Minimilist Level Shifting...

On Sun, 9 Apr 2023 00:29:17 -0400, bitrex <user@example.net> wrote:

On 4/8/2023 10:39 PM, John Larkin wrote:
On Sat, 8 Apr 2023 22:22:44 -0400, bitrex <user@example.net> wrote:

On 4/8/2023 9:22 PM, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


The common base shifter seems a good compromise between cost and
speed/power consumption:

https://next-hack.com/wp-content/uploads/2017/09/A5I9.png

And there\'s Jim Thompson\'s variant on it if the naked topology edges are
too slow:

https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf

Why not one resistor?

One resistor from the 5 volt output to the upper clamp diode of the
following 3.3 V CMOS seems OK for low speeds but RS-422 can go fast, do
we wanna go fast? Don\'t know.
Click to expand...

RS422 isn\'t fast!

It can probably be done with a direct connection, if the driver is
classic TTL and the load is modern CMOS. TTL doesn\'t pull up to +5.

A 200 ohm resistor would limit the ESD diode current if that\'s a
concern.
 
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet
Click to expand...

200 ohms was my suggestion!
 
On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote:
On Sunday, April 9, 2023 at 3:44:30 AM UTC-4, whit3rd wrote:
On Sunday, April 9, 2023 at 12:34:09 AM UTC-7, Ricky wrote:
On Sunday, April 9, 2023 at 3:08:46 AM UTC-4, whit3rd wrote:
On Saturday, April 8, 2023 at 8:12:07 PM UTC-7, Ricky wrote:
On Saturday, April 8, 2023 at 10:22:51 PM UTC-4, bitrex wrote:
On 4/8/2023 9:22 PM, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS.

If you want to power it from 5V, it\'s 5V tolerant; if you power it from 3.3V, it isn\'t. CD4049 and CD4050,
powered from 3V takes 5V input, different input clamp diode structure.

That could work. But I\'d need a circuit to shift from the 5V output of the CD4000 parts to the 3.3V device being driven. > > > > See the problem?

No problem; 4000 series CMOS works at 3.3V just fine (not very fast, as logic goes, though).
CMOS output isn\'t 5V if you don\'t power it from 5V.
Click to expand...

Does your 3.3V end of the problem even have a +5V supply?

> But it\'s also not 5V tolerant.

There\'s 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point.
If, by \'it\' one means a general 4000 series part, or 74HC14, on 3.3V those \'it\' items are not 5V tolerant.

> If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that\'s just excessive.

Yeah, it\'s a nuisance; some of the \'digital transistor\' items can lower the resistor count, or if
you have a 3.3V supply available that eliminates a resistor. The inverter isn\'t required to do
the level translation, it just assures clean output (and inversion, of course).

A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V
logic input, base to +3.3V, and a pullup resistor from collector to +5V is the
level-translator part of the JT design (for 3.3V in, 5V out) that you\'d want. One resistor only.
 
On Sunday, April 9, 2023 at 7:32:48 AM UTC-4, upsid...@downunder.com wrote:
On Sun, 9 Apr 2023 00:29:17 -0400, bitrex <us...@example.net> wrote:

On 4/8/2023 10:39 PM, John Larkin wrote:
On Sat, 8 Apr 2023 22:22:44 -0400, bitrex <us...@example.net> wrote:

On 4/8/2023 9:22 PM, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


The common base shifter seems a good compromise between cost and
speed/power consumption:

https://next-hack.com/wp-content/uploads/2017/09/A5I9.png

And there\'s Jim Thompson\'s variant on it if the naked topology edges are
too slow:

https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf

Why not one resistor?

One resistor from the 5 volt output to the upper clamp diode of the
following 3.3 V CMOS seems OK for low speeds but RS-422 can go fast, do
we wanna go fast? Don\'t know.
Put a small capacitance across the series resistor. This will speed up
the transitions. Keep the capacitance small so that it doesn\'t hurt
the CMOS ESD protection diodes during transitions. A suitable
capacitance would be in the same order as the CMOS input capacitance.

If a two resistor voltage divider is used to drop from 5 V to 3.3 V,
put capacitors in parallel with both resistors so that the reactance
ratio matches the resistor ratio. This doesn\'t risk the CMOS ESD
diodes, but do not use too big capacitors that might limit the TTL
output swing.

