Spartan-3A DSP vs. Cyclone III Power-wise

Hi Manny, Depends if you are working utlizing the suspend mode in Spartan3. Then at least static power will be more or less the same. For running, TSMC lowpower 65 nm will do the trick for Altera. So it all comes down to your power budget. Regards Fredrik

Manny,

Request the C3 vs S3A power comparison slides from your FAE.

To me, while running (dynamic + static), it seems to be a wash (roughly equal, with S3A slightly better).

When not running, S3A has power savings modes, so it is a clear winner if you decide to use one or both of the modes).

Of course, it isn't hard to create a design where one, or the other shows an advantage for dynamic+static power. This slide set attempts to have a apples to apples comparison: you judge how careful we were to make things "the same."

In general, the 90nm node for medium performance transistors is less leaky (less static power), and also has more dynamic power than 65nm.

65nm is generally leakier, unless performance is lessened by increasing the Vt's of the core transistors. In fact, at 65nm, even the gates leak (this static current is independent of temperature).

The steepness of the increase in static current of 65nm is much greater than at 90nm with increasing junction temperature. Be sure to use a die temperature that is in keeping with your real application.

Austin

First, I will disclose that I work for Altera. My advice is to try the comparisons yourself and you will find out which family is best for you.

Altera has made it very easy to verify power consumption claims yourself using the Cyclone III FPGA Starter Kit. It provides an application note and circuitry to easily measure Cyclone III FPGA static and dynamic power consumption.

You can also purchase the EP3C120 device for measurement using your own board. A board including the largest Cyclone III EP3C120 is in development.

Using the power estimator results from both vendors does not substantiate this claim. For example, looking at static power only, The Cyclone III EP3C120 has roughly 119K logic elements and 3,888 kits of memory while its static power consumption is just 0.169 whether it is active or suspended.

The Xilinx XC3SD3400A has 23,872 slices which is roughly equivalent to just 48K logic elements and has 2268 kbits of memory. Static power for the low power version of this part in active mode is 0.451W. In suspend mode it drops to 0.234W while the part is suspended.

These comparisons used the latest versions of both spreadsheet estimators and assume 85C junction temperature and still air.

Altera devices include an ALTCLKCNTRL function that can be used to shut off clock networks to lower power consumption. Altera users are not penalized with higher static power consumption when the clocks resume operation. This function provides fine grained control over what is shut on/off and how much power savings is achieved. You can shut down the whole part or just selected functions/clocks which provides a lot of flexibility.

In general these statements are true if nothing is done to mitigate these issues. However, the Altera Cyclone III architecture is built on TSMC's low power process and uses an intelligent selection of low Vt's for fast performance only where speed critical. In non-speed critical circuits slower, higher Vt transistors are used. Altera also uses low Vt transistors coupled with longer channel lengths to get a balance between good performance and low leakage.

The proof is in the pudding. Regardless of what Austin or I say, perform your own comparisons and I am confident you will be impressed by the combination of relatively higher performance, higher functionality, and lower power consumption Cyclone III FPGAs deliver.

Regards, Rob

Many thanks all for the input.

At some stage, once I'm done with designing the system, I'll try to gauge things in real-world and hope for the best.

Regards,

-Manny

Hi,

Rob hit most of the points I would make. I would also refer you to

-- this is our take on the Cyclone *II* vs. Spartan-3 power story. Regardless of how much of that material you believe (it can be independently verified with hardware from the two vendors), when you extend this comparison to Cyclone III devices, Cyclone III will clearly have lower power. I'll focus on dynamic power, since Rob didn't touch on it much.

Both chips have the same Vdd supply voltage (1.2V). However, Cyclone III devices are manufactured in a 65 nm process vs. a 90 nm process in the case of Spartan-3 (and Cyclone II devices). This results in (a) a reduction in the linear dimensions of the chip and thus metal capacitance, and (b) a reduction in the size of the transistors and resulting capacitance. Overall, this leads to a ~30% (off the top of my head) dynamic power reduction compared to Cyclone II devices.

On top of this, the Cyclone device family has been highly optimized for low area (e.g. cost). One side-effect of this is that the dynamic power for a given amount of logic tends to be lower than higher- performance families. On top of this, we found in Cyclone II device vs. Spartan-3 comparisons (referenced above) that the Cyclone II device's power was significantly lower (~1/2) than that of Spartan-3, despite being in the same process and with the same supply voltage.

Bottom line: You should expect much lower power in Cyclone III vs. Cyclone II (you can trust us on that!), and thus it should not be hard to convince yourself that Cyclone III should have signficantly less power consumption than Spartan-3 (which is harder to trust us on :-)).

Regards,

Paul Leventis Altera Corp.

Does anyone know if Spartan 3A-DSP is based of Virtex4SX, or is Virtex4SX?

That might explain why the power is higher.

x

Spartan 3AD is based on Spartan 3A,

Austin