Forums

Input protection for 3.3V FPGA in a TTL world...

Started by John Robertson October 14, 2021
I am still working with a friend on a TTL level replacement for some 
chips on a pinball board and we have a nice FPGA (MAX 10 based) but it 
wants (of course) 3.3V I/O.

I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS 
arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof 
1/8W or less resistor on the outside world interface would do for 
protection.

Any other suggestions to reduce real estate, but give us both TTL/3.3 
interfacing as well as protecting the SN74CBT16211C where it interfaces 
to switches, etc. in the 'real world' of a pinball game cabinet.

Thanks!

John :-#)#

PS, my apologies for being On-Topic!
-- 
(Please post followups or tech inquiries to the USENET newsgroup)
                      John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
          (604)872-5757 (Pinballs, Jukes, Video Games)
                      www.flippers.com
        "Old pinballers never die, they just flip out."
On Thu, 14 Oct 2021 11:29:43 -0700, John Robertson <spam@flippers.com>
wrote:

>I am still working with a friend on a TTL level replacement for some >chips on a pinball board and we have a nice FPGA (MAX 10 based) but it >wants (of course) 3.3V I/O. > >I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS >arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof >1/8W or less resistor on the outside world interface would do for >protection. > >Any other suggestions to reduce real estate, but give us both TTL/3.3 >interfacing as well as protecting the SN74CBT16211C where it interfaces >to switches, etc. in the 'real world' of a pinball game cabinet. > >Thanks! > >John :-#)# > >PS, my apologies for being On-Topic!
For an input to the FPGA, you could use just a series resistor and let any overvoltage or spikes hit the ESD diodes. Debounce as needed. 10K or something. Paranoids could add a cap to ground, 1 nf or so. Topic? We have a topic? -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
On 14/10/2021 8:32 pm, John Larkin wrote:
> > For an input to the FPGA, you could use just a series resistor and let > any overvoltage or spikes hit the ESD diodes. Debounce as needed. > > 10K or something. Paranoids could add a cap to ground, 1 nf or so. >
The MAX 10 datasheet has very specific input overvoltage specs, for instance 4.27V for <4.3% of the time over 11.4 years, equally bizarrely they specifiy the supply voltage to three decimal places. I think Intel really do not want the ESD diodes to be touched? piglet
On Thu, 14 Oct 2021 21:16:03 +0100, piglet <erichpwagner@hotmail.com>
wrote:

