ISL12059 Datasheet, PDF (10/11 Pages) Intersil Corporation – Low Cost and Low Power I2C Bus™ Real Time Clock/Calendar

ISL12059
Application Section
R/W BIT = “0”
R/W BIT = “1”
SIGNALS ^S
S
FROM THE ^T IDENTIFICATION
T _{IDENTIFICATION}
S
MASTER ^A
R
T
BYTE WITH
R/W = 0
ADDRESS
BYTE
A _{BYTE WITH}
R
R/W = 1
T
A
A
T
C
C
O
K
K
P
SIGNAL AT
SDA
11010000
A
SIGNALS FROM
C
THE SLAVE
K
11010001
A
A
C
C _{FIRST READ}
K
K _{DATA BYTE}
LAST READ
DATA BYTE
FIGURE 9. MULTIPLE BYTES READ SEQUENCE
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.
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
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.
FIGURE 10. SUGGESTED LAYOUT FOR ISL12059 AND
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
V_DD pins can be treated as a ground, and should be routed
around the crystal.
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:
10
FN6757.0
June 15, 2009