ
DATA BYTE
A
C
K
10
1
1000
10
1
10 00
R/W BIT = “0”
R/W BIT = “1”
TABLE 3. SUGGESTED SURFACE MOUNT CRYSTALS
MANUFACTURER
PART NUMBER
Citizen
CM200S
MicroCrystal
MS3V
Raltron
RSM-200S
SaRonix
32S12
Ecliptek
ECPSM29T-32.768K
ECS
ECX-306
Fox
FSM-327
FIGURE 10. SUGGESTED LAYOUT FOR ISL12059 AND


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