ISL12020MIRZ-T7A Datasheet, PDF (29/34 Pages) Intersil Corporation – Low Power RTC with Battery Backed SRAM, Integrated ±5ppm Temperature Compensation and Auto Daylight Saving

ISL12020M
2. Add a ground trace around the device with one end
terminated at the chip ground. This guard ring will provide
termination for emitted noise in the vicinity of the RTC device.
3. Do not run a ground or power plane immediately under the
RTC. This will add capacitance to the X1/X2 pins and change
the trimmed frequency of the oscillator. Instead, try to leave a
gap in any planes under the RTC device.
.
GROUND
RING
F_OUT
SCL
SDA
FIGURE 22. SUGGESTED LAYOUT FOR ISL12020M
The best way to run clock lines around the RTC is to stay outside
of the ground ring by at least a few millimeters. Also, use the
V_BAT and V_DD as guard ring lines as well, they can isolate clock
lines from the X1 and X2 pins. In addition, if the IRQ/F_OUT pin is
used as a clock, it should be routed away from the RTC device as
well.
Measuring Oscillator Accuracy
The best way to analyze the ISL12020M frequency accuracy is to
set the IRQ/F_OUT pin for a specific frequency, and look at the
output of that pin on a high accuracy frequency counter (at least
7 digits accuracy). Note that the IRQ/F_OUT is an drain output and
will require a pull-up resistor.
Using the 1.0Hz output frequency is the most convenient as the
ppm error is just as shown in Equation 7:
ppm error = F_OUT – 1  1e6
(EQ. 7)
Other frequencies may be used for measurement but the error
calculation becomes more complex.
When the proper layout guidelines above are observed, the
oscillator should start-up in most circuits in less than one second.
When testing RTC circuits, a common impulse is to apply a scope
probe to the circuit at the X2 pin (oscillator output) and observe
the waveform. DO NOT DO THIS! Although in some cases you may
see a usable waveform, due to the parasitics (usually 10pF to
ground) applied with the scope probe, there will be no useful
information in that waveform other than the fact that the circuit
is oscillating. The X2 output is sensitive to capacitive impedance
so the voltage levels and the frequency will be affected by the
parasitic elements in the scope probe. Use the F_OUT output and a
frequency counter for the most accurate results.
Temperature Compensation Operation
The ISL12020M temperature compensation feature needs to be
enabled by the user. This must be done in a specific order as
follows.
1. Read register 0Dh, the BETA register. This register contains
the 5-bit BETA trimmed value, which is automatically loaded
on initial power-up. Mask off the 5LSB’s of the value just read.
2. Bit 7 of the BETA register is the master enable control for
temperature sense operation. Set this to “1” to allow
continuous temperature frequency correction. Frequency
correction will then happen every 60s with V_DD applied.
3. Bits 5 and 6 of the BETA register control temperature
compensation in battery-backup mode (see Table 16 on
page 21). Set the values for the operation desired.
4. Write back to register 0Dh making sure not to change the 5
LSB values, and include the desired compensation control
bits.
Note that every time the BETA register is written with the TSE bit
= 1, a temperature compensation cycle is instigated and a new
correction value will be loaded into the FATR/FDTR registers (if
the temperature changed since the last conversion).
Also note that registers 0Bh and 0Ch, the ITR0 and ALPHA
registers, should not be changed. If they must be written be sure
to write the same values that are recalled from initial power-up.
The ITR0 register may be written if the user wishes to re-calibrate
the oscillator frequency at room temperature for aging or board
mounting. The original recalled value can be re-written if desired
after testing.
Daylight Savings Time (DST) Example
DST involves setting the forward and back times and allowing the
RTC device to automatically advance the time or set the time
back. This can be done for current year, and future years. Many
regions have DST rules that use standard months, weeks and
time of the day, which permit a pre-programmed, permanent
setting.
An example setup for the ISL12020M is in Table 28.
TABLE 28. DST EXAMPLE
VARIABLE
VALUE
REGISTER VALUE
Month Forward and DST April
Enable
15h
84h
Week and Day Forward 1st Week and 16h
48h
and select Day/Week, not Sunday
Date
Date Forward
not used
17h
00h
Hour Forward
2am
18h
02h
Month Reverse
October
19h
10h
Week and Day Reverse Last Week and 1Ah
78h
and select Day/Week, not Sunday
Date
Date Reverse
not used
1Bh
00h
Hour Reverse
2am
1Ch
02h
29
FN6667.5
December 13, 2011