Prototyping a little toy for the kiddos out of junk box parts... I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts is boosted to around 5 to power the 74LS and Numitron segments via an eBay special micro boost converter with shutdown pin. The test code right now simply outputs a BCD to the '47 inputs and pulls the shutdown pin on the boost high to turn it on. The problem is the uP starts up fine and all the output voltages on the 4 BCD pins looks good in isolation, but when the circuit is powered up with the Numitron and '47 connected up the bench supply immediately falls over into overcurrent protection at about 80 mA, way more than say two incandescent segments to form the numeral 1 should be drawing at 5 volts, I believe. The boost has plenty enough oomph to power all the Numitron digits from either the LiPo or the bench supply, I tested that in isolation. Tested two different '47s with the same result. Not sure exactly what's going on here...could there be some ESD/clamp diode pathway that's being driven that I don't know about? Or a startup sequencing issue?
Driving 74LS from 3.3V uP
Started by ●July 12, 2016
Reply by ●July 12, 20162016-07-12
On 07/12/2016 01:52 PM, bitrex wrote:> Prototyping a little toy for the kiddos out of junk box parts... > > I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 > segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) > powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts > is boosted to around 5 to power the 74LS and Numitron segments via an > eBay special micro boost converter with shutdown pin. > > The test code right now simply outputs a BCD to the '47 inputs and pulls > the shutdown pin on the boost high to turn it on. > > The problem is the uP starts up fine and all the output voltages on the > 4 BCD pins looks good in isolation, but when the circuit is powered up > with the Numitron and '47 connected up the bench supply immediately > falls over into overcurrent protection at about 80 mA, way more than say > two incandescent segments to form the numeral 1 should be drawing at 5 > volts, I believe. > > The boost has plenty enough oomph to power all the Numitron digits from > either the LiPo or the bench supply, I tested that in isolation. > > Tested two different '47s with the same result. Not sure exactly what's > going on here...could there be some ESD/clamp diode pathway that's being > driven that I don't know about? Or a startup sequencing issue? >I should add to be clear that there's no output on the display when the bench supply goes into overcurrent, the open-collector output drivers are not turning on.
Reply by ●July 12, 20162016-07-12
On Tue, 12 Jul 2016 13:52:29 -0400, bitrex wrote:> Prototyping a little toy for the kiddos out of junk box parts... > > I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 > segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) > powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts > is boosted to around 5 to power the 74LS and Numitron segments via an > eBay special micro boost converter with shutdown pin. > > The test code right now simply outputs a BCD to the '47 inputs and pulls > the shutdown pin on the boost high to turn it on. > > The problem is the uP starts up fine and all the output voltages on the > 4 BCD pins looks good in isolation, but when the circuit is powered up > with the Numitron and '47 connected up the bench supply immediately > falls over into overcurrent protection at about 80 mA, way more than say > two incandescent segments to form the numeral 1 should be drawing at 5 > volts, I believe. > > The boost has plenty enough oomph to power all the Numitron digits from > either the LiPo or the bench supply, I tested that in isolation. > > Tested two different '47s with the same result. Not sure exactly what's > going on here...could there be some ESD/clamp diode pathway that's being > driven that I don't know about? Or a startup sequencing issue?Double-check connections? Measure power supply voltage to the chips themselves? -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Reply by ●July 12, 20162016-07-12
In article <Hbahz.1$Dh7.0@fx13.iad>, bitrex <bitrex@de.lete.earthlink.net> wrote:>> Prototyping a little toy for the kiddos out of junk box parts... >> >> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >> is boosted to around 5 to power the 74LS and Numitron segments via an >> eBay special micro boost converter with shutdown pin. >> >> The test code right now simply outputs a BCD to the '47 inputs and pulls >> the shutdown pin on the boost high to turn it on. >> >> The problem is the uP starts up fine and all the output voltages on the >> 4 BCD pins looks good in isolation, but when the circuit is powered up >> with the Numitron and '47 connected up the bench supply immediately >> falls over into overcurrent protection at about 80 mA, way more than say >> two incandescent segments to form the numeral 1 should be drawing at 5 >> volts, I believe.Are the ATTiny's pins 5-volt tolerant, when the chip is operated from a 3.3-volt supply? Based on what I see in the 74LS74 data sheet, the flip-flop's input pins are almost certainly going to be trying to pull the Tiny's output pins up to around 4.3 volts. This may very well be turning on some overvoltage-protection diodes in the Tiny. The 74LS74 data sheet suggests that Iil is limited to around 3.2 mA per pin, worst case, so 80 mA surprises me. ... Looking at the ATTiny85 data, it appears to me that it is *not* tolerant in this way. Absolute maximum voltage on any pin is limited to -0.5V, to Vcc+0.5V. So, you're overdriving it. You'll need a different sort of driver circuit to do this safely.
