question: multiple crystals or one?
- From: DJ Delorie <dj@xxxxxxxxxxx>
- Date: 08 Jan 2007 11:20:30 -0500
Relating to EMI and/or reliability. Refresher: this board will be
bolted to my furnace, to control it and the central air. Yeah, I'm
still working on it. Old board: http://www.delorie.com/house/furnace/
Background: the board has six MCUs on it. One is a gumstix (200 or
200MHz XScale). Four are R8C/1B's and one is a R8C/26. These five
R8C's use a 20MHz crystal clock source (the usual xin/xout setup).
I've currently got five crystals on the board, one next to each MCU.
The crystals are almost as big as the chips! It's a four layer board.
The question is: Is it better to have one crystal per MCU, right at
the MCU, or to have one master clock and run it to each MCU? The
distance is about four inches, and the MCU's don't have to be
synchronized (they talk over I2C). My primary concern is correct
operation of the board, as it's in a high-noise environment, so I'm
wondering if the five crystals are going to mess each other (or
anything else) up, or if the long clock line is going to pick up EMI,
and which is the worse of the two.
DJ
.
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