Post by thread:[OT]More than 8 LED's from a single 8-bit port

OK, I got some food in my stomach so I can think a little more clearly now. You'll only need 4 pins to give you 12 LED's with this method. You can light any LED at any time OR if you need all on, you'll need to multiplex them. The pins that are not actively driving the LED in question (only two pins are active at any one time) need to be set to tri-state (inputs). Let me try and draw a picture: First, each LED is back to back with the other, such as: ->|- and -|<- meaning each anode is hooked to the cathode of the other in each two LED group. I will represent this as a -**- in my drawing, where each "**" represents two LED's back to back. O is a PIC PIN So here we go: |------------**------------| | | | |-------**--------| | | | |-------**--------| | | | | | |---**---|---**---|---**---| | | | | O O O O 12 Leds from 4 pins. I left out the currently limiting resistors, but you'll need to add them directly at the PIC pins... don't forget the current will be going through two resistors, so make sure each is half of the final value. Jeff

Don McKenzie wrote: > Jeff King wrote: > > > > > > If you need all 10 on at once, your going to have to multiplex > > them....might be messy but should be doable. > > Not really messy, you should be able to do a matrix of 4 by 4 giving 16 > LEDs. By messy I meant the multiplex code on a PIC that doesn't have a TMR overflow IRQ. My "assumption" is he was trying to get the absolute lowest price.... meaning, no TMR overflow IRQ.. but been a while since I worked on a real low end PIC.... do they all have TMR overflow? Then not messy at all. The 12" turkey sub is starting to kick in so the brain is coming alive. -Jeff

Jeff King wrote: > O is a PIC PIN > > So here we go: > > |------------**------------| > | | > | |-------**--------| > | | | > |-------**--------| | > | | | | > |---**---|---**---|---**---| > | | | | > O O O O > > 12 Leds from 4 pins. > > I left out the currently limiting resistors, but you'll > need to add them directly at the PIC pins... don't That's basically what I meant Jeff, I just can't draw as well. It must be that Turkey Sub. :-) Of course you can do it with max-2719's and 164 shift registers and multiplexing, and etc, but if it's a simple circuit to get 10 LEDs out of 8 pins, then the above is fine. Don McKenzie spam_OUTdonTakeThisOuTdontronics.com http://www.dontronics.com World's Largest Range of Atmel/AVR and PICmicro Hardware and Software. Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

On Thu, 6 Jan 2000, Jeff King wrote: {Quote hidden}> OK, I got some food in my stomach so I can think a little > more clearly now. > > You'll only need 4 pins to give you 12 LED's with this > method. You can light any LED at any time OR if you > need all on, you'll need to multiplex them. > > The pins that are not actively driving the LED in question > (only two pins are active at any one time) need to be set > to tri-state (inputs). > > Let me try and draw a picture: > > First, each LED is back to back with the other, such as: > > ->|- and > -|<- meaning each anode is hooked to the cathode of the other > in each two LED group. I will represent this as a -**- in my drawing, > where each "**" represents two LED's back to back. > > O is a PIC PIN > > So here we go: > > > |------------**------------| > | | > | |-------**--------| > | | | > |-------**--------| | > | | | | > |---**---|---**---|---**---| > | | | | > O O O O > > 12 Leds from 4 pins. > > I left out the currently limiting resistors, but you'll > need to add them directly at the PIC pins... don't > forget the current will be going through two resistors, > so make sure each is half of the final value. You know, I've been doing a lot of programming lately - too much in fact. But the last time I played with leds you couldn't get current to flow through a 'back-to-back' configuration. Maybe I should just stick to programming. :). Scott

Scott Dattalo wrote: > > First, each LED is back to back with the other, such as: > > > > ->|- and > > -|<- meaning each anode is hooked to the cathode of the other > > in each two LED group. snip--- > You know, I've been doing a lot of programming lately - too much in fact. > But the last time I played with leds you couldn't get current to flow > through a 'back-to-back' configuration. Maybe I should just stick to > programming. :). > > Scott He may have used the wrong terminology 'back to back' but what Jeff said was anode of one to cathode of the other in each 2 group, meaning across each other, just like a dual or tri-color LED configuration. Don McKenzie .....donKILLspam@spam@dontronics.com http://www.dontronics.com World's Largest Range of Atmel/AVR and PICmicro Hardware and Software. Free Basic Compiler and Programmer http://www.dontronics.com/runavr.html

