AN4494 datasheet(21/34 Pages) STMICROELECTRONICS

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AN4494

LSOSC centering test

The LSOSC is used to have a reference time clock. The advantage of using the external

32.768 kHz clock is that it consumes less power than internal RO and it is more accurate

(50 ppm). This test permits to center its oscillator frequency, changing the crystal

capacitance.

6.1

Test case specification identifier

LSOSC_center_TEST

6.2

Test prerequisite

For this test, a test point in the pin 14 (TEST9) is required.

6.3

Test description

There is a way, using the IFR tool of the BlueNRG GUI, to put out the LSOSC signal in the

pin 14.

By measuring its frequency with an oscilloscope, the frequency offset can easily be

measured.

6.4

Test setup

6.4.1

Hardware

An oscilloscope is required for this test.

6.4.2

Software

ST BlueNRG GUI.

6.5

Test procedure

Connect an oscilloscope’s probe in the pin 14 (TEST9) test point.

Power up the BlueNRG platform.

Set the scope to capture a consistent number of 32 KHz waveform periods (for example 64

cycles, so set the time base at 200us). In this way, the influence of the jitter in the measure

is minimized.

In the IFR tool of the GUI make a “Read” of the current IFR configuration, then tick the

check-box “LS crystal measure” and then make a “Write” operation.

Now a power cycle is required to let the new IFR be operative.

At this point, the 32.768 KHz waveform will be visible on the oscilloscope screen.

Perform the measurement of the frequency: the difference between the target value

(f=32.768 KHz) and the measured one is the frequency offset Δf.

6.6

Expected results

If DUT freq > 32.768 KHz → increase XTAL caps

If DUT freq < 32.768 KHz → decrease XTAL caps

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