AN2441 datasheet(9/14 Pages) STMICROELECTRONICS

9 / 14 page

AN2441

Pierce oscillator design

9/14

The recommended way to optimize Rs is to first choose C1 and C2 values as explained

earlier and connect a potentiometer in place of Rs. Its initial setting should be set to be

approximately equal to C2 capacitive reactance, and then adjusted if required until an

acceptable output and crystal drive level are found. The output signal should also be in line

with the minimum and maximum VIL and VIH voltages applied on the STR71x CK pin.

To have a square signal, a schmidt trigger device can be added at the inverter’s output.

Figure 7.

Square wave oscillator

The STR71x device embeds an on-chip schmidt trigger on the CK pin so is not needed in

this example, thus making the solution more cost-effective.

The designed oscillator is mounted on an STR7 generic board. The STR71x PLL is

configured to generate 48 MHz from the 16-MHz oscillator output and a GPIO pin level is

toggled before and after PLL lock. This test is passed successfully.

For design robustness, the signal obtained should be tested in a worst case scenario:

●

Application at highest temperature and lowest VDD voltage.

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Application at minimum temperature and highest VDD voltage.

Square output signal

AN2441 数据表(PDF) 9 Page - STMicroelectronics