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Anonymous
Not applicable
5,125 Views

## Saving a 50MHz oscillator using Spartan 3AN?

In a new design we will need an internal 50MHz in our Spartan 3AN.

Of cource, I could just add a 50MHz oscillator externally, but I would rather save that extra cost and space if possible

Available clocks are:

- 24MHz  (with LVCMOS 3.3V levels) - Challange: How to multiply by 50/24?

- 25MHz  (with LVCMOS 1.2V levels) - Challange: How to interface single-ended 1.2V to a 3.3V bank?

All the 4 blocks of our device need to have VCCO = 3.3V

There are only about 3-4 unused pins on the device.

Any suggestions on how to best solve this?

It sounds easiest to use the 1.2V 25MHz clock and just multiply by two. But I need a way of interfacing the low voltage to a 3.3V bank.

My first thought is then:

- Can PCI33 input be defined for just one single GCLK input? If so, I assume I can pass the 1.2V clock through a capacitor and center the 1.2V clock around 0.4 x 3.3V with a resistor divider.

- Other ideas?

The board-design is not finished, so we can still do changes.

Br.

Bjorn

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1 Solution

Accepted Solutions
Observer
6,114 Views
Registered: ‎05-05-2009

Hi bjorn,

use the 24MHz input and use a DCM. If you use the IP core Single DCM_SP (for Spartan 3an) you can type in your input frequency and your output frequency and the IP core calculate the factors (in your case multiply by 25 and divide by 12)and create the DCM modul.

best regards,

Matthias

3 Replies
Observer
6,115 Views
Registered: ‎05-05-2009

Hi bjorn,

use the 24MHz input and use a DCM. If you use the IP core Single DCM_SP (for Spartan 3an) you can type in your input frequency and your output frequency and the IP core calculate the factors (in your case multiply by 25 and divide by 12)and create the DCM modul.

best regards,

Matthias

Xilinx Employee
5,104 Views
Registered: ‎11-28-2007

You can use LVCMOS12 input in a 3.3V bank in S3AN (see the table from UG331):

Cheers,

Jim

Cheers,
Jim
Anonymous
Not applicable
5,078 Views

Thank you, both guys! This is just getting better and better. :-)

This table is more clear about the avialability of LVCMOS12 as an input  than the similar one one shown DS557-3 that focuses more on the outputs.

Regards,

Bjorn