Reply by ●July 12, 20162016-07-12
On 07/12/2016 02:39 PM, Dave Platt wrote:> In article <Hbahz.1$Dh7.0@fx13.iad>, > bitrex <bitrex@de.lete.earthlink.net> wrote: > >>> Prototyping a little toy for the kiddos out of junk box parts... >>> >>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>> is boosted to around 5 to power the 74LS and Numitron segments via an >>> eBay special micro boost converter with shutdown pin. >>> >>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>> the shutdown pin on the boost high to turn it on. >>> >>> The problem is the uP starts up fine and all the output voltages on the >>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>> with the Numitron and '47 connected up the bench supply immediately >>> falls over into overcurrent protection at about 80 mA, way more than say >>> two incandescent segments to form the numeral 1 should be drawing at 5 >>> volts, I believe. > > Are the ATTiny's pins 5-volt tolerant, when the chip is operated from > a 3.3-volt supply? > > Based on what I see in the 74LS74 data sheet, the flip-flop's input > pins are almost certainly going to be trying to pull the Tiny's > output pins up to around 4.3 volts. This may very well be turning on > some overvoltage-protection diodes in the Tiny. The 74LS74 > data sheet suggests that Iil is limited to around 3.2 mA per pin, > worst case, so 80 mA surprises me. > > ... Looking at the ATTiny85 data, it appears to me that it is *not* > tolerant in this way. Absolute maximum voltage on any pin is limited > to -0.5V, to Vcc+0.5V. So, you're overdriving it. > > You'll need a different sort of driver circuit to do this safely. >Ah, thank you. Seems like I should be able to swap in a 74HC4511 and run it off the 3.3 instead?
Reply by ●July 12, 20162016-07-12
On Tue, 12 Jul 2016 11:39:18 -0700, dplatt@coop.radagast.org (Dave Platt) wrote:>In article <Hbahz.1$Dh7.0@fx13.iad>, >bitrex <bitrex@de.lete.earthlink.net> wrote: > >>> Prototyping a little toy for the kiddos out of junk box parts... >>> >>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>> is boosted to around 5 to power the 74LS and Numitron segments via an >>> eBay special micro boost converter with shutdown pin. >>> >>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>> the shutdown pin on the boost high to turn it on. >>> >>> The problem is the uP starts up fine and all the output voltages on the >>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>> with the Numitron and '47 connected up the bench supply immediately >>> falls over into overcurrent protection at about 80 mA, way more than say >>> two incandescent segments to form the numeral 1 should be drawing at 5 >>> volts, I believe. > >Are the ATTiny's pins 5-volt tolerant, when the chip is operated from >a 3.3-volt supply? > >Based on what I see in the 74LS74 data sheet, the flip-flop's input >pins are almost certainly going to be trying to pull the Tiny's >output pins up to around 4.3 volts. This may very well be turning on >some overvoltage-protection diodes in the Tiny. The 74LS74 >data sheet suggests that Iil is limited to around 3.2 mA per pin, >worst case, so 80 mA surprises me. > >... Looking at the ATTiny85 data, it appears to me that it is *not* >tolerant in this way. Absolute maximum voltage on any pin is limited >to -0.5V, to Vcc+0.5V. So, you're overdriving it. > >You'll need a different sort of driver circuit to do this safely.Some of those old LS parts may draw significant crowbar currents as the supply is ramped up... choking the "eBay special". ...Jim Thompson -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I'm looking for work... see my website.
Reply by ●July 12, 20162016-07-12
On Tue, 12 Jul 2016 11:39:18 -0700, dplatt@coop.radagast.org (Dave Platt) wrote:>In article <Hbahz.1$Dh7.0@fx13.iad>, >bitrex <bitrex@de.lete.earthlink.net> wrote: > >>> Prototyping a little toy for the kiddos out of junk box parts... >>> >>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>> is boosted to around 5 to power the 74LS and Numitron segments via an >>> eBay special micro boost converter with shutdown pin. >>> >>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>> the shutdown pin on the boost high to turn it on. >>> >>> The problem is the uP starts up fine and all the output voltages on the >>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>> with the Numitron and '47 connected up the bench supply immediately >>> falls over into overcurrent protection at about 80 mA, way more than say >>> two incandescent segments to form the numeral 1 should be drawing at 5 >>> volts, I believe. > >Are the ATTiny's pins 5-volt tolerant, when the chip is operated from >a 3.3-volt supply? > >Based on what I see in the 74LS74 data sheet, the flip-flop's input >pins are almost certainly going to be trying to pull the Tiny's >output pins up to around 4.3 volts. This may very well be turning on >some overvoltage-protection diodes in the Tiny. The 74LS74 >data sheet suggests that Iil is limited to around 3.2 mA per pin, >worst case, so 80 mA surprises me.It's an LS47, not an LS74. But that doesn't matter.> >... Looking at the ATTiny85 data, it appears to me that it is *not* >tolerant in this way. Absolute maximum voltage on any pin is limited >to -0.5V, to Vcc+0.5V. So, you're overdriving it. > >You'll need a different sort of driver circuit to do this safely.An LS input pulls itself up to about 1 volt, and weakly at that. That shouldn't bother the uP. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●July 12, 20162016-07-12
