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Reduce voltage from 5V to 3.3V 10MHz TTL for HSE in STM32L1

ninjamakaron
Associate
Posted on June 09, 2015 at 17:24

Hello, 

I would like to clock RTC with external clocking signal. The problem is, I have 10MHz TTL 0-5V signal at disposal but pins OSC_IN and OSC_OUT have TC pin structure (standart 3.3V I/O). 

I cannot simply reduce voltage by a resistor divider. While the diviter works fine with a direct current, it does not give expected result with 10MHz TTL signal (I am reading noise on oscilloscope). I read somewhere that parasitic capacitance on resistors can spoil high frequency signals.

My question is either: 1) cannot I simply use the 0-5 TTL signal ?  2) how do I reduce voltage of high frequency signal without spoiling its shape significantly ? 

#rtc #stm32l1 #hse
2 REPLIES 2
Uwe Bonnes
Principal II
Posted on June 09, 2015 at 18:29

Use a 5 Volt tolerant part, like SN74LVC1G125 to level translate the part. If you can live with some load on the 5 Volt clock line, you can also use a well designed resistive divider.

Posted on June 09, 2015 at 18:38

+1 CMOS Inverter

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