On-Chip Oscillator

I vaguely remember trying something like this a few years ago by creating two inverters and using them to implement an oscillator with a crystal and a couple of resistors.

Leon

It is possible to make ring oscillators in CPLD, using a series of inverters. These will have wide process/vcc/temp variations, and should ideally be started with a NAND + Reset structures, to avoid possible stable harmonic cases.

Most CPLDs have a choice of OP Slew rates, with slow bein used to reduce the bounce and RFI effects. In the FAST mode, actual speed is largely a function of load capacitance. What is it that requires tr < 2ns ?

-jg

jg,

My output is a clock which eventually will control A/D converter. Now, the clock for converter needs to have < 2ns Rise Time (For faster sampling rate, in my case 10MSamples/s) by the spec of it.

Nobody really replied to my other question, can we generate clock on the Programmable Device (Max3032)? I want to generate 20MHz clock on the EPLD.

Thanks, Drew

You asked an Altera-specific question, which delayed my response just a bit :-) Any modern programmable device from any of the major manufacturers will give you a 1 ns or faster rise/fall time, and we all consider 20 MHz a low frequency. You should not have any problem with your design, just stay away from ancient parts. Peter Alfke, Xilinx

Just make sure you select the FAST edge option on that pin, and keep the traces as short as possible.

I thought I had. The answer is yes, it can be done, (we have done various delay line clocks inside CPLD ) but you get low performance. If you are running an ADC where low noise matters, I'd avoid the on-chip oscillator.

If you are using a CPLD with hysteresis, you can build RC oscillators, (again, low performance) but I do not think the Altera 3032 meets that.

If you mean attach a crystal to the CPLD, that can also be done, but again at high risk. Spurious Oscillations, 'edge fur', and incorrect harmonics are all possible outcomes.

If you want the smallest/cheapest Xtal Oscillator for CPLD/FPGA drive, look at

Or, you can use an oscillator module.

-jg

Do not use an on-chip oscillator (they are not stable). Do not use an external RC, (that's better, but still not stable) Do not use a crystal driven by a CPLD or FPGA, because it's more trouble and problems than it is worth. Spend a whole dollar ( or $ 1.50) and buy yourself a crystal oscillator module and live happily ever after ! Peter Alfke

As noted, but there are applications where that is perfectly acceptable.

There are also application where this is OK. Truckloads of uC ship these days with calibrated RC Osc, that give ~2% precision. Newer CPLDs include hysteresis, to allow 2 & 3 terminal oscillator designs, if one wishes.

As noted.

Unless Size, Frequency, or power considerations exclude that. I have not seen many Modules at 32Khz, for example. Modules under around 10mA are also rare, so you can easily find your Osc Module chews far more power than your Xilinx Coolrunner, for example. Depends on the design, but that could be a killer.

We have tried the 74lvc1gx04 I mentioned, (Philips have a little Eval PCB ) and it is quite impressive. You can vary Vcc, for quite wide Freq/Power trade offs, and it comes in a tiny SOT23 sized package.

The Altera MAX II CPLD family does have a general purpose on-chip oscillator. The on-chip oscillator is part of the User Flash Memory block feature but is available as a general purpose clock for the core logic or off-chip. The frequency variation on this oscillator is between 4.6 and 7.4 MHz. See the MAX II handbook, chapter 2 page 24 and chapter 10 page 7 for more information.