External Oscillator Voltage?

Discussion in 'Embedded Systems and Microcontrollers' started by klangst, Aug 30, 2014.

  1. klangst

    Thread Starter Member

    Jul 5, 2013
    I am just learning MCUs and am working with a PIC24EP. I am in the process of learning about external crystal oscillators. I have a HC49US 20MHz crystal, but some things I have read online talk about making sure the output is compatible with the MCU voltage input. I am confused by this. The PIC24EP is a 3.3v device. The HC49US crystal datasheet does not say anything about voltage. Is there something I am missing here? Can this crystal be used with any PIC that can accept a 20MHz input regardless of whether it is 5V or 3.3V?
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    I have used 20MHz crystals on 3.3V and 5V PIC's with no problems.
    You will need two low value pf Caps from the PIC Xtal pins to 0V, 12pF thru 22pF.
  3. shteii01

    AAC Fanatic!

    Feb 19, 2010
    I think they are talking about crystal vs external clock.

    With crystal the highest frequency you can get is 25 MHz.

    With external clock you can get highest frequency of 60 MHz. In this case the external clock is probably a very specialized ic with its own circuit, which means that this special ic will have a certain output and you the user should carefully choose which ic to use. Since you have 3.3 volt system, when looking at these specialized ic, you should be looking for one that outputs clock signal with 3.3 volt amplitude.

    See this doc, Section 7.5: http://ww1.microchip.com/downloads/en/DeviceDoc/70580C.pdf
  4. klangst

    Thread Starter Member

    Jul 5, 2013
    Thank you for clarifying.