Scott Dattalo wrote: > You know, I've been doing a lot of programming lately - too much in fact. > But the last time I played with leds you couldn't get current to flow > through a 'back-to-back' configuration. Maybe I should just stick to > programming. :). No, just send me more turkey subs. Back to back was confusing. Is there another way to say connect them across each other? (anode to cathode with BOTH pins). When I think of back to back, I see either ---->|--|<---- or: ----------|<---->|----- which of course I did not mean at all. I meant: |----|<-----| -| |- |---->|-----| Is there another term then back to back for this configuration? -Jeff

parallell led's.................but opposite polarity....... -----Original Message----- From: Jeff King <jeffKILLspamaerodata.net> To: .....PICLISTKILLspam.....MITVMA.MIT.EDU <EraseMEPICLISTspam_OUTTakeThisOuTMITVMA.MIT.EDU> Date: Thursday, January 06, 2000 5:51 PM Subject: Re: [OT]More than 8 LED's from a single 8-bit port {Quote hidden}>Scott Dattalo wrote: > >> You know, I've been doing a lot of programming lately - too much in fact. >> But the last time I played with leds you couldn't get current to flow >> through a 'back-to-back' configuration. Maybe I should just stick to >> programming. :). > >No, just send me more turkey subs. Back to back was confusing. Is >there another way to say connect them across each other? (anode to >cathode with BOTH pins). > >When I think of back to back, I see either ---->|--|<---- >or: ----------|<---->|----- >which of course I did not mean at all. I meant: > > |----|<-----| >-| |- > |---->|-----| > >Is there another term then back to back for this configuration? > >-Jeff

Try saying Inverse Parallel. (words like parallel and series go over much better than back to back and side by side, etc) -Adam Jeff King wrote: {Quote hidden}> > Scott Dattalo wrote: > > > You know, I've been doing a lot of programming lately - too much in fact. > > But the last time I played with leds you couldn't get current to flow > > through a 'back-to-back' configuration. Maybe I should just stick to > > programming. :). > > No, just send me more turkey subs. Back to back was confusing. Is > there another way to say connect them across each other? (anode to > cathode with BOTH pins). > > When I think of back to back, I see either ---->|--|<---- > or: ----------|<---->|----- > which of course I did not mean at all. I meant: > > |----|<-----| > -| |- > |---->|-----| > > Is there another term then back to back for this configuration? > > -Jeff

On Thu, 6 Jan 2000, Jeff King wrote: {Quote hidden}> Scott Dattalo wrote: > > > You know, I've been doing a lot of programming lately - too much in fact. > > But the last time I played with leds you couldn't get current to flow > > through a 'back-to-back' configuration. Maybe I should just stick to > > programming. :). > > No, just send me more turkey subs. Back to back was confusing. Is > there another way to say connect them across each other? (anode to > cathode with BOTH pins). > > When I think of back to back, I see either ---->|--|<---- > or: ----------|<---->|----- > which of course I did not mean at all. I meant: > > |----|<-----| > -| |- > |---->|-----| > > Is there another term then back to back for this configuration? After I sent the message, I thought about how you would do this and then understood what you are talking about. The term you're looking for is 'anti-parallel'. But it has been a looonngg day. Regards, Scott

