isl12057 Intersil Corporation, isl12057 Datasheet - Page 14

• Manufacturer Part Number: isl12057
• Description: Low Cost And Low Power I 2c Rtc Real Time Clock/calendar
• Manufacturer: Intersil Corporation

Application Section
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.
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
MANUFACTURER
lines in the vicinity of the crystal. These logic level lines
can induce noise in the oscillator circuit to cause
misclocking.
TABLE 8. SUGGESTED SURFACE MOUNT CRYSTALS
FROM THE
SIGNALS
MASTER
SIGNALS FROM
MicroCrystal
SIGNAL AT
Citizen
THE SLAVE
ECS
SDA
S
A
R
T
T
1
IDENTIFICATION
1
BYTE WITH
0
R/W = 0
1 0 0 0
14
0
PART NUMBER
A
C
K
ECX-306
CM200S
MS3V
ADDRESS
BYTE
FIGURE 11. READ SEQUENCE
A
C
K
ISL12057
S
A
R
T
T
IDENTIFICATION
1
BYTE WITH
1
R/W = 1
0
1 0 0 0
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
routed away from the RTC device as well. The trace for the
V
around the crystal.
2. Add a ground trace around the crystal with one end
CC
FIGURE 12. SUGGESTED LAYOUT FOR ISL12057 AND
terminated at the chip ground. This will provide
termination for emitted noise in the vicinity of the RTC
device.
pins can be treated as a ground, and should be routed
1
A
C
K
FIRST READ
CRYSTAL
DATA BYTE
OUT
pin is used as a clock, it should be
A
C
K
A
C
K
LAST READ
DATA BYTE
June 15, 2009
FN6755.0
S
O
P
T