Driving LEDs from PIC weak pullup, saving resistors?

[Jan Panteltje]

What I don't know is what other logic circuit inside the device is
looking at that level ~ wondering what to do with it ~ I already
closed the data sheet.

Looks like you have to open it again, best before you critizise.

 

[John Larkin]

On a sunny day (Sun, 6 Jan 2008 10:56:06 -0800 (PST)) it happened
<[email protected]>:



Hm, so your drawing 200uA, what's the voltage drop across the diode?

1.77V @ 200uA red LED.

Click <here> for pizza

Ok, that's to high to be a valid low voltage, is that a valid high
voltage?
Data sheet says a valid high is 2v or [.25xVdd +.8] or are you
operating in an invalid voltage range?

http://www.interfacebus.com/voltage_threshold.html

I think your input is oscillating and burning up that pin, but that's
just me......

Interesting point. Even if it's not oscillating, the input cmos
transistors may be biased in their linear range and wasting a lot more
current than the chip is dumping into the led.

We recently had some tiny-logic cmos gates powered from +5 but
logically driven from +3.3. They work, but glow nicely on a thermal
imager.

John

I think you should take into account that many of the PORT inputs also can be
configured as AD channels,
and the diagram shows some sort of analog mux at the input, then followed by a
D flop, and also branched to a schmitt trigger for the other logs.
Page 75 etc of the pdf.

Well, it's worth considering. If you can turn on the pullup but not
enable the normal logic input path, it won't be an issue.

Can you configure the pin as an output and pwm it? That gives you
brightness control!

John

 

[www.interfacebus.com]

Looks like you have to open it again, best before you critizise.

What did I miss? You have it configured as a digital input pin right?
I did see a late post about an A/D function, I assumed you had that
turned of......
Or should I be apologizing now

 

[Jan Panteltje]

On Sun, 6 Jan 2008 11:31:12 -0800 (PST), "www.interfacebus.com"

On a sunny day (Sun, 6 Jan 2008 10:56:06 -0800 (PST)) it happened
<[email protected]>:



Hm, so your drawing 200uA, what's the voltage drop across the diode?

1.77V @ 200uA red LED.

Click <here> for pizza

Ok, that's to high to be a valid low voltage, is that a valid high
voltage?
Data sheet says a valid high is 2v or [.25xVdd +.8] or are you
operating in an invalid voltage range?

http://www.interfacebus.com/voltage_threshold.html

I think your input is oscillating and burning up that pin, but that's
just me......

Interesting point. Even if it's not oscillating, the input cmos
transistors may be biased in their linear range and wasting a lot more
current than the chip is dumping into the led.

We recently had some tiny-logic cmos gates powered from +5 but
logically driven from +3.3. They work, but glow nicely on a thermal
imager.

John

I think you should take into account that many of the PORT inputs also can be
configured as AD channels,
and the diagram shows some sort of analog mux at the input, then followed by a
D flop, and also branched to a schmitt trigger for the other logs.
Page 75 etc of the pdf.

Well, it's worth considering. If you can turn on the pullup but not
enable the normal logic input path, it won't be an issue.

Can you configure the pin as an output and pwm it? That gives you
brightness control!

There is a PWM output, I have implented it in the soft, but it is intended to
drive high power stuff (MOSFET).
The issue here is not so much variable brightness, but sparing out X resistors.
As Vladimir ... think of all the energy needed to:
1) make the 8 resistors.
2) drive out and buy the 8 resistors.
3) To solder the 8 resistors.
4) To tin the extra tracks, mine the silver, etc.
5) The time spend with a high wattage lamp to see, and soldering iron fixing
the 8 tiny SMD thingies.
6) The manufacturing of the money needed to buy the 8 resistors.
7) The extra accounting, extra paper, paperwork, BOM, design time, board space,
documentation, extra bits in pictures, added fault probability, all ask
for no resistors.
So it is a green solution, even with blue or red LEDs.

 

[Jan Panteltje]

What did I miss? You have it configured as a digital input pin right?
I did see a late post about an A/D function, I assumed you had that
turned of......
Or should I be apologizing now

DO YOUR HOMEWORK AND LOOK AT THE PDF.

 

[Anthony Fremont]

Now that I found that most people drive LEDs (the bright ones)
with less then half a milli amp, I just did a test with a PIC:

Configured pin as input, activated weak pull up, LED between
ground and pin, LED burns nice low brightness.
To switch the LED off, configure port as out and level zero, or
perhaps disable weak pull up.

This save any resistors driving LEDs

The weak pull ups are specified from 50 to 400uA, with 250uA as
typical.
The difference in one PIC is probably very low, making nice equal LED
current. I tried with a red high brightness LED and it looks really
nice,
measured about 200uA.

Any comments? This bad?

