Two Xilinx CPLDs questions

[kender]

Colleagues,

I’m starting to design a small instrument around a CoolRunner XPLA3, and I have two questions (so far):

- Can I use a watch crystal? Can I use an RC oscillator? If yes, then how? What are other ways to make a small low power oscillator for a CPLD? Unfortunately, I don’t have the room and power for a full blown oscillator.

- I need to make analog measurements. They can be slow and low resolution. But they too have to be low power and low part count.

Do you know any techniques for these?
Could anyone recommend a specialized forum (or a web group) for Xilinx CPLDs?

Cheers,
Nick

 

[echo47]

The Xilinx CPLD FAQ says crystal and RC oscillator are not recommended:
**broken link removed**

Maybe you could build a delta-sigma A/D converter into the CPLD by adding only a few external analog parts such as a filter and comparator. However, that may require too much logic for a small CPLD.

Here are some Xilinx discussion forums:
**broken link removed**

You can ask Xilinx a question by submitting a WebCase. They usually respond in a day or two:
https://www.xilinx.com/support/clearexpress/websupport.htm

 

[kender]

The Xilinx CPLD FAQ says crystal and RC oscillator are not recommended:
**broken link removed**

Thanks for the link to the Xilinx knowledge base. Timing is not that critical in my application. At the same time the temperature will be nearly constant (within, say, 5 degC) and the supply voltage will be regulated.

Following your advice I will leave provisions for a crystal and oscillator. Well, oscillator is easy. Is there anything that would prevent the topology shown below from working with a CPLD?

Cheers,
Nick

 

[echo47]

Your diagram shows one inverter, however inside the CPLD there will be several high-speed gates in series. I doubt they would behave like a single CMOS gate biased into its linear region. The oscillator may suffer high-frequency instabilities, may consume lots of power, may have startup problems, or it might work fine. I've never tried it.

You may find helpful old discussions by searching Google Groups for the words CPLD oscillator
**broken link removed**