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.
Minimilist Level Shifting
Started by ●April 8, 2023
Reply by ●April 9, 20232023-04-09
Reply by ●April 9, 20232023-04-09
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
Reply by ●April 9, 20232023-04-09
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
Reply by ●April 9, 20232023-04-09
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.
Reply by ●April 9, 20232023-04-09
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
Reply by ●April 9, 20232023-04-09
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. --- Get 1,000 miles of free Supercharging --- Tesla referral code - https://ts.la/richard11209
Reply by ●April 9, 20232023-04-09
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
Reply by ●April 9, 20232023-04-09
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.
Reply by ●April 9, 20232023-04-09
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
Reply by ●April 9, 20232023-04-09
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