>On 14/10/2021 8:32 pm, John Larkin wrote: >> >> For an input to the FPGA, you could use just a series resistor and let >> any overvoltage or spikes hit the ESD diodes. Debounce as needed. >> >> 10K or something. Paranoids could add a cap to ground, 1 nf or so. >> > >The MAX 10 datasheet has very specific input overvoltage specs, for >instance 4.27V for <4.3% of the time over 11.4 years, equally bizarrely >they specifiy the supply voltage to three decimal places. > >I think Intel really do not want the ESD diodes to be touched? > >piglet >
What are they for? -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
On Thursday, October 14, 2021 at 2:29:54 PM UTC-4, John Robertson wrote:
> I am still working with a friend on a TTL level replacement for some > chips on a pinball board and we have a nice FPGA (MAX 10 based) but it > wants (of course) 3.3V I/O. > > I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS > arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof > 1/8W or less resistor on the outside world interface would do for > protection. > > Any other suggestions to reduce real estate, but give us both TTL/3.3 > interfacing as well as protecting the SN74CBT16211C where it interfaces > to switches, etc. in the 'real world' of a pinball game cabinet. > > Thanks! > > John :-#)# > > PS, my apologies for being On-Topic! > -- > (Please post followups or tech inquiries to the USENET newsgroup) > John's Jukes Ltd. > MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 > (604)872-5757 (Pinballs, Jukes, Video Games) > www.flippers.com > "Old pinballers never die, they just flip out."
There's this that does what you want, maybe too much. https://store.open-electronics.org/LLCTTL If you click on the schematic link, you get some MOSFET level shifters: https://www.sparkfun.com/datasheets/BreakoutBoards/Level-Converter-v10.pdf Since you're too busy to tell anyone whether you're talking about 8 I/O or 500 I/O, tough shit if this doesn't work for you.
On 2021/10/14 4:03 p.m., Fred Bloggs wrote:
> On Thursday, October 14, 2021 at 2:29:54 PM UTC-4, John Robertson wrote: >> I am still working with a friend on a TTL level replacement for some >> chips on a pinball board and we have a nice FPGA (MAX 10 based) but it >> wants (of course) 3.3V I/O. >> >> I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS >> arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof >> 1/8W or less resistor on the outside world interface would do for >> protection. >> >> Any other suggestions to reduce real estate, but give us both TTL/3.3 >> interfacing as well as protecting the SN74CBT16211C where it interfaces >> to switches, etc. in the 'real world' of a pinball game cabinet. >> >> Thanks! >> >> John :-#)# >> >> PS, my apologies for being On-Topic! >> -- >> (Please post followups or tech inquiries to the USENET newsgroup) >> John's Jukes Ltd. >> MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 >> (604)872-5757 (Pinballs, Jukes, Video Games) >> www.flippers.com >> "Old pinballers never die, they just flip out." > > There's this that does what you want, maybe too much. > https://store.open-electronics.org/LLCTTL > > If you click on the schematic link, you get some MOSFET level shifters: > https://www.sparkfun.com/datasheets/BreakoutBoards/Level-Converter-v10.pdf > > Since you're too busy to tell anyone whether you're talking about 8 I/O or 500 I/O, tough shit if this doesn't work for you. >
Ah, I didn't think to give an IO count. It is around 50 in total, with 20 or so actually going to the outside world. Thanks! John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."
On Friday, October 15, 2021 at 6:32:28 AM UTC+11, John Larkin wrote:
> On Thu, 14 Oct 2021 11:29:43 -0700, John Robertson <sp...@flippers.com> > wrote: > >I am still working with a friend on a TTL level replacement for some > >chips on a pinball board and we have a nice FPGA (MAX 10 based) but it > >wants (of course) 3.3V I/O. > > > >I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS > >arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof > >1/8W or less resistor on the outside world interface would do for > >protection. > > > >Any other suggestions to reduce real estate, but give us both TTL/3.3 > >interfacing as well as protecting the SN74CBT16211C where it interfaces > >to switches, etc. in the 'real world' of a pinball game cabinet. > > > >Thanks! > > > >John :-#)# > > > >PS, my apologies for being On-Topic! > For an input to the FPGA, you could use just a series resistor and let > any overvoltage or spikes hit the ESD diodes. Debounce as needed. > > 10K or something. Paranoids could add a cap to ground, 1 nf or so.
Not an entirely safe approach. When you let ESD diode dump current into the substrate it can come out in odd places, and the FPGA could start behaving strangely. I've never tried it with an FPGA but I did get my fingers burnt with a less complex integrated circuit. -- Bill Sloman, Sydney
On Friday, October 15, 2021 at 7:21:36 AM UTC+11, John Larkin wrote:
> On Thu, 14 Oct 2021 21:16:03 +0100, piglet <erichp...@hotmail.com> > wrote: > >On 14/10/2021 8:32 pm, John Larkin wrote: > >> > >> For an input to the FPGA, you could use just a series resistor and let > >> any overvoltage or spikes hit the ESD diodes. Debounce as needed. > >> > >> 10K or something. Paranoids could add a cap to ground, 1 nf or so. > >> > > > >The MAX 10 datasheet has very specific input overvoltage specs, for > >instance 4.27V for <4.3% of the time over 11.4 years, equally bizarrely > >they specifiy the supply voltage to three decimal places. > > > >I think Intel really do not want the ESD diodes to be touched? > > > >piglet > > > What are they for?
They protect the circuit from catastrophic and irreversible failure. There's no guarantee that the circuit will work to specification when there's current flowing through the protection diodes, and the one time I tried it, the integrated circuit didn't - and it took fairly careful measurements to show that it wasn't. -- Bill Sloman, Sydney
On 10/14/2021 4:24 PM, John Robertson wrote:
> Ah, I didn't think to give an IO count. It is around 50 in total, with 20 or so > actually going to the outside world.
ISTR most schematics are available, "somewhere". You might be better served posting a link to the actual board you're trying to replace (or patch) so folks can see what the I/Os actually look like. E.g., a sound board interfacing to the main board has a different notion of "the outside world" than one that monitors live rubber.
On Thursday, October 14, 2021 at 11:29:54 AM UTC-7, John Robertson wrote:
> I am still working with a friend on a TTL level replacement for some > chips on a pinball board and we have a nice FPGA (MAX 10 based) but it > wants (of course) 3.3V I/O. > > I was thinking that SN74CBT16211C (24 x IO level shifter) and a TVS > arrays like the D3V3X8U9LP3810-7 with a low Ohm (1 - 10R) Flame-Proof > 1/8W or less resistor on the outside world interface would do for > protection. > > Any other suggestions to ...protecting the SN74CBT16211C where it interfaces > to switches, etc. in the 'real world' of a pinball game cabinet.
Depending on how many 'switches' there are, the venerable MC1489 tolerates lots of input volts and is a multiple-sourced jellybean. I"m wary of diode-to-Vdd clamps, though, since that just puts trash currents into your 3.3V regulator; the chip can survive, but will regulation suffer? No need for a series resistor, that's built-in, and '1489 input clamping is to GND.