FN6667 datasheet(29/34 Pages) RENESAS

29 / 34 page

ISL12020M

FN6667 Rev 6.00

Page 29 of 34

January 9, 2015

Figure 22 shows a suggested layout for the ISL12020M device.

Three main precautions should be followed:

1. Do not run the serial bus lines or any high speed logic lines in

the vicinity of the X1 and X2 pins. These logic level lines can

induce noise in the oscillator circuit, causing misclocking.

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.

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

VBAT and VDD as guard ring lines as well, they can isolate clock

lines from the X1 and X2 pins. In addition, if the IRQ/FOUT 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/FOUT 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/FOUT 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:

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 FOUT 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 VDD 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 recalibrate

the oscillator frequency at room temperature for aging or board

mounting. The original recalled value can be rewritten 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 preprogrammed, permanent

setting.

An example setup for the ISL12020M is in Table 29.

FIGURE 22. SUGGESTED LAYOUT FOR ISL12020M

FOUT

SCL

SDA

GROUND

RING

ppm error

FOUT 1

–

 1e6



(EQ. 7)

TABLE 29. DST EXAMPLE

VARIABLE

VALUE

Month Forward and DST

Enable

April

15h

84h

Week and Day Forward

and select Day/Week, not

Date

1st Week and

Sunday

16h

48h

Date Forward

not used

17h

00h

Hour Forward

2am

18h

02h

Month Reverse

October

19h

10h

Week and Day Reverse

and select Day/Week, not

Date

Last Week and

Sunday

1Ah

78h

Date Reverse

not used

1Bh

00h

FN6667 Datasheet(PDF) 29 Page - Renesas Technology Corp

FN6667 Datasheet(HTML) 29 Page - Renesas Technology Corp

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Datasheet Download

Link URL