Timeline for Why are LEDs on most embedded designs inverted?
Current License: CC BY-SA 3.0
15 events
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| May 24, 2017 at 9:27 | comment | added | Lundin | Mostly the core issue here is that Microchip is peddling old 1970s technology and pretends that it is something modern. That's in fact their whole business model. | |
| May 24, 2017 at 2:27 | comment | added | Nick Alexeev | Somewhat similar question about driving an LED by sourcing or sinking current: Driving LED with PIC 5V | |
| May 23, 2017 at 22:02 | comment | added | D.A.S. | @OlinLathrop "6F1xxx series is a recent sequal to the old 16Fxxx series." Yes I am aware of that, but they chose not to upgrade the RdsOn specs so Vol,Voh are identical so that Q would not change in legacy board designs which affects stripline characteristics and ringing. Changing driver Z in half on uncontrolled impedance tracks ( read inductive) may cause false edges from ringing) hence, grandfathered driver specs for those operating at max f.clk. Q=2pi*f * L(f)/ESR for source drive | |
| May 23, 2017 at 21:54 | comment | added | D.A.S. | The lower the Vdd MAX , the more symmetrical they design N and Pch RdsOn.. In particular because of 74HCT technology which is asymmetrical threshold 5V logic designed to also match TTL V thresholds of 1.4V, but there is no TTL logic rated for 1.x to 3.x V so all these use symmetrical input threshold (Vdd/2) and symmetrical RdsOn drivers. (even though 5V logic input for 74HCxxx is also Vdd/2 ) hope this makes logical sense.) there is a lot of standards hidden to users and Shoothru current is one of them. Hence why you can still use logic inverters as analog gain amps , with negative feedback | |
| May 23, 2017 at 21:46 | comment | added | D.A.S. | In general the LOWER the Vdd (or Vcc) max e.g. 3.8 vs 6 vs 15V then the LOWER the Vth of the FET and smaller lithography (newer) and ALSO the Lower RdsOn in order to get faster Tr rise time or max clock speed. yet -10% tolerance on that Vdd in that chip always yields higher RdsOn or ESR.. HENCE, 3.3V logic is ~25 Ohms and 5V logic ~50 Ohms or ~75 Ohms at 3.3 with a 25% tolerance on ESR approx. Speeds are always rated for Cload max such as 30 or 50pF (limited by Id for Vdd/RdsOn) thus you can see why for same f why they design symmetrical RdsOn and reduce RdsOn with Vdd !!! | |
| May 23, 2017 at 21:33 | comment | added | TonyM | Thanks for straightening the text up, downvote now upvote. They're clearly not predominant like they used to be - they're around but the diminishing minority. Good thing too, it was always a rubbish part of electronics that we all had to design around. | |
| May 23, 2017 at 19:26 | history | edited | Olin Lathrop | CC BY-SA 3.0 |
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| May 23, 2017 at 19:22 | comment | added | Olin Lathrop | @Tony: The PIC 16F1xxx series is a recent sequal to the old 16Fxxx series. It has a updated memory architecture, some new instructions, etc. New ones are being designed now. Certainly they can change the I/O ring if they want to. The one I showed happened to be the first PIC datasheet I looked in. PICs generally have higher current drive, which may be why most of them are asymmetrical. There are still a lot of asymmetrical outputs out there. I'll add some weasel words, but "fairly common" is still correct. | |
| May 23, 2017 at 18:58 | comment | added | D.A.S. | that's how old 16F1 must be .. I have written extensively in this forum about the RdsOn vs Vdd and CD4xxx, 74HCxxx 74 LV, ALC AVLC2 are ALL more symmetrical by design with vintage and lower ESR or Rs or RdsOn = Vol/Iol and (Vdd-Voh)/Ioh but no one else has given kudos for this revelation. | |
| May 23, 2017 at 18:30 | comment | added | TonyM | Thanks for looking but Microchip part looks misleading. I looked up: NXP P89LPC933 (8051, 2004) with Iol=Ioh=20 mA; NXP LPC1111 (ARM, 2010) with Iol=Ioh=4 mA; TI OMAP5910 with Iol=Ioh=same (configurable) mA; TI TMS320C620 with Iol=Ioh=8 mA; Silicon Labs EFM32GG380 (2014) with Iol=Ioh=same (configurable) mA. Ignore the missing '-' on sinks. Could have kept going, just quick 5 mins in my datasheet library...personally, I haven't seen an unbalanced one in decades. Please can you re-edit your answer similar to my earlier comment, good to give the full picture and harmless to your answer to do so. | |
| May 23, 2017 at 18:23 | comment | added | D.A.S. | The PIC16F1xxxx are an upgrade from the old PIC16Fxxx line but still ancient technology. They are all symmetrical now as close as possible with wide tolerances 25% typ. and 1/Vdd sensitive. | |
| May 23, 2017 at 17:07 | history | edited | Olin Lathrop | CC BY-SA 3.0 |
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| May 23, 2017 at 16:59 | comment | added | Olin Lathrop | @Tony: Asymmetric source/sink is still common, although less than it used to be. I just checked one of the PIC 16F1xxx (16F1359 specifically) parts, which are fairly new. With 5 V Vdd a high output can source 3.5 mA with 700 mV drop. A low output can sink 8 mA with 600 mV drop. This is far from gone, even in modern micros. | |
| May 23, 2017 at 16:41 | comment | added | TonyM | Hi, a correction for your answer: [it was fairly common that microcontroller output pins could sink more current in the low state than they can source in the high state. Microcontrollers of the last 10 years or more have balanced outputs that source as much as they sink] I agree completely it was the case with 8048, 8051, 6811 and all the old'uns but not so with post-2005 or thereabouts-ish stuff like all the ARMs. Thanks. | |
| May 23, 2017 at 16:29 | history | answered | Olin Lathrop | CC BY-SA 3.0 |