Regarding RS-422 transceivers, don\'t use full speed devices, unless it
is strictly required. The high speed devices connect a lot of high
speed line noise into the receivers. There are a lot of transceivers
that are speed limited to 250 kbit/s which is especially usable for
hundred of meters long lines. The RS-422 specifies up to 100 kbit/s up
to 1200 m long lines.
Click to expand...

As I\'ve said, I have to support MHz on the RS-422 signal. Slow devices are not suitable.

Two resistors and two capacitors would start to be significant real estate. Ii could use 0402 devices, with 1.6 x 0.7 mm pads. Just two such devices are the size of a 6-XSON and the eight I would need would be four times larger. I think using a LVC2G14 is a good way to go.

I\'d like to get the RS-422 device in a QFN, but supply is not clear at this point. But heck, this is the one part in the BoM that continues to be a problem in any package. Most of the parts I\'m using seem to have settled down and I can get what I will need for this year. I may be stuck with the TSSOP package for this part.

--

Rick C.

--+ Get 1,000 miles of free Supercharging
--+ Tesla referral code - https://ts.la/richard11209
 
On Sunday, April 9, 2023 at 5:58:01 AM UTC-4, Fred Bloggs wrote:
On Saturday, April 8, 2023 at 9:22:56 PM UTC-4, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget..

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?
There\'s tons of -422 level shifting transceivers for 3.3V in stock. The primitive kluges you have in mind don\'t come close to the same performance. Most of that junk with the MOSFETs, CB transistors, and HC14s, from the days when 1Mbps was considered lightning fast, are no longer suitable for a modern application.
Click to expand...

\"In stock\"? Perhaps, but nothing suitable for this project. I think you are trying to design without knowing the requirements. Never a good idea.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209
 
Am 09.04.23 um 20:16 schrieb whit3rd:
On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote:

Does your 3.3V end of the problem even have a +5V supply?

But it\'s also not 5V tolerant.

There\'s 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point.
If, by \'it\' one means a general 4000 series part, or 74HC14, on 3.3V those \'it\' items are not 5V tolerant.

If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that\'s just excessive.

Yeah, it\'s a nuisance; some of the \'digital transistor\' items can lower the resistor count, or if
you have a 3.3V supply available that eliminates a resistor. The inverter isn\'t required to do
the level translation, it just assures clean output (and inversion, of course).

A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V
logic input, base to +3.3V, and a pullup resistor from collector to +5V is the
level-translator part of the JT design (for 3.3V in, 5V out) that you\'d want. One resistor only.
Click to expand...

< https://assets.nexperia.com/documents/data-sheet/74LVC1G125.pdf >

There is also an 1G04.

Gerhard
 
On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote:
søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget..

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?
plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistor
Click to expand...

What value? Do you guarantee this for the part I\'m using? Does it matter how many lines are terminated this way?

That is the sort of short cut that I don\'t like to do. Injecting current into the ESD diodes always has risk, from unexpected events. Two resistors for two channels, is about the same board space as a single X2-DFN1410-6 which is very similar to the SOT886, 1 x 1.5 mm. I think the 74LVC2G14 is probably the best solution without playing games with ESD diodes. If the chip maker of the receiving part had an app note for interfacing to 5V this way, I might do it. But I have no interest in taking liability for replacing thousands of boards that start failing in the field after some months or years.

So other than one less resistor, how is the series resistor better than a voltage divider? The 200 ohm value I see recommended, would result in around 10 mA of high level current. I\'d like to use much less than this.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209
 
On Sunday, April 9, 2023 at 2:16:38 PM UTC-4, whit3rd wrote:
On Sunday, April 9, 2023 at 1:23:21 AM UTC-7, Ricky wrote:
On Sunday, April 9, 2023 at 3:44:30 AM UTC-4, whit3rd wrote:
On Sunday, April 9, 2023 at 12:34:09 AM UTC-7, Ricky wrote:
On Sunday, April 9, 2023 at 3:08:46 AM UTC-4, whit3rd wrote:
On Saturday, April 8, 2023 at 8:12:07 PM UTC-7, Ricky wrote:
On Saturday, April 8, 2023 at 10:22:51 PM UTC-4, bitrex wrote:
On 4/8/2023 9:22 PM, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS.
If you want to power it from 5V, it\'s 5V tolerant; if you power it from 3.3V, it isn\'t. CD4049 and CD4050,
powered from 3V takes 5V input, different input clamp diode structure.

That could work. But I\'d need a circuit to shift from the 5V output of the CD4000 parts to the 3.3V device being driven. > > > > See the problem?

