Clock input with resistor divider?

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Hello!

I have an on-board oscillator (not a crystal, a full oscillator), but it outputs 3V3 or 5V clock signal.

For my design (it's an FPGA, not AVR-related, but it's electronics, so you might be able to help me out) I need 2V5 inputs, so my question is how can I use the oscillator as clock input.

Someone suggested using a simple resistor divider to lower the input voltage from 3V3 to 2V5, but for an 50MHz signal this sounds fishy for me. Would it really work?

Regards,

axos88

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At 50 MHz there are more concerns, like layout. Make sure you keep oscillator and FPGA close together. A resistive divider is OK, but make your layout in such way that a small capacitor can be put in parallel over the feeding resistor. When the board is made, check the clock signal with a scope, and see if that small cap (10's of pF) is necessary.

Nard

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Have you checked if the chip will take a higher voltage on the clock pin?

There are level converter chips around, don't know if they will run at 50Mhz.

You can always try the resistor divider, 2K4 at the input and 780R at the output.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Okay, thanks :)

axos88

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Check the stickie at the top of the list about "tricks". There is a post, there, about level translation and most of those devices will run to 50MHz or better.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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A resistor divider may slow the edges of your signal and cause excessive jitter. Keeping the resistors to a small value will improve things, but at the cost of power consumption.

Personally I'd look at level translators. (Texas Instruments do a whole bunch of them. I'm sure others do too.) I've translated a whole address / data bus at 60MHz without problem using TI converters.

-Tim

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I agree with the last two post. You have capacitance on the gate you are driving and at 50Mhz I would think the resistor would give problems. Find a level translator. You might find one in the Tiny Gate family which is a single driver in a small SMD with 4 or so pins.