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need L290 substitute

Hello, all,

Why is it whenever I find the PERFECT chip for something, it turns out to be discontinued? I found the L290 from SGS-Thompson, an old part that converts quadrature encoder signals to bipolar voltage proportinal to frequency. It looks like it could be real difficult to get this part, except from the vultures. It is basically a double F-V converter, so it produces a voltage proportional to count rate, with the sign of the output indicating the direction the encoder is turning. I think I know how to do this with 2 F-V converters and some logic, but I can't get it below about 6 chips. Does anyone know of a replacement for the L290 or L290B that does the same quadrature to bipolar voltage conversion?

Just to prevent bad suggestions, you can't use one F-V and a sign inverter after it because of encoder dithering. The balanced approach will always see equal numbers of steps in + and - direction and cancel them out. The single scheme would produce a rapidly flickering non-zero voltage proportional to the dithering rate, which would drive a servo system crazy.

Thanks in advance for any helpful ideas!

Jon

Reply to
Jon Elson
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CPLD?

How fast do the bits change? Can you do it in software with a PIC/AVR/8051? Pick your favorite.

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Reply to
Hal Murray

That can get you from one obsolescence problem into the next. Programmable logic doesn't have a particularly long life span and it's expensive.

Way to go.

Which one comes with teriyaki sauce?

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Reply to
Joerg

Maybe, but it's a different dimension of obsolescence. It's very likely that similar CPLDs will be available. You might have to recompile or even spin the board, but you are very likely to be able to get a similar chip.

Only if it's fast enough.

What is the track record on lifetime of PICs or AVRs or 8051s vs CPLDs?

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Reply to
Hal Murray

Seems likely! At a trade show yesterday I picked up a sample of a Silicon Labs 336D development "stick". The daughter board's micro is a QFN20 device with an 8051-compatible core, 16 kB of flash code space,

768 bytes of RAM, a ten-bit 200 ksps 16-channel AC, and a single 10-bit current-output DAC, runs at up to 25 MIPS using its onboard oscillator or at lower speeds (onboard or RC or crystal).

Writing a lookup-table-based quadrature-to-value loop with a DAC output would be easy and fast. Cost for the bare chips from Mouser is $4.31 in onesies, under $3 in hundreds.

I'm sure that similar capabilities are available in other micro families (e.g. PIC, TI MSP340, etc.). If you can accept a coarser DAC output (e.g. a low-pass-filtered PWM or PDM output) then even a pretty tiny jellybean PIC or AVR could do the job.

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Reply to
Dave Platt

I don't know PICs and AVRs but one of my 8051-based designs from the early 90's is still in full prodcution with no end in sight. Some younger folks scoff at the 8051 but that's because they have never been through an obsolescence situation where da boss paces down the hallway all hollering and chewing everyone out ;-)

The MSP430 has a pretty good track record but lately TI's support has become quite pathetic. They don't even answer serious requests after several reminders. Very sad because their 16-bit architecture is pretty nice and fast. I hope this won't become another case where management destroys an otherwise excellent company.

CPLD? Not with a 10ft pole, unless I absoutely have to. And I usually do not have to.

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Reply to
Joerg

That's one of the issues, the 8051 species are usually not exaclty a bargain. There is a price to be paid for reducing the effects of obsolenscence.

The MSP430F2002 can be had for close to a buck in quantities. If only TI wouldn't have dropped the ball on support. How is SiLabs' support?

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Reply to
Joerg

OK, its gonna cost you 3$ a chip, plus a counter, a dac , a jim williams or Pease V/F design, and some glue logic,

but start here:

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Steve

Reply to
osr

When they do obsolete that 8051 of yours you can just tell the boss to stick the core in an FPGA and keep on truckin'.

(Ducking... :-) )

Reply to
Joel Koltner

Digikey still keeps thousands of them in stock just like years ago. And they command a pretty penny, seems a lot of people are willing to pay for peace of mind. It may not be the right part for a very cost sensitive mass product but this was higher end medical and it sure worked out well. 15 years and no redesign.

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Reply to
Joerg

Well, the problem is this is in a servo application, and the encoder can be moving from one count every few seconds to, say,

100,000 counts a second. It should be pretty linear over that range. Lets see, a 300,000:1 dynamic range needs an 18+ bit DAC. Also, it needs to have bipolar output. Pretty hard to do with PIC or AVR. If that is all the micro was doing, the phase lag might not be to bad, but it needs to make a clean switch between frequency counting and pulse timing, and produce an output with minimal ripple.

Jon

Reply to
Jon Elson

The problem is encoder jitter. if you use ONE V/F converter, it sees the jitter both ways as counts, and rapidly switches back and forth between + and - outputs of non-zero voltage. it SHOULD produce an output very close to zero, as the + and - movements all cancel out. So, it takes TWO V/Fs which go to a

+/- summer. That is exactly what the L290 did, all so neatly in one chip! I'm trying to keep this circuit small, and was grumbling about the obsolete L290 only being available in DIP-16.

Jon

Reply to
Jon Elson

They *are* obselete, that's why they are expensive. :)

[...] 

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John Devereux
Reply to
John Devereux

Then why does Digikey stock thousands? And why haven't my clients received any notices about that? One 8051 design has now been in production for over 15 years, no end in sight. And no design changes so far.

Oh, and when I repaired our pellet stove guess what uC was in there?

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Joerg
[snip]
[snip]

That is the _main_ problem with the world, substituting a uC for good design practices.

If you car quits running, can you lift the hood, twiddle a bit and get it running? Not a chance ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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Reply to
Jim Thompson

But then why are they expensive? Shouldn't they be a "commodity" part like a LM324?

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John Devereux
Reply to
John Devereux

Yes :-(

If that pellet stove didn't have a uC I would be able to correct some design deficiencies. IMHO pellet stoves have a lot of design deficiencies.

I made sure the car has no more than the mandatory ECU. The dealer was quite stunned. "You mean you don't even want central lock and electric windows?" ... "Nope."

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Reply to
Joerg

They were always expensive. Good old capitalism I guess. If they can get the money they keep the prices up. Despite the price that company has saved big time by not having to redesign a thing in over 15 years. And since it's medical electronics the slightest code change would entail a whole slew of bureaucratic procedures (it's a patient contact device).

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