fpga to 5v ttl logic

what shall I use to convert FPGA i/o pins to 5v TTL compatible levels?

50 i/o pins, tri-state, TTL R
Reply to
aiiadict
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What FPGA? What I/O supply voltage? How fast?

A few (older ones mostly) you can directly connect.

For some such as spartan 3 you can use series resistors. (Typically ones which have internal protection diodes - the resistor limits the current to prevent burning out the diode, which limits the voltage to the input itself)

For others you may want to use special level translators or perhaps 3.3 volt buffers with 5v compatible I/O.

Also, what logic family are you using on the 5v side - TTL, 5v CMOS, or

5v TTL-compatible CMOS? Ordinary 5v CMOS may not see 3.3v referenced '1's as '1's - though 5V TTL and TTL compatible CMOS should.
Reply to
cs_posting

There are two directions: FPGA to TTL, and TTL to FPGA Today, all the FPGAs I know still support 3.3 V as I/O supply voltage. (That may change in the future, for 3.6-V tolerance is not natural or easy in the best and fastest processes) TTL-to-FPGA: Old bipolar TTL generally stayed 1 or 2 diode drops below Vcc, but CMOS variants can swing all the way to Vcc=5.5Vmax. Most FPGAs do not tolerate >3.6 V on their pins, but most also have a clamp diode to their own Vcc. If you can rely on that diode, then use 2.5 V or 3.3 V as Vccio and put a series resistor of 100 or 220 Ohm between the FPGA and TTL pin, to limit any clamp current.

FPGA-to-TTL: Usually no problem, since TTL sees anything above 2.4 V as High. There are, however, "TTL" derivatives with CMOS input thresholds that might be up to 3.5 V. In that case, you should 3-state the FPGA output, and use a pull-up resistor to 5 V (relying on the clamp diode to the 3.3 V Vcco. This is a slow pull-up, and there is a trick to temporarily enable the active pull-up to generate the first 2 V of voltage swing.

5V should today be considered an obsolete and awkward supply voltage, although it has served us well for 40 years.( In a few years, 3.3V will cause the same grief...) Peter Alfke, from home.
Reply to
Peter Alfke

There is also a growing range of Dual-Vcc Translators, to service this market that refuses to die... TI has quite a number now, and I see Philips following them.

You need dual supply, if power supply drain is any sort of a convern.The Clamp diode tricks above are simple, but do not do good things to the power drain figures.....

Sorry Peter, but much as the FPGA sector wants 5V to go away, it's still here. In fact, the newest devices from Infineon and Freescale have 5V ports !

Yes, they have lower voltage cores, but that is hidden from the designer. ie, the Silicon vendor takes the trouble!

ISTR the Freescale one impressed me, as it appeasrs to not need a core decoupling pin - not sure how they managed that.

Why ? - noise immunity, ease of interface : have you ever tried to find a power MOSFET that can be driven from 3.3V ?

-jg

Reply to
Jim Granville

If building in 5v tolerance for that reason, why not build for say 9 volt or 15 volt?

Even 5v gate drive is marginal for power MOSFETs, in that it's only enough for some parts.

Reply to
cs_posting

On a sunny day (26 Feb 2006 07:45:40 -0800) it happened cs snipped-for-privacy@hotmail.com wrote in :

Yes but at least you can get these.

3.3 V parts, 1.5 V 100% on, are rare.

Reply to
Jan Panteltje

Yes, it is possible to make modern ICs 5-V tolerant, while the internal logic operatesoff 1 to 1.5 V. But the high Vcc and thick oxide in the I/O circuits severely reduces their speed. And we see far more of our customers clamoring for high performance than for 5-V compatibility. We try to serve a very broad range of customer requirements, but sometimes we have to make a choice. 5-V tolerance was sacrificed for higher I/O performance and lower power. Peter Alfke, Xilinx Applications

Reply to
Peter Alfke

4000 series logic had higher voltage operation, and is still widely used. We still use 4000 series gates in new designs, but choose to operate at 5V, not higher.

One of the driving forces :) in the 5V standard is Automotive. Here, 9 and 15V regulated rails are clearly too high, as the canking voltage would cause dropout.

A strange argument for removing it, if that is what you are saying ?

