Minimilist Level Shifting...

[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?

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[bitrex]

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>

 

[John Larkin]

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?

 

[Ricky]

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. 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

Maybe I\'m missing something, but the transistor circuit seems to be equivalent to an open collector circuit. I suppose the transistor action on the rising edge in this configuration does pump current into the 3.3V side until the BE is no longer forward biased, so a speed up over a simple pullup. In this case the actual pullup just assures the signal stays high when the transistor is cut off. Still, that\'s two resistors and a transistor, so more circuitry than a pair of resistors or a resistor-Zener combination. I can get two Zeners in a SOT-363 package which is just 2 x 2 mm. Also, the two resistors are likely to draw more current that a simple resistor or Zener circuit.

I can\'t say I understand the other circuit. Should I assume the inverter is powered from 3.3V? It would appear that R3 is important to the delay of the circuit and presents the same problems of current draw as the Zener or resistor circuits. The 74HC14 is 5V tolerant. Why not just use the inverter by itself?

What am I missing?

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

[Clifford Heath]

On 09/04/23 11:22, 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?

I use this arrangement:
<https://www.digikey.com/en/blog/logic-level-shifting-basics?

Works in both directions, speed-limited by the pullup R on one side.

 

[bitrex]

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.

 

[Ricky]

On Sunday, April 9, 2023 at 12:29:25 AM UTC-4, bitrex 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.

They don\'t call me Ricky Bobby for nothin\'! Yeah, I don\'t know if it is used much, but the RS-422 device is rated for 20 MHz or more and there\'s no reason for this circuit to slow it down. I don\'t recall the speed limitation inside the FPGA. Not because of logic delays, but there\'s a circuit to extract a clock from data transitions which can\'t run faster than a third the FPGA clock. Sort of a Nyquist like thing.

I\'ll probably just use a couple of LVC inverters. It\'s probably just easier to do that, than to fuss over the limitations of a circuit that will be twice the size on the board.

Generally, I like to optimize designs for costs in manufacturing, and of course, the board is going to be very, very tight. At least the parts shortages seem to be loosening up a bit. Right now, I don\'t have any parts in the design that I can\'t buy. That\'s a big plus!

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Sunday, April 9, 2023 at 12:04:51 AM UTC-4, Clifford Heath wrote:

On 09/04/23 11:22, 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?

I use this arrangement:
https://www.digikey.com/en/blog/logic-level-shifting-basics?

Works in both directions, speed-limited by the pullup R on one side.

Yes, I\'ve used that sort of circuit before, but with the switch inside a chip and 10 bits wide. It was always easy to buy and very cheap. So, of course, in this design I don\'t need it anymore. I do still need two bits. I was hoping to use a Greenpak device for this, but they have various capabilities in various chips. I can\'t find a chip that even includes two of the several jelly bean functions I\'d like to include, of which this is one. These functions just are not compatible in a Greenpak device.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[bitrex]

On 4/9/2023 1:00 AM, Ricky wrote:

On Sunday, April 9, 2023 at 12:29:25 AM UTC-4, bitrex 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.

They don\'t call me Ricky Bobby for nothin\'! Yeah, I don\'t know if it is used much, but the RS-422 device is rated for 20 MHz or more and there\'s no reason for this circuit to slow it down. I don\'t recall the speed limitation inside the FPGA. Not because of logic delays, but there\'s a circuit to extract a clock from data transitions which can\'t run faster than a third the FPGA clock. Sort of a Nyquist like thing.

I\'ll probably just use a couple of LVC inverters. It\'s probably just easier to do that, than to fuss over the limitations of a circuit that will be twice the size on the board.

Generally, I like to optimize designs for costs in manufacturing, and of course, the board is going to be very, very tight. At least the parts shortages seem to be loosening up a bit. Right now, I don\'t have any parts in the design that I can\'t buy. That\'s a big plus!

12 cent in quantity, makes it pretty easy:

<https://www.mouser.com/ProductDetail/Texas-Instruments/SN74LV1T34DBVR?qs=8sOby8ZxZLE4ekkxA3S6Wg%3D%3D>

That JT circuit (which was originally for 3.3 to 5) I think can be made
to work at speed by running everything at 3.3 volts and putting 5 volts
to the emitter, but it will be as you say hard to make low-power and
it\'s a fair number of parts

 

[bitrex]

On 4/9/2023 1:17 AM, bitrex wrote:

On 4/9/2023 1:00 AM, Ricky wrote:
On Sunday, April 9, 2023 at 12:29:25 AM UTC-4, bitrex 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.