No problem; 4000 series CMOS works at 3.3V just fine (not very fast, as logic goes, though).
CMOS output isn\'t 5V if you don\'t power it from 5V.
Does your 3.3V end of the problem even have a +5V supply?
But it\'s also not 5V tolerant.
There\'s 5V tolerant inputs on the particular parts, CD4049 and CD4050, when running on 3.3V, was the point.
Click to expand...

Ah, I was not aware that any parts in the CD4000 series were 5V tolerant when run from 3.3V. So that\'s like a 74LVC2G14 then, which is the part I\'m thinking of using, but in a MUCH smaller package.


If, by \'it\' one means a general 4000 series part, or 74HC14, on 3.3V those \'it\' items are not 5V tolerant.
If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that\'s just excessive.
Yeah, it\'s a nuisance; some of the \'digital transistor\' items can lower the resistor count, or if
you have a 3.3V supply available that eliminates a resistor. The inverter isn\'t required to do
the level translation, it just assures clean output (and inversion, of course).
Click to expand...

The circuit with the transistor, inverter and four resistors, is a bit absurd. If the design had an unused inverter and lots of room, fine. I have neither.


A common-base digital transistor like Rohm DTC014Y with emitter to the 3.3V
logic input, base to +3.3V, and a pullup resistor from collector to +5V is the
level-translator part of the JT design (for 3.3V in, 5V out) that you\'d want. One resistor only.
Click to expand...

But the pullup is either slow, or consumes excess current, or both. The 74LVC2G14 is a much better solution for the two channels I need (I can even buy them). Part of my frustration is the incredible plethora of logic families. I find it hard to track what is what and why one is better than another. Maybe I\'m just getting old. I remember when 74LS was the hot thing and then they came out with 74ALS! WOW! I think we had one design that used a 74S part for speed!

Thanks for the comments.

--

Rick C.

+-- Get 1,000 miles of free Supercharging
+-- Tesla referral code - https://ts.la/richard11209
 
søndag den 9. april 2023 kl. 20.45.32 UTC+2 skrev Ricky:
On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote:
søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes..

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?
plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistor
What value? Do you guarantee this for the part I\'m using? Does it matter how many lines are terminated this way?
Click to expand...

you already said two lines ...
 
On Sunday, April 9, 2023 at 3:00:33 PM UTC-4, Lasse Langwadt Christensen wrote:
søndag den 9. april 2023 kl. 20.45.32 UTC+2 skrev Ricky:
On Sunday, April 9, 2023 at 8:31:46 AM UTC-4, Lasse Langwadt Christensen wrote:
søndag den 9. april 2023 kl. 03.22.56 UTC+2 skrev Ricky:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?
plenty of parts are specified for injecting several mA into the ESD diodes so all you need going from 5V to 3.3V is single resistor
What value? Do you guarantee this for the part I\'m using? Does it matter how many lines are terminated this way?
you already said two lines ...
Click to expand...

And...

--

Rick C.

+-+ Get 1,000 miles of free Supercharging
+-+ Tesla referral code - https://ts.la/richard11209
 
Ricky <gnuarm.deletethisbit@gmail.com> wrote:
I\'m tired of digging around looking for an optimal chip to level shift a
pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There
are too many devices, and no small number of them are impacted by the
semiconductor shortage. But resistors are pretty available, as are Zener
diodes.

So, I could use a pair of resistors to simply divide the TTL output
voltage to something safe for CMOS. But there\'s not much wiggle room,
given that the TTL output is not well specified. If the current through
the divider is minimized, this slows the signal edges. So a mA or two
needs to be flowing when the output is high, but not much more, because of
the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to
20 mA of current. Running at 1 mA is into the knee as best I can tell, not
desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the
Zener might not do much better than resistors.

Any thoughts?
Click to expand...

I have 2 full unopened reels (2,500 pcs each) of SN74CBTD3384CDBQR parts
that I have no use for if somebody needs those :) And yet another partial
reel of 2,000 on top of those. That is 7K parts total.

They are dirt chip, probably even cheaper than single 74LVC1G parts... And
they are fast enough for PCI bus and beyond...

