Is using a single 25MHz oscillator feasible

[Altera_Forum]

We're starting to design a custom board using a Stratix II part and we'd like the board to be as small as possible. The board will have an ethernet device, SRAM, CFI flash, and the MAX programming CPLD from the dev board.  

 Assuming we go with the LAN91C111 for the ethernet device, would it be feasible to use a single 25MHz oscillator for the entire board? The MAX's design only runs at 3.125MHz so that shouldn't be an issue, and a PLL can be used to make a custom target design so the flash programmer can run at 50MHz.   Anyone else tried this, or know of a potential issue with using a single oscillator for the entire board?

[Altera_Forum]

--- Quote Start ---  originally posted by bkucera@Jul 4 2006, 12:05 AM we&#39;re starting to design a custom board using a stratix ii part and we&#39;d like the board to be as small as possible. the board will have an ethernet device, sram, cfi flash, and the max programming cpld from the dev board.   assuming we go with the lan91c111 for the ethernet device, would it be feasible to use a single 25mhz oscillator for the entire board? the max&#39;s design only runs at 3.125mhz so that shouldn&#39;t be an issue, and a pll can be used to make a custom target design so the flash programmer can run at 50mhz.   anyone else tried this, or know of a potential issue with using a single oscillator for the entire board? <div align='right'><{post_snapback}> (index.php?act=findpost&pid=16635)  --- quote end ---    --- Quote End ---   Hi! I think you have not any problem for realize it! 1)You should use PLL for getting 50MHz from 25 MHz 2) 3.125 Mhz is equal 50Mhz divide by 16. It means simple counter with module 2

[Altera_Forum]

--- Quote Start ---  originally posted by bkucera@Jul 3 2006, 04:05 PM we&#39;re starting to design a custom board using a stratix ii part and we&#39;d like the board to be as small as possible. the board will have an ethernet device, sram, cfi flash, and the max programming cpld from the dev board.   assuming we go with the lan91c111 for the ethernet device, would it be feasible to use a single 25mhz oscillator for the entire board? the max&#39;s design only runs at 3.125mhz so that shouldn&#39;t be an issue, and a pll can be used to make a custom target design so the flash programmer can run at 50mhz.   anyone else tried this, or know of a potential issue with using a single oscillator for the entire board? <div align='right'><{post_snapback}> (index.php?act=findpost&pid=16635)  --- quote end ---    --- Quote End ---    Ya. there won&#39;t be any problem in doing that. We have also tried the inverse: Using a 50MHz oscillator for NIOS and, using this 50MHz, deriving 25MHz LAN clock from PLL.  Sincerely,

[Altera_Forum]

The only potential issue I have had was deriving the right frequencies; that&#39;s why I usually have two slots, one for a 50M and one for a 48M... they can get you different time bases. If you can get all of your frqeuencies with the one, then you should be ok. The only case I could see forming a problem was if you needed multiple output clocks from the FPGA; you would ideally use the outputs of a PLL for these clocks, and theres only one external output per pll... so keep that in mind too

[Altera_Forum]

--- Quote Start ---  originally posted by jdhar@Jul 5 2006, 01:51 PM the only potential issue i have had was deriving the right frequencies; that&#39;s why i usually have two slots, one for a 50m and one for a 48m... they can get you different time bases. if you can get all of your frqeuencies with the one, then you should be ok. the only case i could see forming a problem was if you needed multiple output clocks from the fpga; you would ideally use the outputs of a pll for these clocks, and theres only one external output per pll... so keep that in mind too <div align='right'><{post_snapback}> (index.php?act=findpost&pid=16676)  --- quote end ---    --- Quote End ---    We&#39;re driving all the internal logic with the same clock so that shouldn&#39;t be an issue. The only frequencies we&#39;ll need are 25MHz and 50, 75, or 100.