If I was Apple I sure would patent this.
Claim, anyone attempting to now patent this, will be faced with prior
art.
I am releasing this solution IN THE PUBLIC DOMAIN.
Jan Panteltje (c) 2008
Sun Jan 6 14:22:55 CET 2008

Cool, but it still wastes power in the resistor. You can have your cake and
eat it too by using an 18F (most seem to have >100R output impedance on the
i/o pins) and PWM the LED. No resistor, no wasted power. ;-)

 

[John Larkin]

On a sunny day (Sun, 06 Jan 2008 11:39:57 -0800) it happened John Larkin
<[email protected]>:

On Sun, 6 Jan 2008 11:31:12 -0800 (PST), "www.interfacebus.com"

On a sunny day (Sun, 6 Jan 2008 10:56:06 -0800 (PST)) it happened
<[email protected]>:



Hm, so your drawing 200uA, what's the voltage drop across the diode?

1.77V @ 200uA red LED.

Click <here> for pizza

Ok, that's to high to be a valid low voltage, is that a valid high
voltage?
Data sheet says a valid high is 2v or [.25xVdd +.8] or are you
operating in an invalid voltage range?

http://www.interfacebus.com/voltage_threshold.html

I think your input is oscillating and burning up that pin, but that's
just me......

Interesting point. Even if it's not oscillating, the input cmos
transistors may be biased in their linear range and wasting a lot more
current than the chip is dumping into the led.

We recently had some tiny-logic cmos gates powered from +5 but
logically driven from +3.3. They work, but glow nicely on a thermal
imager.

John

I think you should take into account that many of the PORT inputs also can be
configured as AD channels,
and the diagram shows some sort of analog mux at the input, then followed by a
D flop, and also branched to a schmitt trigger for the other logs.
Page 75 etc of the pdf.

Well, it's worth considering. If you can turn on the pullup but not
enable the normal logic input path, it won't be an issue.

Can you configure the pin as an output and pwm it? That gives you
brightness control!

There is a PWM output, I have implented it in the soft, but it is intended to
drive high power stuff (MOSFET).
The issue here is not so much variable brightness, but sparing out X resistors.
As Vladimir ... think of all the energy needed to:
1) make the 8 resistors.
2) drive out and buy the 8 resistors.
3) To solder the 8 resistors.
4) To tin the extra tracks, mine the silver, etc.
5) The time spend with a high wattage lamp to see, and soldering iron fixing
the 8 tiny SMD thingies.
6) The manufacturing of the money needed to buy the 8 resistors.
7) The extra accounting, extra paper, paperwork, BOM, design time, board space,
documentation, extra bits in pictures, added fault probability, all ask
for no resistors.
So it is a green solution, even with blue or red LEDs.

John

I was thinking of pwm *without* resistors.

John

 

[www.interfacebus.com]

DO YOUR HOMEWORK AND LOOK AT THE PDF.

You mean this typo:
"... and that pin (RB1) will be configured as input.
..... PIC 16F690. ..."

Maybe that should be 'RA1'

I'm going for wings and a beer ~ have a better one

 

[Jan Panteltje]

Cool, but it still wastes power in the resistor. You can have your cake and
eat it too by using an 18F (most seem to have >100R output impedance on the
i/o pins) and PWM the LED. No resistor, no wasted power. ;-)

PWM is not always nice, the thing I do not like is that you see a zillion LEds
when it moves fast.
Als to do PWM on each pin takes a lot of resources in a PIC.

 

[Jan Panteltje]

I was thinking of pwm *without* resistors.

OK.

I was thinking if I would win the lottery (have not looked yet).

 

[www.interfacebus.com]

DO YOUR HOMEWORK AND LOOK AT THE PDF.

Dude you post 300 to 700 times a month, what is up with that?
Do you work or what?
My wings were goog btw.......

 

[Don Klipstein]

Now that I found that most people drive LEDs (the bright ones)
with less then half a milli amp, I just did a test with a PIC:

Configured pin as input, activated weak pull up, LED between
ground and pin, LED burns nice low brightness.
To switch the LED off, configure port as out and level zero, or perhaps
disable weak pull up.

This save any resistors driving LEDs

The weak pull ups are specified from 50 to 400uA, with 250uA as typical.
The difference in one PIC is probably very low, making nice equal LED current.
I tried with a red high brightness LED and it looks really nice,
measured about 200uA.

Any comments? This bad?

If I was Apple I sure would patent this.
Claim, anyone attempting to now patent this, will be faced with prior art.
I am releasing this solution IN THE PUBLIC DOMAIN.
Jan Panteltje (c) 2008
Sun Jan 6 14:22:55 CET 2008

I have yet to read the 30 or so articles down the thread as of my late
entry into this thread.

However, I do suggest that InGaN LEDs (blue, blue-green,
non-yellowish-green and white, with typical forward voltage drop
3.something volts at "characterization current") do very well at low
currents. I see this even more true for blue, blue-green and green ones
with nominal wavelength 470 to 530 nm and "characterization current" of 20
mA.

I am aware of some red ones being less-bad than the above at losing
efficiency once the current is down to the roughly-10-microamp ballpark.
However, if current will be in the ballpark of a couple hundred microamps,
InGaN blue, blue-green and green LEDs are the champs!

Beware of color shift from such low current: Most green ones become
"lime green" to "yellowish green", most blue-green ones get closer to
"emerald green", and most blue ones shift at least to "turquoise",
sometimes to a hue closer to "cyan".

- Don Klipstein ([email protected])