oscillator 25MHz

[uros]

Hi,

thanks again for this awesome card! I'm doing tests with Drasko, also
present in this group.
I was just wondering whenever is possible to export 25Mhz clock for
LAN8720 chip from LPC? In that case we can omit one oscillator in
design. Is that possible? Is that a good idea?

Is there any special reason why not using small package oscillators?
maybe price?

Best
Uros

[Casainho]

2011/6/21 uros <uros.pe...@gmail.com>:

> Hi,
>
> thanks again for this awesome card! I'm doing tests with Drasko, also
> present in this group.
> I was just wondering whenever is possible to export 25Mhz clock for
> LAN8720 chip from LPC? In that case we can omit one oscillator in
> design. Is that possible? Is that a good idea?

Good question I also would like to know :-)

[Opendous Inc.]

Small Industrial Temperature Range (-40C to 85C) crystals are at
least twice as expensive as HC49US crystals. This is also a standard
footprint available from many manufacturers.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=535-9869-1-ND

There is also the issue of lead. Small crystals are RoHS-Compliant
due to an exemption but they contain lead which I am trying to avoid:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=535-9836-1-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=535-10261-1-ND

The LPC17xx has a clock output pin and it can handle up to 50MHz (Pg.
66 of UM10360.pdf). If your CPU clock is set to 100MHz and you divide
it by 2 for CLKOUT then it should be possible to use it for the
Ethernet PHY which needs 50MHz for single-ended clocks. The only
concern is whether the CLKOUT pin has the ability to drive the PHY
clock input. To be safe I would try it with a buffer such as a
NC7SZ125M5X or SN74LVC1G17DBVR which are pin-compatible.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=296-11933-1-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=NC7SZ125M5XCT-ND