On 07/12/2016 03:59 PM, John Larkin wrote:> On Tue, 12 Jul 2016 11:39:18 -0700, dplatt@coop.radagast.org (Dave > Platt) wrote: > >> In article <Hbahz.1$Dh7.0@fx13.iad>, >> bitrex <bitrex@de.lete.earthlink.net> wrote: >> >>>> Prototyping a little toy for the kiddos out of junk box parts... >>>> >>>> I have a IV-9 Numitron connected up on a PCB to a 74LS47 BCD to 7 >>>> segment driver. uP is a 3.3 volt ATTiny (Adafruit "Trinket" devboard) >>>> powered via an on-board LDO directly off a 3.7 V 150mAh lipo. 3.7 volts >>>> is boosted to around 5 to power the 74LS and Numitron segments via an >>>> eBay special micro boost converter with shutdown pin. >>>> >>>> The test code right now simply outputs a BCD to the '47 inputs and pulls >>>> the shutdown pin on the boost high to turn it on. >>>> >>>> The problem is the uP starts up fine and all the output voltages on the >>>> 4 BCD pins looks good in isolation, but when the circuit is powered up >>>> with the Numitron and '47 connected up the bench supply immediately >>>> falls over into overcurrent protection at about 80 mA, way more than say >>>> two incandescent segments to form the numeral 1 should be drawing at 5 >>>> volts, I believe. >> >> Are the ATTiny's pins 5-volt tolerant, when the chip is operated from >> a 3.3-volt supply? >> >> Based on what I see in the 74LS74 data sheet, the flip-flop's input >> pins are almost certainly going to be trying to pull the Tiny's >> output pins up to around 4.3 volts. This may very well be turning on >> some overvoltage-protection diodes in the Tiny. The 74LS74 >> data sheet suggests that Iil is limited to around 3.2 mA per pin, >> worst case, so 80 mA surprises me. > > It's an LS47, not an LS74. But that doesn't matter. > > >> >> ... Looking at the ATTiny85 data, it appears to me that it is *not* >> tolerant in this way. Absolute maximum voltage on any pin is limited >> to -0.5V, to Vcc+0.5V. So, you're overdriving it. >> >> You'll need a different sort of driver circuit to do this safely. > > An LS input pulls itself up to about 1 volt, and weakly at that. That > shouldn't bother the uP. > >I'm currently rewriting the code to test if it might be a "sequencing" issue. Hold the outputs tri-state, bring up the boost supply via the enable pin, then set the pins to output and latch the data.
Reply by ●July 12, 20162016-07-12
In article <1siaobh469ijefbhiunll9l6hllnuvofer@4ax.com>, John Larkin <jjlarkin@highlandtechnology.com> wrote:>It's an LS47, not an LS74. But that doesn't matter.You're right, I mis-read. But, as you say, it doesn't matter.>An LS input pulls itself up to about 1 volt, and weakly at that. That >shouldn't bother the uP.The TI datasheet for the 74LS47 shows the equivalent input circuit as having a resistor (nominal 20k) tied to Vcc, and then fed through a zener to the input pin. It looks as if it'll try to pull up to "Vcc minus one diode drop" - not all that strongly, but that may be enough to pull the ATTiny's pin up above its absolute-maximum limit. The O.P. may want to measure the Tiny's output-pin voltages, with the 5-volt boost both on and off, and see if the chip's spec is being violated. Assuming that the problem isn't due to a short circuit or miswiring somewhere, the idea of switching to a CMOS 7-segment driver and running the chip from the 3.3-volt supply would not be a bad idea... this would honor the Tiny's absolute-maximum voltage ratings. If I understand correctly, the 7-segment drivers are open-collector / open-drain, and so the 7-segment display itself could be run directly from the LiPo battery. This would avoid the need for a boost converter, and should increase electrical efficiency - fewer unnecessary volts to dissipate in the 7-segment current limiting resistors, and so smaller resistors could be used.
Reply by ●July 12, 20162016-07-12
On Tue, 12 Jul 2016 13:54:37 -0700, dplatt@coop.radagast.org (Dave Platt) wrote:>In article <1siaobh469ijefbhiunll9l6hllnuvofer@4ax.com>, >John Larkin <jjlarkin@highlandtechnology.com> wrote: > >>It's an LS47, not an LS74. But that doesn't matter. > >You're right, I mis-read. But, as you say, it doesn't matter. > >>An LS input pulls itself up to about 1 volt, and weakly at that. That >>shouldn't bother the uP. > >The TI datasheet for the 74LS47 shows the equivalent input circuit as >having a resistor (nominal 20k) tied to Vcc, and then fed through a >zener to the input pin. It looks as if it'll try to pull up to "Vcc >minus one diode drop" - not all that strongly, but that may be enough >to pull the ATTiny's pin up above its absolute-maximum limit.That's not a zener, it's a Schottky diode. But the resistor-schottky node is clamped to ground through two more diode drops, so it can't go very positive. TTL and LSTTL don't pull up very far, around +1 volt. I suspect something else is going on. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
