Driving LEDs from PIC weak pullup, saving resistors?

[Jan Panteltje]

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

end of posting

 

[Fred Bloggs]

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

end of posting

What voltage does that weak pull-up current source develop across the
LED and what internal CMOS is looking at it?

 

[www.interfacebus.com]

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

end of posting

Your driving the LED with an input pin? That must be a mistype.

http://www.interfacebus.com/

 

[Vladimir Vassilevsky]

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,

This should be prohibited under the law since it contributes to the global
warming.
You should drive the ROHS LED by the green PWM at maximum environmentally
friendly current, so the energy waste would be minimal.

VLV

 

[Jan Panteltje]

Your driving the LED with an input pin? That must be a mistype.

No it is no typo, the inputs can have a weak pull up MOSFET, and
that current is enough to light a modern high brightnes LED.
The pull ups can be enabled and disabled via software too.

 

[www.interfacebus.com]

No it is no typo, the inputs can have a weak pull up MOSFET, and
that current is enough to light a modern high brightnes LED.
The pull ups can be enabled and disabled via software too.- Hide quoted text -

- Show quoted text -

Many IC pins have programmable input resistors, there for line
termination and to control floating lines and so on.

Hmm, it's called an "input" for a reason, I'm unsure why you would do
that. Do you have the PIC number so I can take a quick look at the
data sheet.

http://www.serialphy.com/

 

[Jan Panteltje]

Many IC pins have programmable input resistors, there for line
termination and to control floating lines and so on.

Hmm, it's called an "input" for a reason, I'm unsure why you would do
that. Do you have the PIC number so I can take a quick look at the
data sheet.

It goes for most, if not all, PICs I know, I am using a 16F690 now.
The reason is, as I stated, that moderne high brightness LEDs are
way too bright at say even 1mA, and people like to drive these at
levels as low as 400uA these days, to reduce eye damage
This requires some resistors, but using the weak pullups has enough
current for normal LED signalling use, and saves resistors.
This became only possible now the LEDs shine bright already at 200uA.

 

[www.interfacebus.com]

It goes for most, if not all, PICs I know, I am using a 16F690 now.
The reason is, as I stated, that moderne high brightness LEDs are
way too bright at say even 1mA, and people like to drive these at
levels as low as 400uA these days, to reduce eye damage
This requires some resistors, but using the weak pullups has enough
current for normal LED signalling use, and saves resistors.
This became only possible now the LEDs shine bright already at 200uA.

Ok, most of those pins appear to be I/O pins. What pin are you using?

http://www.serialphy.com/

 

[Jan Panteltje]

Ok, most of those pins appear to be I/O pins. What pin are you using?

Yes, they are almost all IO pins.
The TRIS (TRISB for port B) bits indicate if the pin is an input or output.

So you can do:
bsf TRISB, 1
and that pin (RB1) will be configured as input.
You need to globally enable weak pull ups with bit NOT_RABPU in OPTION_REG,
then have individual control for each pin in the WPUB register.
See page 72 of the 294 page document on the PIC 16F690.

 

[John Larkin]

This should be prohibited under the law since it contributes to the global
warming.
You should drive the ROHS LED by the green PWM at maximum environmentally
friendly current, so the energy waste would be minimal.

VLV

You could certainly configure the port to be an output and drive the
led pwm to control brightness, but I think the energy used would be
the same.

Unless you add a series inductor, in which case the whole thing
becomes a synchronous switcher.

John

 

[www.interfacebus.com]

Yes, they are almost all IO pins.
The TRIS (TRISB for port B) bits indicate if the pin is an input or output..

So you can do:
 bsf TRISB, 1
and that pin (RB1) will be configured as input.
You need to globally enable weak pull ups with bit NOT_RABPU in OPTION_REG,
then have individual control for each pin in the WPUB register.
See page 72 of the 294 page document on the PIC 16F690.- Hide quoted text -

- Show quoted text -

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

 

[Vladimir Vassilevsky]

You could certainly configure the port to be an output and drive the
led pwm to control brightness, but I think the energy used would be
the same.

It is better then shorting a pullup to the ground; as for the PWM
control, it is not too obvious because of the nonlinear conductance and
capacitance of the LED.

Unless you add a series inductor, in which case the whole thing
becomes a synchronous switcher.

Did you account for the energy required to build the inductor and to put
it on the board?

VLV

 

[Jan Panteltje]

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

1.77V @ 200uA red LED.

Click <here> for pizza

 

[John Larkin]

It is better then shorting a pullup to the ground; as for the PWM
control, it is not too obvious because of the nonlinear conductance and
capacitance of the LED.

Jan mentioned that he got dark by reconfiguring the port, so no power
is wasted then.

Did you account for the energy required to build the inductor and to put
it on the board?

I assumed that the copper was recycled from the recent remodel of Al
Gore's mansion.

John

 

[www.interfacebus.com]

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......

 

[John Larkin]

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

 

[Jan Panteltje]

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?

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

I did not notice any current increase, you cannot burn a PIC with 2.5 mA
Idd.
In fact I think any voltage on the input is valid, but some thresholds are
set for logic 0 and logic 1.
It is irrelevant if it is a valid high if you drive a LED, and no, it is
not oscillating.

 

[Jan Panteltje]

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.

 

[John Popelish]

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.

Perhaps using PIC inputs that include Schmitt triggers would
keep the supply current down. To be sure, I would have to
dive into the data sheet for the particular PIC in question.

 

[www.interfacebus.com]

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

I did not notice any current increase, you cannot burn a PIC with 2.5 mA
Idd.
In fact I think any voltage on the input is valid, but some thresholds are
set for logic 0 and logic 1.
It is irrelevant if it is a valid high if you drive a LED, and no, it is
not oscillating.- Hide quoted text -

- Show quoted text -

Well that's up to you. This is digital Logic right, so you can only
have a high or low. Any other voltage level is undefined, and the IC
is no longer guaranteed to function, that's why they put those numbers
in the data sheet.

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. Your not really driving the LED, your sourcing
current out of an input pin ~ could be 50 other CMOS FETs inside with
invalid input voltages.

Leroy
ElectricaL Engineer; 1984