There are a raft of TTL compatible MOSFET driver chips, typically SO8, if you really need 10V drive - but a large portion of MOSFETS in use are for relay/Solenoid/Small motor control, where 5V gate drive is fine. Last time I checked, it was really only the "final milliohm" that needed 10V gate drive - eg a Philips PH2625L is 4.5 milliOhms at 4.5V Vgs, and 2.7 MilliOhms at 10V Vgs

White and Blue LEDS are another common (and growing) load, that struggles with 3.3V and lower supply rails.

You CAN get 3.3V relays (I have a design now that will use 3V relays) but they are not common, and restricted to smaller models.

-jg

Reply to
Jim Granville

It's a relatively small market comparted to new designs, but 5V legacy designs in the field will be around for another decade or so, where speed has never been the issue.

The original posters history shows interest in Apple II development, and may be hobby only. On the other hand, there remains 5V industrial development around ISA and PC104, which isn't likely to go completely

3.3V for a while.
Reply to
fpga_toys

All engineering decisions, and also all business decisions are a trade-off between conflicting goals and requirements. While the two big gorillas in FPGA-land emphasize performance, density, and low cost, perhaps the little guys (used to be affectionally called the "seven dwarfs" but they are fewer than seven now) can populate the niches left empty by the big guys. Might be their one chance... Peter Alfke

Reply to
Peter Alfke

I believe some of the smaller MCUs have onchip decoupling. They could also have regular caps inside the package, I'm not sure how much cost that adds but for the big powerpcs with megabytes of flash it can't be much.

I'm sure that the 5v output come at a cost, it's just that chips designed for applications that require 5v drive to e.g. mosfets, the cost is less than having to add external circuitry.

I'm sure that if the main application for FPGAs were automotive they would have 5v IOs as well

-Lasse

Reply to
langwadt

One thing to watch out for if you have many inputs and rely on clamp diodes: When the inputs are high they can raise your 3.3v VCCO to unsave levels unless you have a large load on VCCO. Voltage regulators normally work only one way (they supply current)... Peter Wallace

Reply to
Peter Wallace

Have you ever tried to find a 5V MOSFET that wants to be driven by a 24mA logic drive? It seems that FET drivers (standalone parts with 2A or more drive) are better done outside the FPGA.

Reply to
John_H

hi i am using spartan2 XC2S50.i am trying to communicate fpga with a computer , using a lan card. i found out that the voltages levels arent the same. can anyone tell me how i should go about it ....i have read about level translaters. but dont know where to find one and which one to use.......help me please .......anyone thankyou all

Reply to
aayush

That depends very much on the application: FET drivers are used where fast gate dV/dT is essential - commonly in fast SMPS designs.

If you are driving a relay, or a motor on your wing mirror, or door lock, then ns edges are actually a liability. We sometimes use Series Gate resistors, to deliberately slow down the edges. In those cases, you would not use 24mA option, but 8mA instead.

-jg

Reply to
Jim Granville

xilinx xc3s200

3.3v

a bit faster than 1 MHz

Anybody have links to level translator components, or a 3.3v buffer with 5v i/o?

TTL

Rich

Reply to
aiiadict

Reply to
Peter Alfke

ok, I can drive a TTL with an FPGA I/O pin, can I drive an FPGA pin with TTL? will 5v damage a 3.3v input on the FPGA?

Rich

Reply to
aiiadict

Google "TI level Translator" Click first hit, Click again, and you are at

formatting link

Now, simply select the widths/voltages that suit.

-jg

Reply to
Jim Granville

agree 100%

it is the FPGA makers who are awkward; not the supply voltage.

imho, fpga makers have dropped the ball, by totally ignoring the enormous markets of various mixed-signal products; where 5v is VERY common...along with generally noisy environments.

I have always been amazed that not a single fpga-maker has made a line of small-to-medium sized parts targeted not for blazing speed and monster computing functions; but rather, for 5v I/O, all Schmitt-trigger inputs, and very low cost.

Imho, such a line would sell like hotcakes. Instead of hard-silicon MAC's and such, where are the hard-silicon counter/timer modules, etc. ??

I'd love to be able to buy a $5 part with 64 or 128 flops, and a hard block of 8 timer/counters (hardly any chip area in modern silicon). I've got =dozens= of apps for such a part.

In any case, it's not the users who are mistaken about 5v; but rather, the fpga-makers...who are ignoring the -reality- of -ongoing demand- for it, instead of embracing it via a profitable targeted product-line.

just m-h-o as one of those pesky 5v users, of course...

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Reply to
metal

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