---
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* Las Vegas NV, USA < > Miracles require 24-hour notice. *
******************************************************************
 
On 2023-04-08 22:22, bitrex wrote:
On 4/8/2023 9:22 PM, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift
a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS.
There are too many devices, and no small number of them are impacted
by the semiconductor shortage.  But resistors are pretty available, as
are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output
voltage to something safe for CMOS.  But there\'s not much wiggle room,
given that the TTL output is not well specified.  If the current
through the divider is minimized, this slows the signal edges.  So a
mA or two needs to be flowing when the output is high, but not much
more, because of the power budget.

Another option is to use a Zener diode.  Most Zeners are specified at
5 to 20 mA of current. Running at 1 mA is into the knee as best I can
tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output,
the Zener might not do much better than resistors.

Any thoughts?


The common base shifter seems a good compromise between cost and
speed/power consumption:

https://next-hack.com/wp-content/uploads/2017/09/A5I9.png

And there\'s Jim Thompson\'s variant on it if the naked topology edges are
too slow:

https://electrooptical.net/static/oldsite/www.analog-innovations.com/SED/LogicLevelTranslator74HC14-RevA.pdf
Click to expand...

(mirror host takes bow)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Sunday, April 9, 2023 at 4:44:14 PM UTC-4, Sergey Kubushyn wrote:
Ricky <gnuarm.del...@gmail.com> wrote:
I\'m tired of digging around looking for an optimal chip to level shift a
pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There
are too many devices, and no small number of them are impacted by the
semiconductor shortage. But resistors are pretty available, as are Zener
diodes.

So, I could use a pair of resistors to simply divide the TTL output
voltage to something safe for CMOS. But there\'s not much wiggle room,
given that the TTL output is not well specified. If the current through
the divider is minimized, this slows the signal edges. So a mA or two
needs to be flowing when the output is high, but not much more, because of
the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to
20 mA of current. Running at 1 mA is into the knee as best I can tell, not
desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the
Zener might not do much better than resistors.

Any thoughts?
I have 2 full unopened reels (2,500 pcs each) of SN74CBTD3384CDBQR parts
that I have no use for if somebody needs those :) And yet another partial
reel of 2,000 on top of those. That is 7K parts total.

They are dirt chip, probably even cheaper than single 74LVC1G parts... And
they are fast enough for PCI bus and beyond...
Click to expand...

That\'s what I used for the previous revision on the interface to the main board. But that\'s 3.3V now, so no more need. But I was running these to signals through that, which is why I\'m looking for a 2 bit version.

--

Rick C.

++- Get 1,000 miles of free Supercharging
++- Tesla referral code - https://ts.la/richard11209
 
mandag den 10. april 2023 kl. 01.39.08 UTC+2 skrev Ricky:
On Sunday, April 9, 2023 at 4:44:14 PM UTC-4, Sergey Kubushyn wrote:
Ricky <gnuarm.del...@gmail.com> wrote:
I\'m tired of digging around looking for an optimal chip to level shift a
pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There
are too many devices, and no small number of them are impacted by the
semiconductor shortage. But resistors are pretty available, as are Zener
diodes.

So, I could use a pair of resistors to simply divide the TTL output
voltage to something safe for CMOS. But there\'s not much wiggle room,
given that the TTL output is not well specified. If the current through
the divider is minimized, this slows the signal edges. So a mA or two
needs to be flowing when the output is high, but not much more, because of
the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to
20 mA of current. Running at 1 mA is into the knee as best I can tell, not
desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the
Zener might not do much better than resistors.

Any thoughts?
I have 2 full unopened reels (2,500 pcs each) of SN74CBTD3384CDBQR parts
that I have no use for if somebody needs those :) And yet another partial
reel of 2,000 on top of those. That is 7K parts total.

They are dirt chip, probably even cheaper than single 74LVC1G parts... And
they are fast enough for PCI bus and beyond...
That\'s what I used for the previous revision on the interface to the main board. But that\'s 3.3V now, so no more need. But I was running these to signals through that, which is why I\'m looking for a 2 bit version.
Click to expand...

https://www.digikey.dk/da/products/detail/nexperia-usa-inc/74LVC2G34GM-115/1231594
 
On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet



200 ohms was my suggestion!
Click to expand...

Great minds think alike! :)

piglet
 
On Mon, 10 Apr 2023 13:03:46 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichpwagner@hotmail.com
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet



200 ohms was my suggestion!


Great minds think alike! :)

piglet
Click to expand...

And do things the easy way.
 
On Monday, April 10, 2023 at 8:03:54 AM UTC-4, piglet wrote:
On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichp...@hotmail.com
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes..

