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Forum: FPGA, VHDL & Verilog External oscillator on my LatticeXP2 Brevia2


Author: Neoz (Guest)
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Hi !

(Sorry in advance for my english that is not very good)

I'm an apprentice in a French company and I have a project based on an 
FPGA.

The new development board we bought for my tests has an external 
oscillator of 50 MHz unlike the first one i used and started to program 
on (Lattice MachXO2) that has one of 12 MHz.

My current problem is that I need a 1.5 MHz clock (I can't do with 
something else) and 12 MHz was a multiple of this but not 50 ...
So, I tried to use an external oscillator of 36 Mhz: I linked it to one 
of the I/Os. But all other I/Os are parasitized by this clock.

I checked the wiring and it seems good. The clock works normally.

My question : Is there a special I/O made to be a clock ? The actual 
oscillator of 50 Mhz is on pin 21 of the FPGA. I put mine on pin 106.

Thanks !

Author: Lattice User (Guest)
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Neoz wrote:

> But all other I/Os are parasitized by this clock.

What du you mean with "parasitized"?

>
> I checked the wiring and it seems good. The clock works normally.
>
> My question : Is there a special I/O made to be a clock ? The actual
> oscillator of 50 Mhz is on pin 21 of the FPGA. I put mine on pin 106.
>

Pin 106 is a GND Pin. So you missed something while checking.

There are actually pins which have a special funktion for clock input, 
you should use one of them.
On the LFXP2-5E-5TN144C these are: 21,28,56,61,78,88,116,120

But why don't you use a PLL (the LFXP2-5E has two) do create a 12 MHz 
clock from the onboard 50 MHz oscillator?

Author: Neoz (Guest)
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> What du you mean with "parasitized"?

There is lot of noise on input and output signals.

> Pin 106 is a GND Pin. So you missed something while checking.
>
> There are actually pins which have a special funktion for clock input,
> you should use one of them.
> On the LFXP2-5E-5TN144C these are: 21,28,56,61,78,88,116,120

Yep sorry, it is on pin 116, not 106. So this is ok for the input.

> But why don't you use a PLL (the LFXP2-5E has two) do create a 12 MHz
> clock from the onboard 50 MHz oscillator?

I never tried to use this so I'll see how it works ;)


Thanks a lot!

Author: Neoz (Guest)
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I attached a screenshot of my signals.

In blue : the good signal without noise.
In red : the signal I have with this new development board.

Author: Lattice User (Guest)
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Neoz wrote:
> I attached a screenshot of my signals.
>
> In blue : the good signal without noise.
> In red : the signal I have with this new development board.

This looks bad, but i doubt it is caused by the development board 
itself. (unless it is already damaged)

Check the Vccio (3.3V) with the scope. If you power your external 
hardware from the development board, you possible may draw to much 
current. Also keep in mind the limitations for USB power.

Recheck your wiring again, with emphasis on good ground connections. A 
bad ground connection can cause all kinds of weird behavior.

Check if the signals from the FPGA are ok while nothing extrenal is 
connected.

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