Jeff King wrote: > By messy I meant the multiplex code on a PIC that doesn't have a TMR > overflow IRQ. My "assumption" is he was trying to get the absolute > lowest price.... meaning, no TMR overflow IRQ.. but been a while since > I worked on a real low end PIC.... do they all have TMR overflow? No, the low-end devices have neither IRQ, which doesn't matter a hoot, nor TMR0 overflow flag (T0IF) which is *much* more important. But it's still really easy. You must of course use polling, but this is no great hassle. You divide up your "main loop" processing into blocks which are achievable within your multiplex interval, say 2 ms. You intersperse the sequence of these blocks with alternate waits for one and zero on TMR0 bit 7, and arrange so that TMR0 overflows each 4 ms (i.e., bit 7 toggles each 2 ms). IOW, TMR0 is clocked at 64 µs, prescaler set to 32 (option mask b'100') using 4.096 MHz crystal. Program flow goes: waithi macro btfss TMR0,7 goto $-1 endm waitlo macro btfsc TMR0,7 goto $-1 endm loop: do_1st ; Up to approx 2 ms processing waithi strobe_next_digit do_2nd ; Up to approx 2 ms processing waitlo strobe_next_digit do_3rd ; Up to approx 2 ms processing waithi strobe_next_digit do_4th ; Up to approx 2 ms processing waitlo strobe_next_digit goto loop Note that do_1st, do_2nd etc. do not have to always execute in less than 2 ms, as long as they do so on average and never, in combination with strobe_next_digit, approach 4 ms either singly or as a consecutive pair. Hope this is clear Scott Dattalo wrote, quoting Jeff King: >> Is there another term then back to back for this configuration? > The term you're looking for is 'anti-parallel'. Yeah, and various other permutations thereof. One, numeric, sprang to mind but I dare not say... (;-) Jinx wrote: > Combining the 4-pin and 4017 methods together - use a BCD- output > counter (eg 74LS393 or 4040) and set the bit pattern on the o/ps to > match the 4-pin PIC idea. Sorry, it doesn't work that way. The 4-pin trick requires the ability to tri-state (set to input) the I/O lines. Also, if you're going to use an output expander, you want a shift register which loads all your data in 8 clocks, rather than a counter. Updates will be so fast that intermediate states will be invisible, so a latching shift register is unnecessary for display. If each output of a 74HC164 will drive sufficient current (you can have 10 mA, but no more :), then use five of the outputs to drive up to five LEDs out of your ten low simultaneously, and two other outputs (one left over!) to multiplex alternate banks of five LEDs high via a NPN emitter follower with collector supplied from unregulated Vcc. Resistors are only required for the five cathode drives, none are needed for the emitter followers. The same method could be used with seven PIC outputs, which can drive five LEDs low simultaneously with 20 mA. This is similar to the original multiplexed proposal except that the emitter followers allow lots more current - much brighter! -- Cheers, Paul B.

Note I modified drawing below: + = resistor Marcelo Yamamoto wrote: {Quote hidden}> Jeff wrote: > >First, each LED is back to back with the other, such as: > >|------------**------------| > >| | > >| |-------**--------| > >| | | > >|-------**--------| | > >| | | | > >|---**---|---**---|---**---| > >+ + + + > >O O O O > > Pay attention to do not use high bright leds, as the current will flow thru > the desired led and thru 3 other leds in series that will be in parallel > with the first one. > > o--+----L---------+-----o > |---L--L--L---- | As I mentioned my earlier post, I hadn't shown the resistors, but they were there. Each pin has a resistor.... lets say 220ohms for a total of 440ohms. So any LED that is powered across it will see about 2.2volts, or whatever its turnon voltage is. Remember, a LED is still a diode, and the turnon knee is fairly sharp. So, in "theory" the most any other LED will see will be 1.1 volts, so it shouldn't turn on. Of course, if you start mixing technologies..... maybe a blue LED with a red led, since they have varying turn-on voltages, you might have problems. -Jeff

Jeff wrote: >As I mentioned my earlier post, I hadn't shown the resistors, but they >were there. Each pin has a resistor.... lets say 220ohms for a total >of 440ohms. So any LED that is powered across it will see about >2.2volts, or whatever its turnon voltage is. > >Remember, a LED is still a diode, and the turnon knee is fairly >sharp. So, in "theory" the most any other LED will see will be >1.1 volts, so it shouldn't turn on. Of course, if you start mixing >technologies..... maybe a blue LED with a red led, since they >have varying turn-on voltages, you might have problems. Yes, I agree. But I have tried it and you should avoid some High bright leds. They turn on even with 1 volt. Not fully bright, but you can see it is lighted. Marcelo Y.