FPGA as heater

The LM35 is supposedly not c-load stable, but most things are c-load stable with enough c.

It is a kinda tricky part.

He has already said he has some constraint that won't let him use a PLL or DLL inside the FPGA, so I expect he can't use this either. I can't imagine what his constraint is, maybe the clock is not regular like a typical clock but rather is an async data strobe.

I don't recall the upper limits of what can be done with the SERDES that most FPGAs have on chip. But they all run at multi-GHz data rates and are clocked much slower with an internal clock multiplier. Likely that can't be used either.

It does seem pretty silly to be adjusting timing by varying the temperature of the die. Not just crude, but fairly ineffective as delays are controlled by local temperature and a die can have hot spots. If the full problem were explained perhaps a solution could be offered.

Anyone know what a "1 ns CMOS part" means?

LM35 output is an emitter follower with a weak pulldown. An ideal RF detector.

And it latches up if it possibly can.

LM71, SPI interface, is a nice part.

Do you use that property of CMOS (slower the warmer it gets) as an automatic negative feedback loop for your heater?

John

That would likely not work nearly as well as directly measuring the temperature of the die. Notice he said the chip has a built in thermometer. The only issue with using the thermometer is the temperature across the die will vary. Using the delay to control the heater might work well in concept, but would be hard to measure and still only work for the output being measured. Any other outputs will be spread around the die and so not isothermal. But mostly the timing would be hard to measure.

He has an oven/refrigerator. He could measure the change in timing over temperature for some number of samples. This would give him an idea of how much spread is involved. There are multiple sources of timing spread and he can control one and sort of control another. He wants an idea of how much timing spread is left.

Thanks for the explanation. I am not a qualified board designer, just a bit of a hacker designing and making the few I need for my restoration business. It is nice to get the background of design considerations as I find that information useful in determining where designs have gone wrong in our equipment and try to make improvements to the systems.

As an example - Common/Ground designs in early arcade games were not always at their best. A number of early electronic pinball games used to burn up driver transistors and coils at random until I traced the problem - which was a design error where all the commons (MPU, Audio, Solenoids, Lamps) came together at the regulator board through several different pins, and as each pin connection oxidized slightly it allowed ground potential differences between the MPU and the solenoid logic grounds, leading to the transistors being slightly biased on... And this game logic had been designed by Rockwell.

John

This sort of poor design is common in consumer electronics. Likely the engineer wanted more or better ground connections, but got shot down due to increased cost of connectors with either more pins or better plating (silver or gold). As long as the oxidation issue happens _after_ the warranty period expires, there's no incentive to correct the design, either.