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet



200 ohms was my suggestion!

Great minds think alike! :)
Click to expand...

Too bad they come up with the wrong answer. What will be the current when the TTL output is high? Does anyone consider the requirements?

Parameter TEST CONDITIONS MIN TYP(1) MAX UNIT
VOH High-level output voltage VID = 200 mV, IOH = –6 mA 3.8 4.2 V

We\'ll see if that makes it through the usenet filter. Likely not.

--

Rick C.

+++ Get 1,000 miles of free Supercharging
+++ Tesla referral code - https://ts.la/richard11209
 
On 10/04/2023 3:32 pm, Ricky wrote:
On Monday, April 10, 2023 at 8:03:54 AM UTC-4, piglet wrote:
On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichp...@hotmail.com
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet



200 ohms was my suggestion!

Great minds think alike! :)

Too bad they come up with the wrong answer. What will be the current when the TTL output is high? Does anyone consider the requirements?

Parameter TEST CONDITIONS MIN TYP(1) MAX UNIT
VOH High-level output voltage VID = 200 mV, IOH = –6 mA 3.8 4.2 V

We\'ll see if that makes it through the usenet filter. Likely not.
Click to expand...

I don\'t get your point? If the TTL Voh is 4.2V then the current in 200R
would be 1mA (assuming 0.7V drop for the input clamp diode to 3.3Vdd)

If even 1mA is too much then one could increase the resistor until RC
delay becomes significant.

piglet
 
On Mon, 10 Apr 2023 17:30:33 +0100, piglet <erichpwagner@hotmail.com>
wrote:

On 10/04/2023 3:32 pm, Ricky wrote:
On Monday, April 10, 2023 at 8:03:54?AM UTC-4, piglet wrote:
On 09/04/2023 15:44, John Larkin wrote:
On Sun, 9 Apr 2023 08:43:20 +0100, piglet <erichp...@hotmail.com
wrote:

On 09/04/2023 02:22, Ricky wrote:
I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes.

So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget.

Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation.

I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors.

Any thoughts?


If the 3.3V input has a 5V tolerant ESD structure then of course no
level shift is needed. If the 3.3V supply rail is stiff enough to accept
a few mA injection then a series resistor might be all you need. For
example 200 ohms would limit ESD diode current to 5mA and assuming 10pF
input capacity would only slowdown 2ns,

piglet



200 ohms was my suggestion!

Great minds think alike! :)

Too bad they come up with the wrong answer. What will be the current when the TTL output is high? Does anyone consider the requirements?

Parameter TEST CONDITIONS MIN TYP(1) MAX UNIT
VOH High-level output voltage VID = 200 mV, IOH = –6 mA 3.8 4.2 V

We\'ll see if that makes it through the usenet filter. Likely not.


I don\'t get your point? If the TTL Voh is 4.2V then the current in 200R
would be 1mA (assuming 0.7V drop for the input clamp diode to 3.3Vdd)

If even 1mA is too much then one could increase the resistor until RC
delay becomes significant.

piglet
Click to expand...

Real 74xx style TTL doesn\'t pull up hard. Modern CMOS will tolerate 50
mA into its ESD diodes and not latch up. A direct connection would
work fine.

Yes, add 200 ohms for the faint of heart (or the math impaired.)
 
Ricky <gnuarm.deletethisbit@gmail.com> Wrote in message:r
> I\'m tired of digging around looking for an optimal chip to level shift a pair of signals from 5V TTL (an RS-422 transceiver) to 3.3V CMOS. There are too many devices, and no small number of them are impacted by the semiconductor shortage. But resistors are pretty available, as are Zener diodes. So, I could use a pair of resistors to simply divide the TTL output voltage to something safe for CMOS. But there\'s not much wiggle room, given that the TTL output is not well specified. If the current through the divider is minimized, this slows the signal edges. So a mA or two needs to be flowing when the output is high, but not much more, because of the power budget. Another option is to use a Zener diode. Most Zeners are specified at 5 to 20 mA of current. Running at 1 mA is into the knee as best I can tell, not desirable, resulting in poor regulation. I\'m thinking with the variation in output voltage from the TTL output, the Zener might not do much better than resistors. Any thoughts? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209

I use the M74VHC1GT50 an there\'s plenty in stock.

Cheers
--


----Android NewsGroup Reader----
https://piaohong.s3-us-west-2.amazonaws.com/usenet/index.html
 
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