They don\'t call me Ricky Bobby for nothin\'!  Yeah, I don\'t know if it
is used much, but the RS-422 device is rated for 20 MHz or more and
there\'s no reason for this circuit to slow it down.  I don\'t recall
the speed limitation inside the FPGA.  Not because of logic delays,
but there\'s a circuit to extract a clock from data transitions which
can\'t run faster than a third the FPGA clock.  Sort of a Nyquist like
thing.

I\'ll probably just use a couple of LVC inverters.  It\'s probably just
easier to do that, than to fuss over the limitations of a circuit that
will be twice the size on the board.

Generally, I like to optimize designs for costs in manufacturing, and
of course, the board is going to be very, very tight.  At least the
parts shortages seem to be loosening up a bit.  Right now, I don\'t
have any parts in the design that I can\'t buy.  That\'s a big plus!


12 cent in quantity, makes it pretty easy:

https://www.mouser.com/ProductDetail/Texas-Instruments/SN74LV1T34DBVR?qs=8sOby8ZxZLE4ekkxA3S6Wg%3D%3D

That JT circuit (which was originally for 3.3 to 5) I think can be made
to work at speed by running everything at 3.3 volts and putting 5 volts
to the emitter, but it will be as you say hard to make low-power and
it\'s a fair number of parts

Like it might be a more cost-effective hack if you had 20 lines to shift
but if it\'s just two pair of differential send and returns probably best
to just by the chip that does the thing, at that price.

 

[whit3rd]

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.

I can\'t say I understand the other circuit. Should I assume the inverter is powered from 3.3V? It would appear that R3 is important to the delay of the circuit and presents the same problems of current draw as the Zener or resistor circuits. The 74HC14 is 5V tolerant.

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.

 

[Ricky]

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.

I can\'t say I understand the other circuit. Should I assume the inverter is powered from 3.3V? It would appear that R3 is important to the delay of the circuit and presents the same problems of current draw as the Zener or resistor circuits. The 74HC14 is 5V tolerant.
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?

Bit explained that it was actually a 3.3V to 5V circuit. So clearly, it isn\'t going to do a great job of 5V to 3.3V, at least, not much better than an open collector transistor. The inverter is just there to shape up the slow rise time with its Schmitt trigger.

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

 

[piglet]

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

 

[whit3rd]

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.

I can\'t say I understand the other circuit. Should I assume the inverter is powered from 3.3V? It would appear that R3 is important to the delay of the circuit and presents the same problems of current draw as the Zener or resistor circuits. The 74HC14 is 5V tolerant.
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.

 

[Ricky]

On Sunday, April 9, 2023 at 3:43:29 AM UTC-4, piglet 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,

Yeah, on the original iteration of this design, I forgot about the level shift required. I actually used a Zener diode rather than the resistor and got the current down to ~2 mA static. I know people talk about this being acceptable practice, but I\'m not up for that. A single logic part with two inverters (74LCV14) will do the job, and is available in a very small no-lead package. I like to keep a clean BoM with a minimum of parts, but a single inverter chip won\'t be a problem. Lots of stock at Digikey and Mouser and many, many substitutes.

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

 

[Ricky]

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.

I can\'t say I understand the other circuit. Should I assume the inverter is powered from 3.3V? It would appear that R3 is important to the delay of the circuit and presents the same problems of current draw as the Zener or resistor circuits. The 74HC14 is 5V tolerant.
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.

But it\'s also not 5V tolerant. If you are suggesting to use the entire circuit with the inverter, transistor and four resistors, that\'s just excessive. I don\'t know what that circuit was intended for, but maybe it was before there were single chip alternatives, or the 74HC14 was already in the circuit. This design is going to be extremely tight, so I\'m scrubbing all the space I can. The 74LVC14 sounds good because I can get it in a 1 x 1.5 mm package. That\'s about the size of a single 0603 resistor.

--

Rick C.

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[Fred Bloggs]

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.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[server]

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.

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.

 

[Jasen Betts]

On 2023-04-09, 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.

If you\'re not too high in the megabauds that could work.

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.

1mA into a 3.3V zener won\'t get you more than 3.3V, and should get you
enough for a logic high

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

Any thoughts?

Use a 3.3V transceiver.

--
Jasen.
🇺🇦 Слава Україні

 

[Lasse Langwadt Christensen]

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