ISL12057 Datasheet, PDF (14/17 Pages) Intersil Corporation – Low Cost and Low Power I2C RTC Real Time Clock/Calendar

Application Section
ISL12057
SIGNALS
FROM THE
MASTER
S
T _{IDENTIFICATION}
A
BYTE WITH
R
R/W = 0
T
ADDRESS
BYTE
S
T _{IDENTIFICATION}
A _{BYTE WITH}
R
R/W = 1
T
S
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 11. READ SEQUENCE
Oscillator Crystal Requirements
The ISL12057 uses a standard 32.768kHz crystal. Either
through hole or surface mount crystals can be used. Table 8
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 ISL12057 if
their specifications are very similar to the devices listed.
The crystal should have a required parallel load capacitance
of 6pF 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 8. SUGGESTED SURFACE MOUNT CRYSTALS
MANUFACTURER
PART NUMBER
Citizen
CM200S
MicroCrystal
MS3V
ECS
ECX-306
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 12 shows a suggested layout for the ISL12057 device
using a surface mount crystal. Two main precautions should
be followed:
1. 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.
2. 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 12. SUGGESTED LAYOUT FOR ISL12057 AND
CRYSTAL
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 IRQ1/F_OUT pin is used as a clock, it should be
routed away from the RTC device as well. The trace for the
V_CC pins can be treated as a ground, and should be routed
around the crystal.
14
FN6755.0
June 15, 2009