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Oscillator versus resonator? — Parallax Forums

Oscillator versus resonator?

btomkobtomko Posts: 10
edited 2008-01-02 14:37 in Propeller 1
Hi,· i'm trying to interface a 5V motorola coprocessor with my propeller.· However, this coprocessor requires a 20 mhz TTL clock which must be driven high to at least 2.4 V and driven low to at most 0.5 V.· After running into some issues with the coprocessor not working, I checked the 5V 20 mhz oscillator I have for the clock signal by putting it on my scope (including taking it off the load to prevent parasitic capacitance).· The scope reads it high at 3.3V and low at around 0.9 to 1 V.· It appears the problem I could be having is that the oscillator is not driving the clock signal low enough to meet specifications.· I've even tried a different 20 MHz oscillator from a different company and get around the same readings.· Both oscillators say in their specs that they have a low output voltage of 0.5 V.· I've tried soldering all the connections on the oscillators to prevent any stray capacitance.· Should I try a resonator instead of an oscillator?

Note: I understand that the propeller is a 3.3V device.· I am using voltage translators between the propeller and the coprocessor for bus communication.

Comments

  • Mike HuseltonMike Huselton Posts: 746
    edited 2007-12-31 15:05
    You wrote: Note: I understand that the propeller is a 3.3V device. I am using voltage translators between the propeller and the coprocessor for bus communication.

    Please describe, in detail, the precise voltage translator you are using.

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    JMH
  • btomkobtomko Posts: 10
    edited 2007-12-31 15:13
    The voltage translators I am using are TI SN74LVC4245a 3.3 to 5v 8-bit bidirectional voltage translators.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2008-01-01 02:14
    Can you provide the part number for your oscillator, and you are talking about an oscillator and not a crystal right? (An ocillator has typically 4 or more pins, while a crystal has 2).

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • btomkobtomko Posts: 10
    edited 2008-01-02 14:10
    I may have figured out the problem.· My oscillator has a rise and fall time of 10ns while the chip requires a maximum rise and fall time of 5ns for the clock signal.· I will try ordering an ECS-2100 series oscillator with a max rise time of 5ns and see if that works.
  • deSilvadeSilva Posts: 2,967
    edited 2008-01-02 14:37
    You can try any 74 AHC chip as a buffer...
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